session.go

// mgo - MongoDB driver for Go
//
// Copyright (c) 2010-2012 - Gustavo Niemeyer <gustavo@niemeyer.net>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this
//    list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright notice,
//    this list of conditions and the following disclaimer in the documentation
//    and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package mgo

import (
	"camlistore.org/third_party/labix.org/v2/mgo/bson"
	"crypto/md5"
	"encoding/hex"
	"errors"
	"fmt"
	"math"
	"net"
	"reflect"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"
)

type mode int

const (
	Eventual  mode = 0
	Monotonic mode = 1
	Strong    mode = 2
)

// When changing the Session type, check if newSession and copySession
// need to be updated too.

type Session struct {
	m            sync.RWMutex
	cluster_     *mongoCluster
	slaveSocket  *mongoSocket
	masterSocket *mongoSocket
	slaveOk      bool
	consistency  mode
	queryConfig  query
	safeOp       *queryOp
	syncTimeout  time.Duration
	sockTimeout  time.Duration
	defaultdb    string
	dialAuth     *authInfo
	auth         []authInfo
}

type Database struct {
	Session *Session
	Name    string
}

type Collection struct {
	Database *Database
	Name     string // "collection"
	FullName string // "db.collection"
}

type Query struct {
	m       sync.Mutex
	session *Session
	query   // Enables default settings in session.
}

type query struct {
	op       queryOp
	prefetch float64
	limit    int32
}

type getLastError struct {
	CmdName  int         "getLastError"
	W        interface{} "w,omitempty"
	WTimeout int         "wtimeout,omitempty"
	FSync    bool        "fsync,omitempty"
	J        bool        "j,omitempty"
}

type Iter struct {
	m              sync.Mutex
	gotReply       sync.Cond
	session        *Session
	server         *mongoServer
	docData        queue
	err            error
	op             getMoreOp
	prefetch       float64
	limit          int32
	docsToReceive  int
	docsBeforeMore int
	timeout        time.Duration
	timedout       bool
}

var ErrNotFound = errors.New("not found")

const defaultPrefetch = 0.25

// Dial establishes a new session to the cluster identified by the given seed
// server(s). The session will enable communication with all of the servers in
// the cluster, so the seed servers are used only to find out about the cluster
// topology.
//
// Dial will timeout after 10 seconds if a server isn't reached. The returned
// session will timeout operations after one minute by default if servers
// aren't available. To customize the timeout, see DialWithTimeout,
// SetSyncTimeout, and SetSocketTimeout.
//
// This method is generally called just once for a given cluster.  Further
// sessions to the same cluster are then established using the New or Copy
// methods on the obtained session. This will make them share the underlying
// cluster, and manage the pool of connections appropriately.
//
// Once the session is not useful anymore, Close must be called to release the
// resources appropriately.
//
// The seed servers must be provided in the following format:
//
//     [mongodb://][user:pass@]host1[:port1][,host2[:port2],...][/database][?options]
//
// For example, it may be as simple as:
//
//     localhost
//
// Or more involved like:
//
//     mongodb://myuser:mypass@localhost:40001,otherhost:40001/mydb
//
// If the port number is not provided for a server, it defaults to 27017.
//
// The username and password provided in the URL will be used to authenticate
// into the database named after the slash at the end of the host names, or
// into the "admin" database if none is provided.  The authentication information
// will persist in sessions obtained through the New method as well.
//
// The following connection options are supported after the question mark:
//
//     connect=direct
//
//         This option will disable the automatic replica set server
//         discovery logic, and will only use the servers provided.
//         This enables forcing the communication with a specific
//         server or set of servers (even if they are slaves).  Note
//         that to talk to a slave you'll need to relax the consistency
//         requirements using a Monotonic or Eventual mode via SetMode.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Connections
//
func Dial(url string) (*Session, error) {
	session, err := DialWithTimeout(url, 10*time.Second)
	if err == nil {
		session.SetSyncTimeout(1 * time.Minute)
		session.SetSocketTimeout(1 * time.Minute)
	}
	return session, err
}

// DialWithTimeout works like Dial, but uses timeout as the amount of time to
// wait for a server to respond when first connecting and also on follow up
// operations in the session. If timeout is zero, the call may block
// forever waiting for a connection to be made.
//
// See SetSyncTimeout for customizing the timeout for the session.
func DialWithTimeout(url string, timeout time.Duration) (*Session, error) {
	uinfo, err := parseURL(url)
	if err != nil {
		return nil, err
	}
	direct := false
	for k, v := range uinfo.options {
		switch k {
		case "connect":
			if v == "direct" {
				direct = true
				break
			}
			if v == "replicaSet" {
				break
			}
			fallthrough
		default:
			return nil, errors.New("Unsupported connection URL option: " + k + "=" + v)
		}
	}
	info := DialInfo{
		Addrs:    uinfo.addrs,
		Direct:   direct,
		Timeout:  timeout,
		Username: uinfo.user,
		Password: uinfo.pass,
		Database: uinfo.db,
	}
	return DialWithInfo(&info)
}

// DialInfo holds options for establishing a session with a MongoDB cluster.
// To use a URL, see the Dial function.
type DialInfo struct {
	// Addrs holds the addresses for the seed servers.
	Addrs []string

	// Direct informs whether to establish connections only with the
	// specified seed servers, or to obtain information for the whole
	// cluster and establish connections with further servers too.
	Direct bool

	// Timeout is the amount of time to wait for a server to respond when
	// first connecting and on follow up operations in the session. If
	// timeout is zero, the call may block forever waiting for a connection
	// to be established.
	Timeout time.Duration

	// Database is the database name used during the initial authentication.
	// If set, the value is also returned as the default result from the
	// Session.DB method, in place of "test".
	Database string

	// Username and Password inform the credentials for the initial
	// authentication done against Database, if that is set,
	// or the "admin" database otherwise. See the Session.Login method too.
	Username string
	Password string

	// Dial optionally specifies the dial function for establishing
	// connections with the MongoDB servers.
	Dial func(addr net.Addr) (net.Conn, error)

	// DialServer optionally specifies the dial function for establishing
	// connections with the MongoDB servers.
	//
	// WARNING: This interface is experimental and may change.
	DialServer func(addr *ServerAddr) (net.Conn, error)
}

// ServerAddr represents the address for establishing a connection to an
// individual MongoDB server.
//
// WARNING: This interface is experimental and may change.
type ServerAddr struct {
	str string
	tcp *net.TCPAddr
}

// String returns the address that was provided for the server before resolution.
func (addr *ServerAddr) String() string {
	return addr.str
}

// TCPAddr returns the resolved TCP address for the server.
func (addr *ServerAddr) TCPAddr() *net.TCPAddr {
	return addr.tcp
}

// DialWithInfo establishes a new session to the cluster identified by info.
func DialWithInfo(info *DialInfo) (*Session, error) {
	addrs := make([]string, len(info.Addrs))
	for i, addr := range info.Addrs {
		p := strings.LastIndexAny(addr, "]:")
		if p == -1 || addr[p] != ':' {
			// XXX This is untested. The test suite doesn't use the standard port.
			addr += ":27017"
		}
		addrs[i] = addr
	}
	cluster := newCluster(addrs, info.Direct, dialer{info.Dial, info.DialServer})
	session := newSession(Eventual, cluster, info.Timeout)
	session.defaultdb = info.Database
	if session.defaultdb == "" {
		session.defaultdb = "test"
	}
	if info.Username != "" {
		db := info.Database
		if db == "" {
			db = "admin"
		}
		session.dialAuth = &authInfo{db, info.Username, info.Password}
		session.auth = []authInfo{*session.dialAuth}
	}
	cluster.Release()

	// People get confused when we return a session that is not actually
	// established to any servers yet (e.g. what if url was wrong). So,
	// ping the server to ensure there's someone there, and abort if it
	// fails.
	if err := session.Ping(); err != nil {
		session.Close()
		return nil, err
	}
	session.SetMode(Strong, true)
	return session, nil
}

func isOptSep(c rune) bool {
	return c == ';' || c == '&'
}

type urlInfo struct {
	addrs   []string
	user    string
	pass    string
	db      string
	options map[string]string
}

func parseURL(url string) (*urlInfo, error) {
	if strings.HasPrefix(url, "mongodb://") {
		url = url[10:]
	}
	info := &urlInfo{options: make(map[string]string)}
	if c := strings.Index(url, "?"); c != -1 {
		for _, pair := range strings.FieldsFunc(url[c+1:], isOptSep) {
			l := strings.SplitN(pair, "=", 2)
			if len(l) != 2 || l[0] == "" || l[1] == "" {
				return nil, errors.New("Connection option must be key=value: " + pair)
			}
			info.options[l[0]] = l[1]
		}
		url = url[:c]
	}
	if c := strings.Index(url, "@"); c != -1 {
		pair := strings.SplitN(url[:c], ":", 2)
		if len(pair) != 2 || pair[0] == "" {
			return nil, errors.New("Credentials must be provided as user:pass@host")
		}
		info.user = pair[0]
		info.pass = pair[1]
		url = url[c+1:]
	}
	if c := strings.Index(url, "/"); c != -1 {
		info.db = url[c+1:]
		url = url[:c]
	}
	info.addrs = strings.Split(url, ",")
	return info, nil
}

func newSession(consistency mode, cluster *mongoCluster, timeout time.Duration) (session *Session) {
	cluster.Acquire()
	session = &Session{cluster_: cluster, syncTimeout: timeout, sockTimeout: timeout}
	debugf("New session %p on cluster %p", session, cluster)
	session.SetMode(consistency, true)
	session.SetSafe(&Safe{})
	session.queryConfig.prefetch = defaultPrefetch
	return session
}

func copySession(session *Session, keepAuth bool) (s *Session) {
	cluster := session.cluster()
	cluster.Acquire()
	if session.masterSocket != nil {
		session.masterSocket.Acquire()
	}
	if session.slaveSocket != nil {
		session.slaveSocket.Acquire()
	}
	var auth []authInfo
	if keepAuth {
		auth = make([]authInfo, len(session.auth))
		copy(auth, session.auth)
	} else if session.dialAuth != nil {
		auth = []authInfo{*session.dialAuth}
	}
	scopy := *session
	scopy.m = sync.RWMutex{}
	scopy.auth = auth
	s = &scopy
	debugf("New session %p on cluster %p (copy from %p)", s, cluster, session)
	return s
}

// LiveServers returns a list of server addresses which are
// currently known to be alive.
func (s *Session) LiveServers() (addrs []string) {
	s.m.RLock()
	addrs = s.cluster().LiveServers()
	s.m.RUnlock()
	return addrs
}

// DB returns a value representing the named database. If name
// is empty, the database name provided in the dialed URL is
// used instead. If that is also empty, "test" is used as a
// fallback in a way equivalent to the mongo shell.
//
// Creating this value is a very lightweight operation, and
// involves no network communication.
func (s *Session) DB(name string) *Database {
	if name == "" {
		name = s.defaultdb
	}
	return &Database{s, name}
}

// C returns a value representing the named collection.
//
// Creating this value is a very lightweight operation, and
// involves no network communication.
func (db *Database) C(name string) *Collection {
	return &Collection{db, name, db.Name + "." + name}
}

// With returns a copy of db that uses session s.
func (db *Database) With(s *Session) *Database {
	newdb := *db
	newdb.Session = s
	return &newdb
}

// With returns a copy of c that uses session s.
func (c *Collection) With(s *Session) *Collection {
	newdb := *c.Database
	newdb.Session = s
	newc := *c
	newc.Database = &newdb
	return &newc
}

// GridFS returns a GridFS value representing collections in db that
// follow the standard GridFS specification.
// The provided prefix (sometimes known as root) will determine which
// collections to use, and is usually set to "fs" when there is a
// single GridFS in the database.
//
// See the GridFS Create, Open, and OpenId methods for more details.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/GridFS
//     http://www.mongodb.org/display/DOCS/GridFS+Tools
//     http://www.mongodb.org/display/DOCS/GridFS+Specification
//
func (db *Database) GridFS(prefix string) *GridFS {
	return newGridFS(db, prefix)
}

// Run issues the provided command against the database and unmarshals
// its result in the respective argument. The cmd argument may be either
// a string with the command name itself, in which case an empty document of
// the form bson.M{cmd: 1} will be used, or it may be a full command document.
//
// Note that MongoDB considers the first marshalled key as the command
// name, so when providing a command with options, it's important to
// use an ordering-preserving document, such as a struct value or an
// instance of bson.D.  For instance:
//
//     db.Run(bson.D{{"create", "mycollection"}, {"size", 1024}})
//
// For privilleged commands typically run against the "admin" database, see
// the Run method in the Session type.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Commands
//     http://www.mongodb.org/display/DOCS/List+of+Database+CommandSkips
//
func (db *Database) Run(cmd interface{}, result interface{}) error {
	if name, ok := cmd.(string); ok {
		cmd = bson.D{{name, 1}}
	}
	return db.C("$cmd").Find(cmd).One(result)
}

// Login authenticates against MongoDB with the provided credentials.  The
// authentication is valid for the whole session and will stay valid until
// Logout is explicitly called for the same database, or the session is
// closed.
//
// Concurrent Login calls will work correctly.
func (db *Database) Login(user, pass string) (err error) {
	session := db.Session
	dbname := db.Name

	socket, err := session.acquireSocket(true)
	if err != nil {
		return err
	}
	defer socket.Release()

	err = socket.Login(dbname, user, pass)
	if err != nil {
		return err
	}

	session.m.Lock()
	defer session.m.Unlock()

	for _, a := range session.auth {
		if a.db == dbname {
			a.user = user
			a.pass = pass
			return nil
		}
	}
	session.auth = append(session.auth, authInfo{dbname, user, pass})
	return nil
}

func (s *Session) socketLogin(socket *mongoSocket) error {
	for _, a := range s.auth {
		if err := socket.Login(a.db, a.user, a.pass); err != nil {
			return err
		}
	}
	return nil
}

// Logout removes any established authentication credentials for the database.
func (db *Database) Logout() {
	session := db.Session
	dbname := db.Name
	session.m.Lock()
	found := false
	for i, a := range session.auth {
		if a.db == dbname {
			copy(session.auth[i:], session.auth[i+1:])
			session.auth = session.auth[:len(session.auth)-1]
			found = true
			break
		}
	}
	if found {
		if session.masterSocket != nil {
			session.masterSocket.Logout(dbname)
		}
		if session.slaveSocket != nil {
			session.slaveSocket.Logout(dbname)
		}
	}
	session.m.Unlock()
}

// LogoutAll removes all established authentication credentials for the session.
func (s *Session) LogoutAll() {
	s.m.Lock()
	for _, a := range s.auth {
		if s.masterSocket != nil {
			s.masterSocket.Logout(a.db)
		}
		if s.slaveSocket != nil {
			s.slaveSocket.Logout(a.db)
		}
	}
	s.auth = s.auth[0:0]
	s.m.Unlock()
}

// User represents a MongoDB user.
//
// Relevant documentation:
//
//     http://docs.mongodb.org/manual/reference/privilege-documents/
//     http://docs.mongodb.org/manual/reference/user-privileges/
//
type User struct {
	// Username is how the user identifies itself to the system.
	Username string `bson:"user"`

	// Password is the plaintext password for the user. If set,
	// the UpsertUser method will hash it into PasswordHash and
	// unset it before the user is added to the database.
	Password string `bson:",omitempty"`

	// PasswordHash is the MD5 hash of Username+":mongo:"+Password.
	PasswordHash string `bson:"pwd,omitempty"`

	// UserSource indicates where to look for this user's credentials.
	// It may be set to a database name, or to "$external" for
	// consulting an external resource such as Kerberos. UserSource
	// must not be set if Password or PasswordHash are present.
	UserSource string `bson:"userSource,omitempty"`

	// Roles indicates the set of roles the user will be provided.
	// See the Role constants.
	Roles []Role `bson:"roles"`

	// OtherDBRoles allows assigning roles in other databases from
	// user documents inserted in the admin database. This field
	// only works in the admin database.
	OtherDBRoles map[string][]Role `bson:"otherDBRoles,omitempty"`
}

type Role string

const (
	// Relevant documentation:
	//
	//     http://docs.mongodb.org/manual/reference/user-privileges/
	//
	RoleRead         Role = "read"
	RoleReadAny      Role = "readAnyDatabase"
	RoleReadWrite    Role = "readWrite"
	RoleReadWriteAny Role = "readWriteAnyDatabase"
	RoleDBAdmin      Role = "dbAdmin"
	RoleDBAdminAny   Role = "dbAdminAnyDatabase"
	RoleUserAdmin    Role = "userAdmin"
	RoleUserAdminAny Role = "UserAdminAnyDatabase"
	RoleClusterAdmin Role = "clusterAdmin"
)

// UpsertUser updates the authentication credentials and the roles for
// a MongoDB user within the db database. If the named user doesn't exist
// it will be created.
//
// This method should only be used from MongoDB 2.4 and on. For older
// MongoDB releases, use the obsolete AddUser method instead.
//
// Relevant documentation:
//
//     http://docs.mongodb.org/manual/reference/user-privileges/
//     http://docs.mongodb.org/manual/reference/privilege-documents/
//
func (db *Database) UpsertUser(user *User) error {
	if user.Username == "" {
		return fmt.Errorf("user has no Username")
	}
	if user.Password != "" {
		psum := md5.New()
		psum.Write([]byte(user.Username + ":mongo:" + user.Password))
		user.PasswordHash = hex.EncodeToString(psum.Sum(nil))
		user.Password = ""
	}
	if user.PasswordHash != "" && user.UserSource != "" {
		return fmt.Errorf("user has both Password/PasswordHash and UserSource set")
	}
	if len(user.OtherDBRoles) > 0 && db.Name != "admin" {
		return fmt.Errorf("user with OtherDBRoles is only supported in admin database")
	}
	var unset bson.D
	if user.PasswordHash == "" {
		unset = append(unset, bson.DocElem{"pwd", 1})
	}
	if user.UserSource == "" {
		unset = append(unset, bson.DocElem{"userSource", 1})
	}
	// user.Roles is always sent, as it's the way MongoDB distinguishes
	// old-style documents from new-style documents.
	if len(user.OtherDBRoles) == 0 {
		unset = append(unset, bson.DocElem{"otherDBRoles", 1})
	}
	c := db.C("system.users")
	_, err := c.Upsert(bson.D{{"user", user.Username}}, bson.D{{"$unset", unset}, {"$set", user}})
	return err
}

// AddUser creates or updates the authentication credentials of user within
// the db database.
//
// This method is obsolete and should only be used with MongoDB 2.2 or
// earlier. For MongoDB 2.4 and on, use UpsertUser instead.
func (db *Database) AddUser(user, pass string, readOnly bool) error {
	psum := md5.New()
	psum.Write([]byte(user + ":mongo:" + pass))
	digest := hex.EncodeToString(psum.Sum(nil))
	c := db.C("system.users")
	_, err := c.Upsert(bson.M{"user": user}, bson.M{"$set": bson.M{"user": user, "pwd": digest, "readOnly": readOnly}})
	return err
}

// RemoveUser removes the authentication credentials of user from the database.
func (db *Database) RemoveUser(user string) error {
	c := db.C("system.users")
	return c.Remove(bson.M{"user": user})
}

type indexSpec struct {
	Name, NS       string
	Key            bson.D
	Unique         bool ",omitempty"
	DropDups       bool "dropDups,omitempty"
	Background     bool ",omitempty"
	Sparse         bool ",omitempty"
	Bits, Min, Max int  ",omitempty"
	ExpireAfter    int  "expireAfterSeconds,omitempty"
}

type Index struct {
	Key        []string // Index key fields; prefix name with dash (-) for descending order
	Unique     bool     // Prevent two documents from having the same index key
	DropDups   bool     // Drop documents with the same index key as a previously indexed one
	Background bool     // Build index in background and return immediately
	Sparse     bool     // Only index documents containing the Key fields

	ExpireAfter time.Duration // Periodically delete docs with indexed time.Time older than that.

	Name string // Index name, computed by EnsureIndex

	Bits, Min, Max int // Properties for spatial indexes
}

func parseIndexKey(key []string) (name string, realKey bson.D, err error) {
	var order interface{}
	for _, field := range key {
		raw := field
		if name != "" {
			name += "_"
		}
		var kind string
		if field != "" {
			if field[0] == '$' {
				if c := strings.Index(field, ":"); c > 1 && c < len(field)-1 {
					kind = field[1:c]
					field = field[c+1:]
					name += field + "_" + kind
				}
			}
			switch field[0] {
			case '$':
				// Logic above failed. Reset and error.
				field = ""
			case '@':
				order = "2d"
				field = field[1:]
				// The shell used to render this field as key_ instead of key_2d,
				// and mgo followed suit. This has been fixed in recent server
				// releases, and mgo followed as well.
				name += field + "_2d"
			case '-':
				order = -1
				field = field[1:]
				name += field + "_-1"
			case '+':
				field = field[1:]
				fallthrough
			default:
				if kind == "" {
					order = 1
					name += field + "_1"
				} else {
					order = kind
				}
			}
		}
		if field == "" || kind != "" && order != kind {
			return "", nil, fmt.Errorf(`Invalid index key: want "[$<kind>:][-]<field name>", got %q`, raw)
		}
		realKey = append(realKey, bson.DocElem{field, order})
	}
	if name == "" {
		return "", nil, errors.New("Invalid index key: no fields provided")
	}
	return
}

// EnsureIndexKey ensures an index with the given key exists, creating it
// if necessary.
//
// This example:
//
//     err := collection.EnsureIndexKey("a", "b")
//
// Is equivalent to:
//
//     err := collection.EnsureIndex(mgo.Index{Key: []string{"a", "b"}})
//
// See the EnsureIndex method for more details.
func (c *Collection) EnsureIndexKey(key ...string) error {
	return c.EnsureIndex(Index{Key: key})
}

// EnsureIndex ensures an index with the given key exists, creating it with
// the provided parameters if necessary.
//
// Once EnsureIndex returns successfully, following requests for the same index
// will not contact the server unless Collection.DropIndex is used to drop the
// same index, or Session.ResetIndexCache is called.
//
// For example:
//
//     index := Index{
//         Key: []string{"lastname", "firstname"},
//         Unique: true,
//         DropDups: true,
//         Background: true, // See notes.
//         Sparse: true,
//     }
//     err := collection.EnsureIndex(index)
//
// The Key value determines which fields compose the index. The index ordering
// will be ascending by default.  To obtain an index with a descending order,
// the field name should be prefixed by a dash (e.g. []string{"-time"}).
//
// If Unique is true, the index must necessarily contain only a single
// document per Key.  With DropDups set to true, documents with the same key
// as a previously indexed one will be dropped rather than an error returned.
//
// If Background is true, other connections will be allowed to proceed using
// the collection without the index while it's being built. Note that the
// session executing EnsureIndex will be blocked for as long as it takes for
// the index to be built.
//
// If Sparse is true, only documents containing the provided Key fields will be
// included in the index.  When using a sparse index for sorting, only indexed
// documents will be returned.
//
// If ExpireAfter is non-zero, the server will periodically scan the collection
// and remove documents containing an indexed time.Time field with a value
// older than ExpireAfter. See the documentation for details:
//
//     http://docs.mongodb.org/manual/tutorial/expire-data
//
// Other kinds of indexes are also supported through that API. Here is an example:
//
//     index := Index{
//         Key: []string{"$2d:loc"},
//         Bits: 26,
//     }
//     err := collection.EnsureIndex(index)
//
// The example above requests the creation of a "2d" index for the "loc" field.
//
// The 2D index bounds may be changed using the Min and Max attributes of the
// Index value.  The default bound setting of (-180, 180) is suitable for
// latitude/longitude pairs.
//
// The Bits parameter sets the precision of the 2D geohash values.  If not
// provided, 26 bits are used, which is roughly equivalent to 1 foot of
// precision for the default (-180, 180) index bounds.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Indexes
//     http://www.mongodb.org/display/DOCS/Indexing+Advice+and+FAQ
//     http://www.mongodb.org/display/DOCS/Indexing+as+a+Background+Operation
//     http://www.mongodb.org/display/DOCS/Geospatial+Indexing
//     http://www.mongodb.org/display/DOCS/Multikeys
//
func (c *Collection) EnsureIndex(index Index) error {
	name, realKey, err := parseIndexKey(index.Key)
	if err != nil {
		return err
	}

	session := c.Database.Session
	cacheKey := c.FullName + "\x00" + name
	if session.cluster().HasCachedIndex(cacheKey) {
		return nil
	}

	spec := indexSpec{
		Name:        name,
		NS:          c.FullName,
		Key:         realKey,
		Unique:      index.Unique,
		DropDups:    index.DropDups,
		Background:  index.Background,
		Sparse:      index.Sparse,
		Bits:        index.Bits,
		Min:         index.Min,
		Max:         index.Max,
		ExpireAfter: int(index.ExpireAfter / time.Second),
	}

	session = session.Clone()
	defer session.Close()
	session.SetMode(Strong, false)
	session.EnsureSafe(&Safe{})

	db := c.Database.With(session)
	err = db.C("system.indexes").Insert(&spec)
	if err == nil {
		session.cluster().CacheIndex(cacheKey, true)
	}
	session.Close()
	return err
}

// DropIndex removes the index with key from the collection.
//
// The key value determines which fields compose the index. The index ordering
// will be ascending by default.  To obtain an index with a descending order,
// the field name should be prefixed by a dash (e.g. []string{"-time"}).
//
// For example:
//
//     err := collection.DropIndex("lastname", "firstname")
//
// See the EnsureIndex method for more details on indexes.
func (c *Collection) DropIndex(key ...string) error {
	name, _, err := parseIndexKey(key)
	if err != nil {
		return err
	}

	session := c.Database.Session
	cacheKey := c.FullName + "\x00" + name
	session.cluster().CacheIndex(cacheKey, false)

	session = session.Clone()
	defer session.Close()
	session.SetMode(Strong, false)

	db := c.Database.With(session)
	result := struct {
		ErrMsg string
		Ok     bool
	}{}
	err = db.Run(bson.D{{"dropIndexes", c.Name}, {"index", name}}, &result)
	if err != nil {
		return err
	}
	if !result.Ok {
		return errors.New(result.ErrMsg)
	}
	return nil
}

// Indexes returns a list of all indexes for the collection.
//
// For example, this snippet would drop all available indexes:
//
//   indexes, err := collection.Indexes()
//   if err != nil {
//       return err
//   }
//   for _, index := range indexes {
//       err = collection.DropIndex(index.Key...)
//       if err != nil {
//           return err
//       }
//   }
//
// See the EnsureIndex method for more details on indexes.
func (c *Collection) Indexes() (indexes []Index, err error) {
	query := c.Database.C("system.indexes").Find(bson.M{"ns": c.FullName})
	iter := query.Sort("name").Iter()
	for {
		var spec indexSpec
		if !iter.Next(&spec) {
			break
		}
		index := Index{
			Name:        spec.Name,
			Key:         simpleIndexKey(spec.Key),
			Unique:      spec.Unique,
			DropDups:    spec.DropDups,
			Background:  spec.Background,
			Sparse:      spec.Sparse,
			ExpireAfter: time.Duration(spec.ExpireAfter) * time.Second,
		}
		indexes = append(indexes, index)
	}
	err = iter.Close()
	return
}

func simpleIndexKey(realKey bson.D) (key []string) {
	for i := range realKey {
		field := realKey[i].Name
		vi, ok := realKey[i].Value.(int)
		if !ok {
			vf, _ := realKey[i].Value.(float64)
			vi = int(vf)
		}
		if vi == 1 {
			key = append(key, field)
			continue
		}
		if vi == -1 {
			key = append(key, "-"+field)
			continue
		}
		if vs, ok := realKey[i].Value.(string); ok {
			key = append(key, "$"+vs+":"+field)
			continue
		}
		panic("Got unknown index key type for field " + field)
	}
	return
}

// ResetIndexCache() clears the cache of previously ensured indexes.
// Following requests to EnsureIndex will contact the server.
func (s *Session) ResetIndexCache() {
	s.cluster().ResetIndexCache()
}

// New creates a new session with the same parameters as the original
// session, including consistency, batch size, prefetching, safety mode,
// etc. The returned session will use sockets from the pool, so there's
// a chance that writes just performed in another session may not yet
// be visible.
//
// Login information from the original session will not be copied over
// into the new session unless it was provided through the initial URL
// for the Dial function.
//
// See the Copy and Clone methods.
//
func (s *Session) New() *Session {
	s.m.Lock()
	scopy := copySession(s, false)
	s.m.Unlock()
	scopy.Refresh()
	return scopy
}

// Copy works just like New, but preserves the exact authentication
// information from the original session.
func (s *Session) Copy() *Session {
	s.m.Lock()
	scopy := copySession(s, true)
	s.m.Unlock()
	scopy.Refresh()
	return scopy
}

// Clone works just like Copy, but also reuses the same socket as the original
// session, in case it had already reserved one due to its consistency
// guarantees.  This behavior ensures that writes performed in the old session
// are necessarily observed when using the new session, as long as it was a
// strong or monotonic session.  That said, it also means that long operations
// may cause other goroutines using the original session to wait.
func (s *Session) Clone() *Session {
	s.m.Lock()
	scopy := copySession(s, true)
	s.m.Unlock()
	return scopy
}

// Close terminates the session.  It's a runtime error to use a session
// after it has been closed.
func (s *Session) Close() {
	s.m.Lock()
	if s.cluster_ != nil {
		debugf("Closing session %p", s)
		s.unsetSocket()
		s.cluster_.Release()
		s.cluster_ = nil
	}
	s.m.Unlock()
}

func (s *Session) cluster() *mongoCluster {
	if s.cluster_ == nil {
		panic("Session already closed")
	}
	return s.cluster_
}

// Refresh puts back any reserved sockets in use and restarts the consistency
// guarantees according to the current consistency setting for the session.
func (s *Session) Refresh() {
	s.m.Lock()
	s.slaveOk = s.consistency != Strong
	s.unsetSocket()
	s.m.Unlock()
}

// SetMode changes the consistency mode for the session.
//
// In the Strong consistency mode reads and writes will always be made to
// the master server using a unique connection so that reads and writes are
// fully consistent, ordered, and observing the most up-to-date data.
// This offers the least benefits in terms of distributing load, but the
// most guarantees.  See also Monotonic and Eventual.
//
// In the Monotonic consistency mode reads may not be entirely up-to-date,
// but they will always see the history of changes moving forward, the data
// read will be consistent across sequential queries in the same session,
// and modifications made within the session will be observed in following
// queries (read-your-writes).
//
// In practice, the Monotonic mode is obtained by performing initial reads
// against a unique connection to an arbitrary slave, if one is available,
// and once the first write happens, the session connection is switched over
// to the master server.  This manages to distribute some of the reading
// load with slaves, while maintaining some useful guarantees.
//
// In the Eventual consistency mode reads will be made to any slave in the
// cluster, if one is available, and sequential reads will not necessarily
// be made with the same connection.  This means that data may be observed
// out of order.  Writes will of course be issued to the master, but
// independent writes in the same Eventual session may also be made with
// independent connections, so there are also no guarantees in terms of
// write ordering (no read-your-writes guarantees either).
//
// The Eventual mode is the fastest and most resource-friendly, but is
// also the one offering the least guarantees about ordering of the data
// read and written.
//
// If refresh is true, in addition to ensuring the session is in the given
// consistency mode, the consistency guarantees will also be reset (e.g.
// a Monotonic session will be allowed to read from slaves again).  This is
// equivalent to calling the Refresh function.
//
// Shifting between Monotonic and Strong modes will keep a previously
// reserved connection for the session unless refresh is true or the
// connection is unsuitable (to a slave server in a Strong session).
func (s *Session) SetMode(consistency mode, refresh bool) {
	s.m.Lock()
	debugf("Session %p: setting mode %d with refresh=%v (master=%p, slave=%p)", s, consistency, refresh, s.masterSocket, s.slaveSocket)
	s.consistency = consistency
	if refresh {
		s.slaveOk = s.consistency != Strong
		s.unsetSocket()
	} else if s.consistency == Strong {
		s.slaveOk = false
	} else if s.masterSocket == nil {
		s.slaveOk = true
	}
	s.m.Unlock()
}

// Mode returns the current consistency mode for the session.
func (s *Session) Mode() mode {
	s.m.RLock()
	mode := s.consistency
	s.m.RUnlock()
	return mode
}

// SetSyncTimeout sets the amount of time an operation with this session
// will wait before returning an error in case a connection to a usable
// server can't be established. Set it to zero to wait forever. The
// default value is 7 seconds.
func (s *Session) SetSyncTimeout(d time.Duration) {
	s.m.Lock()
	s.syncTimeout = d
	s.m.Unlock()
}

// SetSocketTimeout sets the amount of time to wait for a non-responding
// socket to the database before it is forcefully closed.
func (s *Session) SetSocketTimeout(d time.Duration) {
	s.m.Lock()
	s.sockTimeout = d
	if s.masterSocket != nil {
		s.masterSocket.SetTimeout(d)
	}
	if s.slaveSocket != nil {
		s.slaveSocket.SetTimeout(d)
	}
	s.m.Unlock()
}

// SetCursorTimeout changes the standard timeout period that the server
// enforces on created cursors. The only supported value right now is
// 0, which disables the timeout. The standard server timeout is 10 minutes.
func (s *Session) SetCursorTimeout(d time.Duration) {
	s.m.Lock()
	if d == 0 {
		s.queryConfig.op.flags |= flagNoCursorTimeout
	} else {
		panic("SetCursorTimeout: only 0 (disable timeout) supported for now")
	}
	s.m.Unlock()
}

// SetBatch sets the default batch size used when fetching documents from the
// database. It's possible to change this setting on a per-query basis as
// well, using the Query.Batch method.
//
// The default batch size is defined by the database itself.  As of this
// writing, MongoDB will use an initial size of min(100 docs, 4MB) on the
// first batch, and 4MB on remaining ones.
func (s *Session) SetBatch(n int) {
	if n == 1 {
		// Server interprets 1 as -1 and closes the cursor (!?)
		n = 2
	}
	s.m.Lock()
	s.queryConfig.op.limit = int32(n)
	s.m.Unlock()
}

// SetPrefetch sets the default point at which the next batch of results will be
// requested.  When there are p*batch_size remaining documents cached in an
// Iter, the next batch will be requested in background. For instance, when
// using this:
//
//     session.SetBatch(200)
//     session.SetPrefetch(0.25)
//
// and there are only 50 documents cached in the Iter to be processed, the
// next batch of 200 will be requested. It's possible to change this setting on
// a per-query basis as well, using the Prefetch method of Query.
//
// The default prefetch value is 0.25.
func (s *Session) SetPrefetch(p float64) {
	s.m.Lock()
	s.queryConfig.prefetch = p
	s.m.Unlock()
}

// See SetSafe for details on the Safe type.
type Safe struct {
	W        int    // Min # of servers to ack before success
	WMode    string // Write mode for MongoDB 2.0+ (e.g. "majority")
	WTimeout int    // Milliseconds to wait for W before timing out
	FSync    bool   // Should servers sync to disk before returning success
	J        bool   // Wait for next group commit if journaling; no effect otherwise
}

// Safe returns the current safety mode for the session.
func (s *Session) Safe() (safe *Safe) {
	s.m.Lock()
	defer s.m.Unlock()
	if s.safeOp != nil {
		cmd := s.safeOp.query.(*getLastError)
		safe = &Safe{WTimeout: cmd.WTimeout, FSync: cmd.FSync, J: cmd.J}
		switch w := cmd.W.(type) {
		case string:
			safe.WMode = w
		case int:
			safe.W = w
		}
	}
	return
}

// SetSafe changes the session safety mode.
//
// If the safe parameter is nil, the session is put in unsafe mode, and writes
// become fire-and-forget, without error checking.  The unsafe mode is faster
// since operations won't hold on waiting for a confirmation.
//
// If the safe parameter is not nil, any changing query (insert, update, ...)
// will be followed by a getLastError command with the specified parameters,
// to ensure the request was correctly processed.
//
// The safe.W parameter determines how many servers should confirm a write
// before the operation is considered successful.  If set to 0 or 1, the
// command will return as soon as the master is done with the request.
// If safe.WTimeout is greater than zero, it determines how many milliseconds
// to wait for the safe.W servers to respond before returning an error.
//
// Starting with MongoDB 2.0.0 the safe.WMode parameter can be used instead
// of W to request for richer semantics. If set to "majority" the server will
// wait for a majority of members from the replica set to respond before
// returning. Custom modes may also be defined within the server to create
// very detailed placement schemas. See the data awareness documentation in
// the links below for more details (note that MongoDB internally reuses the
// "w" field name for WMode).
//
// If safe.FSync is true and journaling is disabled, the servers will be
// forced to sync all files to disk immediately before returning. If the
// same option is true but journaling is enabled, the server will instead
// await for the next group commit before returning.
//
// Since MongoDB 2.0.0, the safe.J option can also be used instead of FSync
// to force the server to wait for a group commit in case journaling is
// enabled. The option has no effect if the server has journaling disabled.
//
// For example, the following statement will make the session check for
// errors, without imposing further constraints:
//
//     session.SetSafe(&mgo.Safe{})
//
// The following statement will force the server to wait for a majority of
// members of a replica set to return (MongoDB 2.0+ only):
//
//     session.SetSafe(&mgo.Safe{WMode: "majority"})
//
// The following statement, on the other hand, ensures that at least two
// servers have flushed the change to disk before confirming the success
// of operations:
//
//     session.EnsureSafe(&mgo.Safe{W: 2, FSync: true})
//
// The following statement, on the other hand, disables the verification
// of errors entirely:
//
//     session.SetSafe(nil)
//
// See also the EnsureSafe method.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/getLastError+Command
//     http://www.mongodb.org/display/DOCS/Verifying+Propagation+of+Writes+with+getLastError
//     http://www.mongodb.org/display/DOCS/Data+Center+Awareness
//
func (s *Session) SetSafe(safe *Safe) {
	s.m.Lock()
	s.safeOp = nil
	s.ensureSafe(safe)
	s.m.Unlock()
}

// EnsureSafe compares the provided safety parameters with the ones
// currently in use by the session and picks the most conservative
// choice for each setting.
//
// That is:
//
//     - safe.WMode is always used if set.
//     - safe.W is used if larger than the current W and WMode is empty.
//     - safe.FSync is always used if true.
//     - safe.J is used if FSync is false.
//     - safe.WTimeout is used if set and smaller than the current WTimeout.
//
// For example, the following statement will ensure the session is
// at least checking for errors, without enforcing further constraints.
// If a more conservative SetSafe or EnsureSafe call was previously done,
// the following call will be ignored.
//
//     session.EnsureSafe(&mgo.Safe{})
//
// See also the SetSafe method for details on what each option means.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/getLastError+Command
//     http://www.mongodb.org/display/DOCS/Verifying+Propagation+of+Writes+with+getLastError
//     http://www.mongodb.org/display/DOCS/Data+Center+Awareness
//
func (s *Session) EnsureSafe(safe *Safe) {
	s.m.Lock()
	s.ensureSafe(safe)
	s.m.Unlock()
}

func (s *Session) ensureSafe(safe *Safe) {
	if safe == nil {
		return
	}

	var w interface{}
	if safe.WMode != "" {
		w = safe.WMode
	} else if safe.W > 0 {
		w = safe.W
	}

	var cmd getLastError
	if s.safeOp == nil {
		cmd = getLastError{1, w, safe.WTimeout, safe.FSync, safe.J}
	} else {
		// Copy.  We don't want to mutate the existing query.
		cmd = *(s.safeOp.query.(*getLastError))
		if cmd.W == nil {
			cmd.W = w
		} else if safe.WMode != "" {
			cmd.W = safe.WMode
		} else if i, ok := cmd.W.(int); ok && safe.W > i {
			cmd.W = safe.W
		}
		if safe.WTimeout > 0 && safe.WTimeout < cmd.WTimeout {
			cmd.WTimeout = safe.WTimeout
		}
		if safe.FSync {
			cmd.FSync = true
			cmd.J = false
		} else if safe.J && !cmd.FSync {
			cmd.J = true
		}
	}
	s.safeOp = &queryOp{
		query:      &cmd,
		collection: "admin.$cmd",
		limit:      -1,
	}
}

// Run issues the provided command against the "admin" database and
// and unmarshals its result in the respective argument. The cmd
// argument may be either a string with the command name itself, in
// which case an empty document of the form bson.M{cmd: 1} will be used,
// or it may be a full command document.
//
// Note that MongoDB considers the first marshalled key as the command
// name, so when providing a command with options, it's important to
// use an ordering-preserving document, such as a struct value or an
// instance of bson.D.  For instance:
//
//     db.Run(bson.D{{"create", "mycollection"}, {"size", 1024}})
//
// For commands against arbitrary databases, see the Run method in
// the Database type.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Commands
//     http://www.mongodb.org/display/DOCS/List+of+Database+CommandSkips
//
func (s *Session) Run(cmd interface{}, result interface{}) error {
	return s.DB("admin").Run(cmd, result)
}

// SelectServers restricts communication to servers configured with the
// given tags. For example, the following statement restricts servers
// used for reading operations to those with both tag "disk" set to
// "ssd" and tag "rack" set to 1:
//
//     session.SelectSlaves(bson.D{{"disk", "ssd"}, {"rack", 1}})
//
// Multiple sets of tags may be provided, in which case the used server
// must match all tags within any one set.
//
// If a connection was previously assigned to the session due to the
// current session mode (see Session.SetMode), the tag selection will
// only be enforced after the session is refreshed.
//
// Relevant documentation:
//
//     http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets
//
func (s *Session) SelectServers(tags ...bson.D) {
	s.m.Lock()
	s.queryConfig.op.serverTags = tags
	s.m.Unlock()
}

// Ping runs a trivial ping command just to get in touch with the server.
func (s *Session) Ping() error {
	return s.Run("ping", nil)
}

// Fsync flushes in-memory writes to disk on the server the session
// is established with. If async is true, the call returns immediately,
// otherwise it returns after the flush has been made.
func (s *Session) Fsync(async bool) error {
	return s.Run(bson.D{{"fsync", 1}, {"async", async}}, nil)
}

// FsyncLock locks all writes in the specific server the session is
// established with and returns. Any writes attempted to the server
// after it is successfully locked will block until FsyncUnlock is
// called for the same server.
//
// This method works on slaves as well, preventing the oplog from being
// flushed while the server is locked, but since only the server
// connected to is locked, for locking specific slaves it may be
// necessary to establish a connection directly to the slave (see
// Dial's connect=direct option).
//
// As an important caveat, note that once a write is attempted and
// blocks, follow up reads will block as well due to the way the
// lock is internally implemented in the server. More details at:
//
//     https://jira.mongodb.org/browse/SERVER-4243
//
// FsyncLock is often used for performing consistent backups of
// the database files on disk.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/fsync+Command
//     http://www.mongodb.org/display/DOCS/Backups
//
func (s *Session) FsyncLock() error {
	return s.Run(bson.D{{"fsync", 1}, {"lock", true}}, nil)
}

// FsyncUnlock releases the server for writes. See FsyncLock for details.
func (s *Session) FsyncUnlock() error {
	return s.DB("admin").C("$cmd.sys.unlock").Find(nil).One(nil) // WTF?
}

// Find prepares a query using the provided document.  The document may be a
// map or a struct value capable of being marshalled with bson.  The map
// may be a generic one using interface{} for its key and/or values, such as
// bson.M, or it may be a properly typed map.  Providing nil as the document
// is equivalent to providing an empty document such as bson.M{}.
//
// Further details of the query may be tweaked using the resulting Query value,
// and then executed to retrieve results using methods such as One, For,
// Iter, or Tail.
//
// In case the resulting document includes a field named $err or errmsg, which
// are standard ways for MongoDB to return query errors, the returned err will
// be set to a *QueryError value including the Err message and the Code.  In
// those cases, the result argument is still unmarshalled into with the
// received document so that any other custom values may be obtained if
// desired.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Querying
//     http://www.mongodb.org/display/DOCS/Advanced+Queries
//
func (c *Collection) Find(query interface{}) *Query {
	session := c.Database.Session
	session.m.RLock()
	q := &Query{session: session, query: session.queryConfig}
	session.m.RUnlock()
	q.op.query = query
	q.op.collection = c.FullName
	return q
}

// FindId is a convenience helper equivalent to:
//
//     query := collection.Find(bson.M{"_id": id})
//
// See the Find method for more details.
func (c *Collection) FindId(id interface{}) *Query {
	return c.Find(bson.D{{"_id", id}})
}

type Pipe struct {
	session    *Session
	collection *Collection
	pipeline   interface{}
}

// Pipe prepares a pipeline to aggregate. The pipeline document
// must be a slice built in terms of the aggregation framework language.
//
// For example:
//
//     pipe := collection.Pipe([]bson.M{{"$match": bson.M{"name": "Otavio"}}})
//     iter := pipe.Iter()
//
// Relevant documentation:
//
//     http://docs.mongodb.org/manual/reference/aggregation
//     http://docs.mongodb.org/manual/applications/aggregation
//     http://docs.mongodb.org/manual/tutorial/aggregation-examples
//
func (c *Collection) Pipe(pipeline interface{}) *Pipe {
	session := c.Database.Session
	return &Pipe{
		session:    session,
		collection: c,
		pipeline:   pipeline,
	}
}

// Iter executes the pipeline and returns an iterator capable of going
// over all the generated results.
func (p *Pipe) Iter() *Iter {
	iter := &Iter{
		session: p.session,
		timeout: -1,
	}
	iter.gotReply.L = &iter.m
	var result struct{ Result []bson.Raw }
	c := p.collection
	iter.err = c.Database.Run(bson.D{{"aggregate", c.Name}, {"pipeline", p.pipeline}}, &result)
	if iter.err != nil {
		return iter
	}
	for i := range result.Result {
		iter.docData.Push(result.Result[i].Data)
	}
	return iter
}

// All works like Iter.All.
func (p *Pipe) All(result interface{}) error {
	return p.Iter().All(result)
}

// One executes the pipeline and unmarshals the first item from the
// result set into the result parameter.
// It returns ErrNotFound if no items are generated by the pipeline.
func (p *Pipe) One(result interface{}) error {
	iter := p.Iter()
	if iter.Next(result) {
		return nil
	}
	if err := iter.Err(); err != nil {
		return err
	}
	return ErrNotFound
}

type LastError struct {
	Err             string
	Code, N, Waited int
	FSyncFiles      int `bson:"fsyncFiles"`
	WTimeout        bool
	UpdatedExisting bool        `bson:"updatedExisting"`
	UpsertedId      interface{} `bson:"upserted"`
}

func (err *LastError) Error() string {
	return err.Err
}

type queryError struct {
	Err           string "$err"
	ErrMsg        string
	Assertion     string
	Code          int
	AssertionCode int        "assertionCode"
	LastError     *LastError "lastErrorObject"
}

type QueryError struct {
	Code      int
	Message   string
	Assertion bool
}

func (err *QueryError) Error() string {
	return err.Message
}

// IsDup returns whether err informs of a duplicate key error because
// a primary key index or a secondary unique index already has an entry
// with the given value.
func IsDup(err error) bool {
	// Besides being handy, helps with https://jira.mongodb.org/browse/SERVER-7164
	// What follows makes me sad. Hopefully conventions will be more clear over time.
	switch e := err.(type) {
	case *LastError:
		return e.Code == 11000 || e.Code == 11001 || e.Code == 12582
	case *QueryError:
		return e.Code == 11000 || e.Code == 11001 || e.Code == 12582
	}
	return false
}

// Insert inserts one or more documents in the respective collection.  In
// case the session is in safe mode (see the SetSafe method) and an error
// happens while inserting the provided documents, the returned error will
// be of type *LastError.
func (c *Collection) Insert(docs ...interface{}) error {
	_, err := c.writeQuery(&insertOp{c.FullName, docs})
	return err
}

// Update finds a single document matching the provided selector document
// and modifies it according to the change document.
// If the session is in safe mode (see SetSafe) a ErrNotFound error is
// returned if a document isn't found, or a value of type *LastError
// when some other error is detected.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Updating
//     http://www.mongodb.org/display/DOCS/Atomic+Operations
//
func (c *Collection) Update(selector interface{}, change interface{}) error {
	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, change, 0})
	if err == nil && lerr != nil && !lerr.UpdatedExisting {
		return ErrNotFound
	}
	return err
}

// UpdateId is a convenience helper equivalent to:
//
//     err := collection.Update(bson.M{"_id": id}, change)
//
// See the Update method for more details.
func (c *Collection) UpdateId(id interface{}, change interface{}) error {
	return c.Update(bson.D{{"_id", id}}, change)
}

// ChangeInfo holds details about the outcome of a change operation.
type ChangeInfo struct {
	Updated    int         // Number of existing documents updated
	Removed    int         // Number of documents removed
	UpsertedId interface{} // Upserted _id field, when not explicitly provided
}

// UpdateAll finds all documents matching the provided selector document
// and modifies them according to the change document.
// If the session is in safe mode (see SetSafe) details of the executed
// operation are returned in info or an error of type *LastError when
// some problem is detected. It is not an error for the update to not be
// applied on any documents because the selector doesn't match.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Updating
//     http://www.mongodb.org/display/DOCS/Atomic+Operations
//
func (c *Collection) UpdateAll(selector interface{}, change interface{}) (info *ChangeInfo, err error) {
	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, change, 2})
	if err == nil && lerr != nil {
		info = &ChangeInfo{Updated: lerr.N}
	}
	return info, err
}

// Upsert finds a single document matching the provided selector document
// and modifies it according to the change document.  If no document matching
// the selector is found, the change document is applied to the selector
// document and the result is inserted in the collection.
// If the session is in safe mode (see SetSafe) details of the executed
// operation are returned in info, or an error of type *LastError when
// some problem is detected.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Updating
//     http://www.mongodb.org/display/DOCS/Atomic+Operations
//
func (c *Collection) Upsert(selector interface{}, change interface{}) (info *ChangeInfo, err error) {
	data, err := bson.Marshal(change)
	if err != nil {
		return nil, err
	}
	change = bson.Raw{0x03, data}
	lerr, err := c.writeQuery(&updateOp{c.FullName, selector, change, 1})
	if err == nil && lerr != nil {
		info = &ChangeInfo{}
		if lerr.UpdatedExisting {
			info.Updated = lerr.N
		} else {
			info.UpsertedId = lerr.UpsertedId
		}
	}
	return info, err
}

// UpsertId is a convenience helper equivalent to:
//
//     info, err := collection.Upsert(bson.M{"_id": id}, change)
//
// See the Upsert method for more details.
func (c *Collection) UpsertId(id interface{}, change interface{}) (info *ChangeInfo, err error) {
	return c.Upsert(bson.D{{"_id", id}}, change)
}

// Remove finds a single document matching the provided selector document
// and removes it from the database.
// If the session is in safe mode (see SetSafe) a ErrNotFound error is
// returned if a document isn't found, or a value of type *LastError
// when some other error is detected.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Removing
//
func (c *Collection) Remove(selector interface{}) error {
	lerr, err := c.writeQuery(&deleteOp{c.FullName, selector, 1})
	if err == nil && lerr != nil && lerr.N == 0 {
		return ErrNotFound
	}
	return err
}

// RemoveId is a convenience helper equivalent to:
//
//     err := collection.Remove(bson.M{"_id": id})
//
// See the Remove method for more details.
func (c *Collection) RemoveId(id interface{}) error {
	return c.Remove(bson.D{{"_id", id}})
}

// RemoveAll finds all documents matching the provided selector document
// and removes them from the database.  In case the session is in safe mode
// (see the SetSafe method) and an error happens when attempting the change,
// the returned error will be of type *LastError.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Removing
//
func (c *Collection) RemoveAll(selector interface{}) (info *ChangeInfo, err error) {
	lerr, err := c.writeQuery(&deleteOp{c.FullName, selector, 0})
	if err == nil && lerr != nil {
		info = &ChangeInfo{Removed: lerr.N}
	}
	return info, err
}

// DropDatabase removes the entire database including all of its collections.
func (db *Database) DropDatabase() error {
	return db.Run(bson.D{{"dropDatabase", 1}}, nil)
}

// DropCollection removes the entire collection including all of its documents.
func (c *Collection) DropCollection() error {
	return c.Database.Run(bson.D{{"drop", c.Name}}, nil)
}

// The CollectionInfo type holds metadata about a collection.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/createCollection+Command
//     http://www.mongodb.org/display/DOCS/Capped+Collections
//
type CollectionInfo struct {
	// DisableIdIndex prevents the automatic creation of the index
	// on the _id field for the collection.
	DisableIdIndex bool

	// ForceIdIndex enforces the automatic creation of the index
	// on the _id field for the collection. Capped collections,
	// for example, do not have such an index by default.
	ForceIdIndex bool

	// If Capped is true new documents will replace old ones when
	// the collection is full. MaxBytes must necessarily be set
	// to define the size when the collection wraps around.
	// MaxDocs optionally defines the number of documents when it
	// wraps, but MaxBytes still needs to be set.
	Capped   bool
	MaxBytes int
	MaxDocs  int
}

// Create explicitly creates the c collection with details of info.
// MongoDB creates collections automatically on use, so this method
// is only necessary when creating collection with non-default
// characteristics, such as capped collections.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/createCollection+Command
//     http://www.mongodb.org/display/DOCS/Capped+Collections
//
func (c *Collection) Create(info *CollectionInfo) error {
	cmd := make(bson.D, 0, 4)
	cmd = append(cmd, bson.DocElem{"create", c.Name})
	if info.Capped {
		if info.MaxBytes < 1 {
			return fmt.Errorf("Collection.Create: with Capped, MaxBytes must also be set")
		}
		cmd = append(cmd, bson.DocElem{"capped", true})
		cmd = append(cmd, bson.DocElem{"size", info.MaxBytes})
		if info.MaxDocs > 0 {
			cmd = append(cmd, bson.DocElem{"max", info.MaxDocs})
		}
	}
	if info.DisableIdIndex {
		cmd = append(cmd, bson.DocElem{"autoIndexId", false})
	}
	if info.ForceIdIndex {
		cmd = append(cmd, bson.DocElem{"autoIndexId", true})
	}
	return c.Database.Run(cmd, nil)
}

// Batch sets the batch size used when fetching documents from the database.
// It's possible to change this setting on a per-session basis as well, using
// the Batch method of Session.
//
// The default batch size is defined by the database itself.  As of this
// writing, MongoDB will use an initial size of min(100 docs, 4MB) on the
// first batch, and 4MB on remaining ones.
func (q *Query) Batch(n int) *Query {
	if n == 1 {
		// Server interprets 1 as -1 and closes the cursor (!?)
		n = 2
	}
	q.m.Lock()
	q.op.limit = int32(n)
	q.m.Unlock()
	return q
}

// Prefetch sets the point at which the next batch of results will be requested.
// When there are p*batch_size remaining documents cached in an Iter, the next
// batch will be requested in background. For instance, when using this:
//
//     query.Batch(200).Prefetch(0.25)
//
// and there are only 50 documents cached in the Iter to be processed, the
// next batch of 200 will be requested. It's possible to change this setting on
// a per-session basis as well, using the SetPrefetch method of Session.
//
// The default prefetch value is 0.25.
func (q *Query) Prefetch(p float64) *Query {
	q.m.Lock()
	q.prefetch = p
	q.m.Unlock()
	return q
}

// Skip skips over the n initial documents from the query results.  Note that
// this only makes sense with capped collections where documents are naturally
// ordered by insertion time, or with sorted results.
func (q *Query) Skip(n int) *Query {
	q.m.Lock()
	q.op.skip = int32(n)
	q.m.Unlock()
	return q
}

// Limit restricts the maximum number of documents retrieved to n, and also
// changes the batch size to the same value.  Once n documents have been
// returned by Next, the following call will return ErrNotFound.
func (q *Query) Limit(n int) *Query {
	q.m.Lock()
	switch {
	case n == 1:
		q.limit = 1
		q.op.limit = -1
	case n == math.MinInt32: // -MinInt32 == -MinInt32
		q.limit = math.MaxInt32
		q.op.limit = math.MinInt32 + 1
	case n < 0:
		q.limit = int32(-n)
		q.op.limit = int32(n)
	default:
		q.limit = int32(n)
		q.op.limit = int32(n)
	}
	q.m.Unlock()
	return q
}

// Select enables selecting which fields should be retrieved for the results
// found. For example, the following query would only retrieve the name field:
//
//     err := collection.Find(nil).Select(bson.M{"name": 1}).One(&result)
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields
//
func (q *Query) Select(selector interface{}) *Query {
	q.m.Lock()
	q.op.selector = selector
	q.m.Unlock()
	return q
}

// Sort asks the database to order returned documents according to the
// provided field names. A field name may be prefixed by - (minus) for
// it to be sorted in reverse order.
//
// For example:
//
//     query1 := collection.Find(nil).Sort("firstname", "lastname")
//     query2 := collection.Find(nil).Sort("-age")
//     query3 := collection.Find(nil).Sort("$natural")
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Sorting+and+Natural+Order
//
func (q *Query) Sort(fields ...string) *Query {
	q.m.Lock()
	var order bson.D
	for _, field := range fields {
		n := 1
		if field != "" {
			switch field[0] {
			case '+':
				field = field[1:]
			case '-':
				n = -1
				field = field[1:]
			}
		}
		if field == "" {
			panic("Sort: empty field name")
		}
		order = append(order, bson.DocElem{field, n})
	}
	q.op.options.OrderBy = order
	q.op.hasOptions = true
	q.m.Unlock()
	return q
}

// Explain returns a number of details about how the MongoDB server would
// execute the requested query, such as the number of objects examined,
// the number of time the read lock was yielded to allow writes to go in,
// and so on.
//
// For example:
//
//     m := bson.M{}
//     err := collection.Find(bson.M{"filename": name}).Explain(m)
//     if err == nil {
//         fmt.Printf("Explain: %#v\n", m)
//     }
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Optimization
//     http://www.mongodb.org/display/DOCS/Query+Optimizer
//
func (q *Query) Explain(result interface{}) error {
	q.m.Lock()
	clone := &Query{session: q.session, query: q.query}
	q.m.Unlock()
	clone.op.options.Explain = true
	clone.op.hasOptions = true
	if clone.op.limit > 0 {
		clone.op.limit = -q.op.limit
	}
	iter := clone.Iter()
	if iter.Next(result) {
		return nil
	}
	return iter.Close()
}

// Hint will include an explicit "hint" in the query to force the server
// to use a specified index, potentially improving performance in some
// situations.  The provided parameters are the fields that compose the
// key of the index to be used.  For details on how the indexKey may be
// built, see the EnsureIndex method.
//
// For example:
//
//     query := collection.Find(bson.M{"firstname": "Joe", "lastname": "Winter"})
//     query.Hint("lastname", "firstname")
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Optimization
//     http://www.mongodb.org/display/DOCS/Query+Optimizer
//
func (q *Query) Hint(indexKey ...string) *Query {
	q.m.Lock()
	_, realKey, err := parseIndexKey(indexKey)
	q.op.options.Hint = realKey
	q.op.hasOptions = true
	q.m.Unlock()
	if err != nil {
		panic(err)
	}
	return q
}

// Snapshot will force the performed query to make use of an available
// index on the _id field to prevent the same document from being returned
// more than once in a single iteration. This might happen without this
// setting in situations when the document changes in size and thus has to
// be moved while the iteration is running.
//
// Because snapshot mode traverses the _id index, it may not be used with
// sorting or explicit hints. It also cannot use any other index for the
// query.
//
// Even with snapshot mode, items inserted or deleted during the query may
// or may not be returned; that is, this mode is not a true point-in-time
// snapshot.
//
// The same effect of Snapshot may be obtained by using any unique index on
// field(s) that will not be modified (best to use Hint explicitly too).
// A non-unique index (such as creation time) may be made unique by
// appending _id to the index when creating it.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/How+to+do+Snapshotted+Queries+in+the+Mongo+Database
//
func (q *Query) Snapshot() *Query {
	q.m.Lock()
	q.op.options.Snapshot = true
	q.op.hasOptions = true
	q.m.Unlock()
	return q
}

// LogReplay enables an option that optimizes queries that are typically
// made against the MongoDB oplog for replaying it. This is an internal
// implementation aspect and most likely uninteresting for other uses.
// It has seen at least one use case, though, so it's exposed via the API.
func (q *Query) LogReplay() *Query {
	q.m.Lock()
	q.op.flags |= flagLogReplay
	q.m.Unlock()
	return q
}

func checkQueryError(fullname string, d []byte) error {
	l := len(d)
	if l < 16 {
		return nil
	}
	if d[5] == '$' && d[6] == 'e' && d[7] == 'r' && d[8] == 'r' && d[9] == '\x00' && d[4] == '\x02' {
		goto Error
	}
	if len(fullname) < 5 || fullname[len(fullname)-5:] != ".$cmd" {
		return nil
	}
	for i := 0; i+8 < l; i++ {
		if d[i] == '\x02' && d[i+1] == 'e' && d[i+2] == 'r' && d[i+3] == 'r' && d[i+4] == 'm' && d[i+5] == 's' && d[i+6] == 'g' && d[i+7] == '\x00' {
			goto Error
		}
	}
	return nil

Error:
	result := &queryError{}
	bson.Unmarshal(d, result)
	logf("queryError: %#v\n", result)
	if result.LastError != nil {
		return result.LastError
	}
	if result.Err == "" && result.ErrMsg == "" {
		return nil
	}
	if result.AssertionCode != 0 && result.Assertion != "" {
		return &QueryError{Code: result.AssertionCode, Message: result.Assertion, Assertion: true}
	}
	if result.Err != "" {
		return &QueryError{Code: result.Code, Message: result.Err}
	}
	return &QueryError{Code: result.Code, Message: result.ErrMsg}
}

// One executes the query and unmarshals the first obtained document into the
// result argument.  The result must be a struct or map value capable of being
// unmarshalled into by gobson.  This function blocks until either a result
// is available or an error happens.  For example:
//
//     err := collection.Find(bson.M{"a", 1}).One(&result)
//
// In case the resulting document includes a field named $err or errmsg, which
// are standard ways for MongoDB to return query errors, the returned err will
// be set to a *QueryError value including the Err message and the Code.  In
// those cases, the result argument is still unmarshalled into with the
// received document so that any other custom values may be obtained if
// desired.
//
func (q *Query) One(result interface{}) (err error) {
	q.m.Lock()
	session := q.session
	op := q.op // Copy.
	q.m.Unlock()

	socket, err := session.acquireSocket(true)
	if err != nil {
		return err
	}
	defer socket.Release()

	op.flags |= session.slaveOkFlag()
	op.limit = -1

	data, err := socket.SimpleQuery(&op)
	if err != nil {
		return err
	}
	if data == nil {
		return ErrNotFound
	}
	if result != nil {
		err = bson.Unmarshal(data, result)
		if err == nil {
			debugf("Query %p document unmarshaled: %#v", q, result)
		} else {
			debugf("Query %p document unmarshaling failed: %#v", q, err)
			return err
		}
	}
	return checkQueryError(op.collection, data)
}

// The DBRef type implements support for the database reference MongoDB
// convention as supported by multiple drivers.  This convention enables
// cross-referencing documents between collections and databases using
// a structure which includes a collection name, a document id, and
// optionally a database name.
//
// See the FindRef methods on Session and on Database.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Database+References
//
type DBRef struct {
	Collection string      `bson:"$ref"`
	Id         interface{} `bson:"$id"`
	Database   string      `bson:"$db,omitempty"`
}

// NOTE: Order of fields for DBRef above does matter, per documentation.

// FindRef returns a query that looks for the document in the provided
// reference. If the reference includes the DB field, the document will
// be retrieved from the respective database.
//
// See also the DBRef type and the FindRef method on Session.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Database+References
//
func (db *Database) FindRef(ref *DBRef) *Query {
	var c *Collection
	if ref.Database == "" {
		c = db.C(ref.Collection)
	} else {
		c = db.Session.DB(ref.Database).C(ref.Collection)
	}
	return c.FindId(ref.Id)
}

// FindRef returns a query that looks for the document in the provided
// reference. For a DBRef to be resolved correctly at the session level
// it must necessarily have the optional DB field defined.
//
// See also the DBRef type and the FindRef method on Database.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Database+References
//
func (s *Session) FindRef(ref *DBRef) *Query {
	if ref.Database == "" {
		panic(errors.New(fmt.Sprintf("Can't resolve database for %#v", ref)))
	}
	c := s.DB(ref.Database).C(ref.Collection)
	return c.FindId(ref.Id)
}

// CollectionNames returns the collection names present in database.
func (db *Database) CollectionNames() (names []string, err error) {
	c := len(db.Name) + 1
	iter := db.C("system.namespaces").Find(nil).Iter()
	var result *struct{ Name string }
	for iter.Next(&result) {
		if strings.Index(result.Name, "$") < 0 || strings.Index(result.Name, ".oplog.$") >= 0 {
			names = append(names, result.Name[c:])
		}
	}
	if err := iter.Close(); err != nil {
		return nil, err
	}
	sort.Strings(names)
	return names, nil
}

type dbNames struct {
	Databases []struct {
		Name  string
		Empty bool
	}
}

// DatabaseNames returns the names of non-empty databases present in the cluster.
func (s *Session) DatabaseNames() (names []string, err error) {
	var result dbNames
	err = s.Run("listDatabases", &result)
	if err != nil {
		return nil, err
	}
	for _, db := range result.Databases {
		if !db.Empty {
			names = append(names, db.Name)
		}
	}
	sort.Strings(names)
	return names, nil
}

// Iter executes the query and returns an iterator capable of going over all
// the results. Results will be returned in batches of configurable
// size (see the Batch method) and more documents will be requested when a
// configurable number of documents is iterated over (see the Prefetch method).
func (q *Query) Iter() *Iter {
	q.m.Lock()
	session := q.session
	op := q.op
	prefetch := q.prefetch
	limit := q.limit
	q.m.Unlock()

	iter := &Iter{
		session:  session,
		prefetch: prefetch,
		limit:    limit,
		timeout:  -1,
	}
	iter.gotReply.L = &iter.m
	iter.op.collection = op.collection
	iter.op.limit = op.limit
	iter.op.replyFunc = iter.replyFunc()
	iter.docsToReceive++
	op.replyFunc = iter.op.replyFunc
	op.flags |= session.slaveOkFlag()

	socket, err := session.acquireSocket(true)
	if err != nil {
		iter.err = err
	} else {
		iter.err = socket.Query(&op)
		iter.server = socket.Server()
		socket.Release()
	}
	return iter
}

// Tail returns a tailable iterator. Unlike a normal iterator, a
// tailable iterator may wait for new values to be inserted in the
// collection once the end of the current result set is reached,
// A tailable iterator may only be used with capped collections.
//
// The timeout parameter indicates how long Next will block waiting
// for a result before timing out.  If set to -1, Next will not
// timeout, and will continue waiting for a result for as long as
// the cursor is valid and the session is not closed. If set to 0,
// Next times out as soon as it reaches the end of the result set.
// Otherwise, Next will wait for at least the given number of
// seconds for a new document to be available before timing out.
//
// On timeouts, Next will unblock and return false, and the Timeout
// method will return true if called. In these cases, Next may still
// be called again on the same iterator to check if a new value is
// available at the current cursor position, and again it will block
// according to the specified timeoutSecs. If the cursor becomes
// invalid, though, both Next and Timeout will return false and
// the query must be restarted.
//
// The following example demonstrates timeout handling and query
// restarting:
//
//    iter := collection.Find(nil).Sort("$natural").Tail(5 * time.Second)
//    for {
//         for iter.Next(&result) {
//             fmt.Println(result.Id)
//             lastId = result.Id
//         }
//         if err := iter.Close(); err != nil {
//             return err
//         }
//         if iter.Timeout() {
//             continue
//         }
//         query := collection.Find(bson.M{"_id": bson.M{"$gt": lastId}})
//         iter = query.Sort("$natural").Tail(5 * time.Second)
//    }
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Tailable+Cursors
//     http://www.mongodb.org/display/DOCS/Capped+Collections
//     http://www.mongodb.org/display/DOCS/Sorting+and+Natural+Order
//
func (q *Query) Tail(timeout time.Duration) *Iter {
	q.m.Lock()
	session := q.session
	op := q.op
	prefetch := q.prefetch
	q.m.Unlock()

	iter := &Iter{session: session, prefetch: prefetch}
	iter.gotReply.L = &iter.m
	iter.timeout = timeout
	iter.op.collection = op.collection
	iter.op.limit = op.limit
	iter.op.replyFunc = iter.replyFunc()
	iter.docsToReceive++
	op.replyFunc = iter.op.replyFunc
	op.flags |= flagTailable | flagAwaitData | session.slaveOkFlag()

	socket, err := session.acquireSocket(true)
	if err != nil {
		iter.err = err
	} else {
		iter.err = socket.Query(&op)
		iter.server = socket.Server()
		socket.Release()
	}
	return iter
}

func (s *Session) slaveOkFlag() (flag queryOpFlags) {
	s.m.RLock()
	if s.slaveOk {
		flag = flagSlaveOk
	}
	s.m.RUnlock()
	return
}

// Err returns nil if no errors happened during iteration, or the actual
// error otherwise.
//
// In case a resulting document included a field named $err or errmsg, which are
// standard ways for MongoDB to report an improper query, the returned value has
// a *QueryError type, and includes the Err message and the Code.
func (iter *Iter) Err() error {
	iter.m.Lock()
	err := iter.err
	iter.m.Unlock()
	if err == ErrNotFound {
		return nil
	}
	return err
}

// Close kills the server cursor used by the iterator, if any, and returns
// nil if no errors happened during iteration, or the actual error otherwise.
//
// Server cursors are automatically closed at the end of an iteration, which
// means close will do nothing unless the iteration was interrupted before
// the server finished sending results to the driver. If Close is not called
// in such a situation, the cursor will remain available at the server until
// the default cursor timeout period is reached. No further problems arise.
//
// Close is idempotent. That means it can be called repeatedly and will
// return the same result every time.
//
// In case a resulting document included a field named $err or errmsg, which are
// standard ways for MongoDB to report an improper query, the returned value has
// a *QueryError type.
func (iter *Iter) Close() error {
	iter.m.Lock()
	iter.killCursor()
	err := iter.err
	iter.m.Unlock()
	if err == ErrNotFound {
		return nil
	}
	return err
}

func (iter *Iter) killCursor() error {
	if iter.op.cursorId != 0 {
		socket, err := iter.acquireSocket()
		if err == nil {
			// TODO Batch kills.
			err = socket.Query(&killCursorsOp{[]int64{iter.op.cursorId}})
			socket.Release()
		}
		if err != nil && (iter.err == nil || iter.err == ErrNotFound) {
			iter.err = err
		}
		iter.op.cursorId = 0
		return err
	}
	return nil
}

// Timeout returns true if Next returned false due to a timeout of
// a tailable cursor. In those cases, Next may be called again to continue
// the iteration at the previous cursor position.
func (iter *Iter) Timeout() bool {
	iter.m.Lock()
	result := iter.timedout
	iter.m.Unlock()
	return result
}

// Next retrieves the next document from the result set, blocking if necessary.
// This method will also automatically retrieve another batch of documents from
// the server when the current one is exhausted, or before that in background
// if pre-fetching is enabled (see the Query.Prefetch and Session.SetPrefetch
// methods).
//
// Next returns true if a document was successfully unmarshalled onto result,
// and false at the end of the result set or if an error happened.
// When Next returns false, the Err method should be called to verify if
// there was an error during iteration.
//
// For example:
//
//    iter := collection.Find(nil).Iter()
//    for iter.Next(&result) {
//        fmt.Printf("Result: %v\n", result.Id)
//    }
//    if err := iter.Close(); err != nil {
//        return err
//    }
//
func (iter *Iter) Next(result interface{}) bool {
	iter.m.Lock()
	iter.timedout = false
	timeout := time.Time{}
	for iter.err == nil && iter.docData.Len() == 0 && (iter.docsToReceive > 0 || iter.op.cursorId != 0) {
		if iter.docsToReceive == 0 {
			if iter.timeout >= 0 {
				if timeout.IsZero() {
					timeout = time.Now().Add(iter.timeout)
				}
				if time.Now().After(timeout) {
					iter.timedout = true
					iter.m.Unlock()
					return false
				}
			}
			iter.getMore()
		}
		iter.gotReply.Wait()
	}

	// Exhaust available data before reporting any errors.
	if docData, ok := iter.docData.Pop().([]byte); ok {
		if iter.limit > 0 {
			iter.limit--
			if iter.limit == 0 {
				if iter.docData.Len() > 0 {
					panic(fmt.Errorf("data remains after limit exhausted: %d", iter.docData.Len()))
				}
				iter.err = ErrNotFound
				if iter.killCursor() != nil {
					return false
				}
			}
		}
		if iter.op.cursorId != 0 && iter.err == nil {
			if iter.docsBeforeMore == 0 {
				iter.getMore()
			}
			iter.docsBeforeMore-- // Goes negative.
		}
		iter.m.Unlock()
		err := bson.Unmarshal(docData, result)
		if err != nil {
			debugf("Iter %p document unmarshaling failed: %#v", iter, err)
			iter.err = err
			return false
		}
		debugf("Iter %p document unmarshaled: %#v", iter, result)
		// XXX Only have to check first document for a query error?
		err = checkQueryError(iter.op.collection, docData)
		if err != nil {
			iter.m.Lock()
			if iter.err == nil {
				iter.err = err
			}
			iter.m.Unlock()
			return false
		}
		return true
	} else if iter.err != nil {
		debugf("Iter %p returning false: %s", iter, iter.err)
		iter.m.Unlock()
		return false
	} else if iter.op.cursorId == 0 {
		iter.err = ErrNotFound
		debugf("Iter %p exhausted with cursor=0", iter)
		iter.m.Unlock()
		return false
	}

	panic("unreachable")
}

// All retrieves all documents from the result set into the provided slice
// and closes the iterator.
//
// The result argument must necessarily be the address for a slice. The slice
// may be nil or previously allocated.
//
// WARNING: Obviously, All must not be used with result sets that may be
// potentially large, since it may consume all memory until the system
// crashes. Consider building the query with a Limit clause to ensure the
// result size is bounded.
//
// For instance:
//
//    var result []struct{ Value int }
//    iter := collection.Find(nil).Limit(100).Iter()
//    err := iter.All(&result)
//    if err != nil {
//        return err
//    }
//
func (iter *Iter) All(result interface{}) error {
	resultv := reflect.ValueOf(result)
	if resultv.Kind() != reflect.Ptr || resultv.Elem().Kind() != reflect.Slice {
		panic("result argument must be a slice address")
	}
	slicev := resultv.Elem()
	slicev = slicev.Slice(0, slicev.Cap())
	elemt := slicev.Type().Elem()
	i := 0
	for {
		if slicev.Len() == i {
			elemp := reflect.New(elemt)
			if !iter.Next(elemp.Interface()) {
				break
			}
			slicev = reflect.Append(slicev, elemp.Elem())
			slicev = slicev.Slice(0, slicev.Cap())
		} else {
			if !iter.Next(slicev.Index(i).Addr().Interface()) {
				break
			}
		}
		i++
	}
	resultv.Elem().Set(slicev.Slice(0, i))
	return iter.Close()
}

// All works like Iter.All.
func (q *Query) All(result interface{}) error {
	return q.Iter().All(result)
}

// The For method is obsolete and will be removed in a future release.
// See Iter as an elegant replacement.
func (q *Query) For(result interface{}, f func() error) error {
	return q.Iter().For(result, f)
}

// The For method is obsolete and will be removed in a future release.
// See Iter as an elegant replacement.
func (iter *Iter) For(result interface{}, f func() error) (err error) {
	valid := false
	v := reflect.ValueOf(result)
	if v.Kind() == reflect.Ptr {
		v = v.Elem()
		switch v.Kind() {
		case reflect.Map, reflect.Ptr, reflect.Interface, reflect.Slice:
			valid = v.IsNil()
		}
	}
	if !valid {
		panic("For needs a pointer to nil reference value.  See the documentation.")
	}
	zero := reflect.Zero(v.Type())
	for {
		v.Set(zero)
		if !iter.Next(result) {
			break
		}
		err = f()
		if err != nil {
			return err
		}
	}
	return iter.Err()
}

func (iter *Iter) acquireSocket() (*mongoSocket, error) {
	socket, err := iter.session.acquireSocket(true)
	if err != nil {
		return nil, err
	}
	if socket.Server() != iter.server {
		// Socket server changed during iteration. This may happen
		// with Eventual sessions, if a Refresh is done, or if a
		// monotonic session gets a write and shifts from secondary
		// to primary. Our cursor is in a specific server, though.
		iter.session.m.Lock()
		sockTimeout := iter.session.sockTimeout
		iter.session.m.Unlock()
		socket.Release()
		socket, _, err = iter.server.AcquireSocket(0, sockTimeout)
		if err != nil {
			return nil, err
		}
		err := iter.session.socketLogin(socket)
		if err != nil {
			socket.Release()
			return nil, err
		}
	}
	return socket, nil
}

func (iter *Iter) getMore() {
	socket, err := iter.acquireSocket()
	if err != nil {
		iter.err = err
		return
	}
	defer socket.Release()

	debugf("Iter %p requesting more documents", iter)
	if iter.limit > 0 {
		limit := iter.limit - int32(iter.docsToReceive) - int32(iter.docData.Len())
		if limit < iter.op.limit {
			iter.op.limit = limit
		}
	}
	if err := socket.Query(&iter.op); err != nil {
		iter.err = err
	}
	iter.docsToReceive++
}

type countCmd struct {
	Count string
	Query interface{}
	Limit int32 ",omitempty"
	Skip  int32 ",omitempty"
}

// Count returns the total number of documents in the result set.
func (q *Query) Count() (n int, err error) {
	q.m.Lock()
	session := q.session
	op := q.op
	limit := q.limit
	q.m.Unlock()

	c := strings.Index(op.collection, ".")
	if c < 0 {
		return 0, errors.New("Bad collection name: " + op.collection)
	}

	dbname := op.collection[:c]
	cname := op.collection[c+1:]

	result := struct{ N int }{}
	err = session.DB(dbname).Run(countCmd{cname, op.query, limit, op.skip}, &result)
	return result.N, err
}

// Count returns the total number of documents in the collection.
func (c *Collection) Count() (n int, err error) {
	return c.Find(nil).Count()
}

type distinctCmd struct {
	Collection string "distinct"
	Key        string
	Query      interface{} ",omitempty"
}

// Distinct returns a list of distinct values for the given key within
// the result set.  The list of distinct values will be unmarshalled
// in the "values" key of the provided result parameter.
//
// For example:
//
//     var result []int
//     err := collection.Find(bson.M{"gender": "F"}).Distinct("age", &result)
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Aggregation
//
func (q *Query) Distinct(key string, result interface{}) error {
	q.m.Lock()
	session := q.session
	op := q.op // Copy.
	q.m.Unlock()

	c := strings.Index(op.collection, ".")
	if c < 0 {
		return errors.New("Bad collection name: " + op.collection)
	}

	dbname := op.collection[:c]
	cname := op.collection[c+1:]

	var doc struct{ Values bson.Raw }
	err := session.DB(dbname).Run(distinctCmd{cname, key, op.query}, &doc)
	if err != nil {
		return err
	}
	return doc.Values.Unmarshal(result)
}

type mapReduceCmd struct {
	Collection string "mapreduce"
	Map        string ",omitempty"
	Reduce     string ",omitempty"
	Finalize   string ",omitempty"
	Limit      int32  ",omitempty"
	Out        interface{}
	Query      interface{} ",omitempty"
	Sort       interface{} ",omitempty"
	Scope      interface{} ",omitempty"
	Verbose    bool        ",omitempty"
}

type mapReduceResult struct {
	Results    bson.Raw
	Result     bson.Raw
	TimeMillis int64 "timeMillis"
	Counts     struct{ Input, Emit, Output int }
	Ok         bool
	Err        string
	Timing     *MapReduceTime
}

type MapReduce struct {
	Map      string      // Map Javascript function code (required)
	Reduce   string      // Reduce Javascript function code (required)
	Finalize string      // Finalize Javascript function code (optional)
	Out      interface{} // Output collection name or document. If nil, results are inlined into the result parameter.
	Scope    interface{} // Optional global scope for Javascript functions
	Verbose  bool
}

type MapReduceInfo struct {
	InputCount  int            // Number of documents mapped
	EmitCount   int            // Number of times reduce called emit
	OutputCount int            // Number of documents in resulting collection
	Database    string         // Output database, if results are not inlined
	Collection  string         // Output collection, if results are not inlined
	Time        int64          // Time to run the job, in nanoseconds
	VerboseTime *MapReduceTime // Only defined if Verbose was true
}

type MapReduceTime struct {
	Total    int64 // Total time, in nanoseconds
	Map      int64 "mapTime"  // Time within map function, in nanoseconds
	EmitLoop int64 "emitLoop" // Time within the emit/map loop, in nanoseconds
}

// MapReduce executes a map/reduce job for documents covered by the query.
// That kind of job is suitable for very flexible bulk aggregation of data
// performed at the server side via Javascript functions.
//
// Results from the job may be returned as a result of the query itself
// through the result parameter in case they'll certainly fit in memory
// and in a single document.  If there's the possibility that the amount
// of data might be too large, results must be stored back in an alternative
// collection or even a separate database, by setting the Out field of the
// provided MapReduce job.  In that case, provide nil as the result parameter.
//
// These are some of the ways to set Out:
//
//     nil
//         Inline results into the result parameter.
//
//     bson.M{"replace": "mycollection"}
//         The output will be inserted into a collection which replaces any
//         existing collection with the same name.
//
//     bson.M{"merge": "mycollection"}
//         This option will merge new data into the old output collection. In
//         other words, if the same key exists in both the result set and the
//         old collection, the new key will overwrite the old one.
//
//     bson.M{"reduce": "mycollection"}
//         If documents exist for a given key in the result set and in the old
//         collection, then a reduce operation (using the specified reduce
//         function) will be performed on the two values and the result will be
//         written to the output collection. If a finalize function was
//         provided, this will be run after the reduce as well.
//
//     bson.M{...., "db": "mydb"}
//         Any of the above options can have the "db" key included for doing
//         the respective action in a separate database.
//
// The following is a trivial example which will count the number of
// occurrences of a field named n on each document in a collection, and
// will return results inline:
//
//     job := &mgo.MapReduce{
//             Map:      "function() { emit(this.n, 1) }",
//             Reduce:   "function(key, values) { return Array.sum(values) }",
//     }
//     var result []struct { Id int "_id"; Value int }
//     _, err := collection.Find(nil).MapReduce(job, &result)
//     if err != nil {
//         return err
//     }
//     for _, item := range result {
//         fmt.Println(item.Value)
//     }
//
// This function is compatible with MongoDB 1.7.4+.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/MapReduce
//
func (q *Query) MapReduce(job *MapReduce, result interface{}) (info *MapReduceInfo, err error) {
	q.m.Lock()
	session := q.session
	op := q.op // Copy.
	limit := q.limit
	q.m.Unlock()

	c := strings.Index(op.collection, ".")
	if c < 0 {
		return nil, errors.New("Bad collection name: " + op.collection)
	}

	dbname := op.collection[:c]
	cname := op.collection[c+1:]

	cmd := mapReduceCmd{
		Collection: cname,
		Map:        job.Map,
		Reduce:     job.Reduce,
		Finalize:   job.Finalize,
		Out:        fixMROut(job.Out),
		Scope:      job.Scope,
		Verbose:    job.Verbose,
		Query:      op.query,
		Sort:       op.options.OrderBy,
		Limit:      limit,
	}

	if cmd.Out == nil {
		cmd.Out = bson.M{"inline": 1}
	}

	var doc mapReduceResult
	err = session.DB(dbname).Run(&cmd, &doc)
	if err != nil {
		return nil, err
	}
	if doc.Err != "" {
		return nil, errors.New(doc.Err)
	}

	info = &MapReduceInfo{
		InputCount:  doc.Counts.Input,
		EmitCount:   doc.Counts.Emit,
		OutputCount: doc.Counts.Output,
		Time:        doc.TimeMillis * 1e6,
	}

	if doc.Result.Kind == 0x02 {
		err = doc.Result.Unmarshal(&info.Collection)
		info.Database = dbname
	} else if doc.Result.Kind == 0x03 {
		var v struct{ Collection, Db string }
		err = doc.Result.Unmarshal(&v)
		info.Collection = v.Collection
		info.Database = v.Db
	}

	if doc.Timing != nil {
		info.VerboseTime = doc.Timing
		info.VerboseTime.Total *= 1e6
		info.VerboseTime.Map *= 1e6
		info.VerboseTime.EmitLoop *= 1e6
	}

	if err != nil {
		return nil, err
	}
	if result != nil {
		return info, doc.Results.Unmarshal(result)
	}
	return info, nil
}

// The "out" option in the MapReduce command must be ordered. This was
// found after the implementation was accepting maps for a long time,
// so rather than breaking the API, we'll fix the order if necessary.
// Details about the order requirement may be seen in MongoDB's code:
//
//     http://goo.gl/L8jwJX
//
func fixMROut(out interface{}) interface{} {
	outv := reflect.ValueOf(out)
	if outv.Kind() != reflect.Map || outv.Type().Key() != reflect.TypeOf("") {
		return out
	}
	outs := make(bson.D, outv.Len())

	outTypeIndex := -1
	for i, k := range outv.MapKeys() {
		ks := k.String()
		outs[i].Name = ks
		outs[i].Value = outv.MapIndex(k).Interface()
		switch ks {
		case "normal", "replace", "merge", "reduce", "inline":
			outTypeIndex = i
		}
	}
	if outTypeIndex > 0 {
		outs[0], outs[outTypeIndex] = outs[outTypeIndex], outs[0]
	}
	return outs
}

type Change struct {
	Update    interface{} // The change document
	Upsert    bool        // Whether to insert in case the document isn't found
	Remove    bool        // Whether to remove the document found rather than updating
	ReturnNew bool        // Should the modified document be returned rather than the old one
}

type findModifyCmd struct {
	Collection                  string      "findAndModify"
	Query, Update, Sort, Fields interface{} ",omitempty"
	Upsert, Remove, New         bool        ",omitempty"
}

type valueResult struct {
	Value     bson.Raw
	LastError LastError "lastErrorObject"
}

// Apply allows updating, upserting or removing a document matching a query
// and atomically returning either the old version (the default) or the new
// version of the document (when ReturnNew is true). If no objects are
// found Apply returns ErrNotFound.
//
// The Sort and Select query methods affect the result of Apply.  In case
// multiple documents match the query, Sort enables selecting which document to
// act upon by ordering it first.  Select enables retrieving only a selection
// of fields of the new or old document.
//
// This simple example increments a counter and prints its new value:
//
//     change := mgo.Change{
//             Update: bson.M{"$inc": bson.M{"n": 1}},
//             ReturnNew: true,
//     }
//     info, err = col.Find(M{"_id": id}).Apply(change, &doc)
//     fmt.Println(doc.N)
//
// This method depends on MongoDB >= 2.0 to work properly.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/findAndModify+Command
//     http://www.mongodb.org/display/DOCS/Updating
//     http://www.mongodb.org/display/DOCS/Atomic+Operations
//
func (q *Query) Apply(change Change, result interface{}) (info *ChangeInfo, err error) {
	q.m.Lock()
	session := q.session
	op := q.op // Copy.
	q.m.Unlock()

	c := strings.Index(op.collection, ".")
	if c < 0 {
		return nil, errors.New("bad collection name: " + op.collection)
	}

	dbname := op.collection[:c]
	cname := op.collection[c+1:]

	cmd := findModifyCmd{
		Collection: cname,
		Update:     change.Update,
		Upsert:     change.Upsert,
		Remove:     change.Remove,
		New:        change.ReturnNew,
		Query:      op.query,
		Sort:       op.options.OrderBy,
		Fields:     op.selector,
	}

	session = session.Clone()
	defer session.Close()
	session.SetMode(Strong, false)

	var doc valueResult
	err = session.DB(dbname).Run(&cmd, &doc)
	if err != nil {
		if qerr, ok := err.(*QueryError); ok && qerr.Message == "No matching object found" {
			return nil, ErrNotFound
		}
		return nil, err
	}
	if doc.LastError.N == 0 {
		return nil, ErrNotFound
	}
	if doc.Value.Kind != 0x0A {
		err = doc.Value.Unmarshal(result)
		if err != nil {
			return nil, err
		}
	}
	info = &ChangeInfo{}
	lerr := &doc.LastError
	if lerr.UpdatedExisting {
		info.Updated = lerr.N
	} else if change.Remove {
		info.Removed = lerr.N
	} else if change.Upsert {
		info.UpsertedId = lerr.UpsertedId
	}
	return info, nil
}

// The BuildInfo type encapsulates details about the running MongoDB server.
//
// Note that the VersionArray field was introduced in MongoDB 2.0+, but it is
// internally assembled from the Version information for previous versions.
// In both cases, VersionArray is guaranteed to have at least 4 entries.
type BuildInfo struct {
	Version       string
	VersionArray  []int  `bson:"versionArray"` // On MongoDB 2.0+; assembled from Version otherwise
	GitVersion    string `bson:"gitVersion"`
	SysInfo       string `bson:"sysInfo"`
	Bits          int
	Debug         bool
	MaxObjectSize int `bson:"maxBsonObjectSize"`
}

// BuildInfo retrieves the version and other details about the
// running MongoDB server.
func (s *Session) BuildInfo() (info BuildInfo, err error) {
	err = s.Run(bson.D{{"buildInfo", "1"}}, &info)
	if len(info.VersionArray) == 0 {
		for _, a := range strings.Split(info.Version, ".") {
			i, err := strconv.Atoi(a)
			if err != nil {
				break
			}
			info.VersionArray = append(info.VersionArray, i)
		}
	}
	for len(info.VersionArray) < 4 {
		info.VersionArray = append(info.VersionArray, 0)
	}
	return
}

// ---------------------------------------------------------------------------
// Internal session handling helpers.

func (s *Session) acquireSocket(slaveOk bool) (*mongoSocket, error) {

	// Read-only lock to check for previously reserved socket.
	s.m.RLock()
	if s.masterSocket != nil {
		socket := s.masterSocket
		socket.Acquire()
		s.m.RUnlock()
		return socket, nil
	}
	if s.slaveSocket != nil && s.slaveOk && slaveOk {
		socket := s.slaveSocket
		socket.Acquire()
		s.m.RUnlock()
		return socket, nil
	}
	s.m.RUnlock()

	// No go.  We may have to request a new socket and change the session,
	// so try again but with an exclusive lock now.
	s.m.Lock()
	defer s.m.Unlock()

	if s.masterSocket != nil {
		s.masterSocket.Acquire()
		return s.masterSocket, nil
	}
	if s.slaveSocket != nil && s.slaveOk && slaveOk {
		s.slaveSocket.Acquire()
		return s.slaveSocket, nil
	}

	// Still not good.  We need a new socket.
	sock, err := s.cluster().AcquireSocket(slaveOk && s.slaveOk, s.syncTimeout, s.sockTimeout, s.queryConfig.op.serverTags)
	if err != nil {
		return nil, err
	}

	// Authenticate the new socket.
	if err = s.socketLogin(sock); err != nil {
		sock.Release()
		return nil, err
	}

	// Keep track of the new socket, if necessary.
	// Note that, as a special case, if the Eventual session was
	// not refreshed (s.slaveSocket != nil), it means the developer
	// asked to preserve an existing reserved socket, so we'll
	// keep a master one around too before a Refresh happens.
	if s.consistency != Eventual || s.slaveSocket != nil {
		s.setSocket(sock)
	}

	// Switch over a Monotonic session to the master.
	if !slaveOk && s.consistency == Monotonic {
		s.slaveOk = false
	}

	return sock, nil
}

// setSocket binds socket to this section.
func (s *Session) setSocket(socket *mongoSocket) {
	info := socket.Acquire()
	if info.Master {
		if s.masterSocket != nil {
			panic("setSocket(master) with existing master socket reserved")
		}
		s.masterSocket = socket
	} else {
		if s.slaveSocket != nil {
			panic("setSocket(slave) with existing slave socket reserved")
		}
		s.slaveSocket = socket
	}
}

// unsetSocket releases any slave and/or master sockets reserved.
func (s *Session) unsetSocket() {
	if s.masterSocket != nil {
		s.masterSocket.Release()
	}
	if s.slaveSocket != nil {
		s.slaveSocket.Release()
	}
	s.masterSocket = nil
	s.slaveSocket = nil
}

func (iter *Iter) replyFunc() replyFunc {
	return func(err error, op *replyOp, docNum int, docData []byte) {
		iter.m.Lock()
		iter.docsToReceive--
		if err != nil {
			iter.err = err
			debugf("Iter %p received an error: %s", iter, err.Error())
		} else if docNum == -1 {
			debugf("Iter %p received no documents (cursor=%d).", iter, op.cursorId)
			if op != nil && op.cursorId != 0 {
				// It's a tailable cursor.
				iter.op.cursorId = op.cursorId
			} else {
				iter.err = ErrNotFound
			}
		} else {
			rdocs := int(op.replyDocs)
			if docNum == 0 {
				iter.docsToReceive += rdocs - 1
				docsToProcess := iter.docData.Len() + rdocs
				if iter.limit == 0 || int32(docsToProcess) < iter.limit {
					iter.docsBeforeMore = docsToProcess - int(iter.prefetch*float64(rdocs))
				} else {
					iter.docsBeforeMore = -1
				}
				iter.op.cursorId = op.cursorId
			}
			// XXX Handle errors and flags.
			debugf("Iter %p received reply document %d/%d (cursor=%d)", iter, docNum+1, rdocs, op.cursorId)
			iter.docData.Push(docData)
		}
		iter.gotReply.Broadcast()
		iter.m.Unlock()
	}
}

// writeQuery runs the given modifying operation, potentially followed up
// by a getLastError command in case the session is in safe mode.  The
// LastError result is made available in lerr, and if lerr.Err is set it
// will also be returned as err.
func (c *Collection) writeQuery(op interface{}) (lerr *LastError, err error) {
	s := c.Database.Session
	dbname := c.Database.Name
	socket, err := s.acquireSocket(dbname == "local")
	if err != nil {
		return nil, err
	}
	defer socket.Release()

	s.m.RLock()
	safeOp := s.safeOp
	s.m.RUnlock()

	if safeOp == nil {
		return nil, socket.Query(op)
	} else {
		var mutex sync.Mutex
		var replyData []byte
		var replyErr error
		mutex.Lock()
		query := *safeOp // Copy the data.
		query.collection = dbname + ".$cmd"
		query.replyFunc = func(err error, reply *replyOp, docNum int, docData []byte) {
			replyData = docData
			replyErr = err
			mutex.Unlock()
		}
		err = socket.Query(op, &query)
		if err != nil {
			return nil, err
		}
		mutex.Lock() // Wait.
		if replyErr != nil {
			return nil, replyErr // XXX TESTME
		}
		if hasErrMsg(replyData) {
			// Looks like getLastError itself failed.
			err = checkQueryError(query.collection, replyData)
			if err != nil {
				return nil, err
			}
		}
		result := &LastError{}
		bson.Unmarshal(replyData, &result)
		debugf("Result from writing query: %#v", result)
		if result.Err != "" {
			return result, result
		}
		return result, nil
	}
	panic("unreachable")
}

func hasErrMsg(d []byte) bool {
	l := len(d)
	for i := 0; i+8 < l; i++ {
		if d[i] == '\x02' && d[i+1] == 'e' && d[i+2] == 'r' && d[i+3] == 'r' && d[i+4] == 'm' && d[i+5] == 's' && d[i+6] == 'g' && d[i+7] == '\x00' {
			return true
		}
	}
	return false
}