memcache.go

/*
Copyright 2011 Google Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Package memcache provides a client for the memcached cache server.
package memcache

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"math/rand"
	"net"
	"strconv"
	"strings"
	"sync"
	"time"
)

// Similar to:
// https://godoc.org/google.golang.org/appengine/memcache

var (
	// ErrCacheMiss means that a Get failed because the item wasn't present.
	ErrCacheMiss = errors.New("memcache: cache miss")

	// ErrCASConflict means that a CompareAndSwap call failed due to the
	// cached value being modified between the Get and the CompareAndSwap.
	// If the cached value was simply evicted rather than replaced,
	// ErrNotStored will be returned instead.
	ErrCASConflict = errors.New("memcache: compare-and-swap conflict")

	// ErrNotStored means that a conditional write operation (i.e. Add or
	// CompareAndSwap) failed because the condition was not satisfied.
	ErrNotStored = errors.New("memcache: item not stored")

	// ErrServer means that a server error occurred.
	ErrServerError = errors.New("memcache: server error")

	// ErrNoStats means that no statistics were available.
	ErrNoStats = errors.New("memcache: no statistics available")

	// ErrMalformedKey is returned when an invalid key is used.
	// Keys must be at maximum 250 bytes long and not
	// contain whitespace or control characters.
	ErrMalformedKey = errors.New("malformed: key is too long or contains invalid characters")

	// ErrNoServers is returned when no servers are configured or available.
	ErrNoServers = errors.New("memcache: no servers configured or available")
)

const (
	// DefaultTimeout is the default socket read/write timeout.
	DefaultTimeout = 100 * time.Millisecond

	// DefaultMaxIdleConns is the default maximum number of idle connections
	// kept for any single address.
	DefaultMaxIdleConns = 2
)

// PoolPolicy decides how connection from free connection lists selected.
type PoolPolicy int

const (
	// PoolPolicyLatest selects the most recently used connection from the pool.
	PoolPolicyLatest PoolPolicy = iota

	// PoolPolicyEarliest selects the least recently used connection from the pool.
	PoolPolicyEarliest

	// PoolPolicyRandom randomly selects the connection from the pool.
	PoolPolicyRandom
)

const buffered = 8 // arbitrary buffered channel size, for readability

// resumableError returns true if err is only a protocol-level cache error.
// This is used to determine whether or not a server connection should
// be re-used or not. If an error occurs, by default we don't reuse the
// connection, unless it was just a cache error.
func resumableError(err error) bool {
	switch err {
	case ErrCacheMiss, ErrCASConflict, ErrNotStored, ErrMalformedKey:
		return true
	}
	return false
}

func legalKey(key string) bool {
	if len(key) > 250 {
		return false
	}
	for i := 0; i < len(key); i++ {
		if key[i] <= ' ' || key[i] == 0x7f {
			return false
		}
	}
	return true
}

var (
	crlf            = []byte("\r\n")
	space           = []byte(" ")
	resultOK        = []byte("OK\r\n")
	resultStored    = []byte("STORED\r\n")
	resultNotStored = []byte("NOT_STORED\r\n")
	resultExists    = []byte("EXISTS\r\n")
	resultNotFound  = []byte("NOT_FOUND\r\n")
	resultDeleted   = []byte("DELETED\r\n")
	resultEnd       = []byte("END\r\n")
	resultOk        = []byte("OK\r\n")
	resultTouched   = []byte("TOUCHED\r\n")

	resultClientErrorPrefix = []byte("CLIENT_ERROR ")
	versionPrefix           = []byte("VERSION")
)

// New returns a memcache client using the provided server(s)
// with equal weight. If a server is listed multiple times,
// it gets a proportional amount of weight.
func New(server ...string) *Client {
	ss := new(ServerList)
	ss.SetServers(server...)
	return NewFromSelector(ss)
}

// NewFromSelector returns a new Client using the provided ServerSelector.
func NewFromSelector(ss ServerSelector) *Client {
	return &Client{selector: ss}
}

// Client is a memcache client.
// It is safe for unlocked use by multiple concurrent goroutines.
type Client struct {
	// Timeout specifies the socket read/write timeout.
	// If zero, DefaultTimeout is used.
	Timeout time.Duration

	// MaxIdleConns specifies the maximum number of idle connections that will
	// be maintained per address. If less than one, DefaultMaxIdleConns will be
	// used.
	//
	// Consider your expected traffic rates and latency carefully. This should
	// be set to a number higher than your peak parallel requests.
	MaxIdleConns int

	// PoolPolicy specifies how connection from free connection lists selected.
	// Default option is PoolPolicyLatest
	PoolPolicy PoolPolicy
	rand       *rand.Rand

	selector ServerSelector

	lk       sync.Mutex
	freeconn map[string][]*conn
}

// Item is an item to be got or stored in a memcached server.
type Item struct {
	// Key is the Item's key (250 bytes maximum).
	Key string

	// Value is the Item's value.
	Value []byte

	// Flags are server-opaque flags whose semantics are entirely
	// up to the app.
	Flags uint32

	// Expiration is the cache expiration time, in seconds: either a relative
	// time from now (up to 1 month), or an absolute Unix epoch time.
	// Zero means the Item has no expiration time.
	Expiration int32

	// Compare and swap ID.
	casid uint64
}

// conn is a connection to a server.
type conn struct {
	nc   net.Conn
	rw   *bufio.ReadWriter
	addr net.Addr
	c    *Client
}

// release returns this connection back to the client's free pool
func (cn *conn) release() {
	cn.c.putFreeConn(cn.addr, cn)
}

func (cn *conn) extendDeadline() {
	cn.nc.SetDeadline(time.Now().Add(cn.c.netTimeout()))
}

// condRelease releases this connection if the error pointed to by err
// is nil (not an error) or is only a protocol level error (e.g. a
// cache miss).  The purpose is to not recycle TCP connections that
// are bad.
func (cn *conn) condRelease(err *error) {
	if *err == nil || resumableError(*err) {
		cn.release()
	} else {
		cn.nc.Close()
	}
}

func (c *Client) putFreeConn(addr net.Addr, cn *conn) {
	c.lk.Lock()
	defer c.lk.Unlock()
	if c.freeconn == nil {
		c.freeconn = make(map[string][]*conn)
	}
	freelist := c.freeconn[addr.String()]
	if len(freelist) >= c.maxIdleConns() {
		cn.nc.Close()
		return
	}
	c.freeconn[addr.String()] = append(freelist, cn)
}

func (c *Client) getFreeConn(addr net.Addr) (cn *conn, ok bool) {
	c.lk.Lock()
	defer c.lk.Unlock()
	if c.freeconn == nil {
		return nil, false
	}
	freelist, ok := c.freeconn[addr.String()]
	if !ok || len(freelist) == 0 {
		return nil, false
	}
	cn, c.freeconn[addr.String()] = c.popFreeConnByPolicy(freelist)
	return cn, true
}

// Caution: popFreeConnByPolicy must be called in critical section
func (c *Client) popFreeConnByPolicy(freelist []*conn) (*conn, []*conn) {
	switch c.PoolPolicy {
	case PoolPolicyLatest:
		conn := freelist[len(freelist)-1]
		return conn, freelist[:len(freelist)-1]
	case PoolPolicyEarliest:
		conn := freelist[0]
		return conn, append(freelist[:0], freelist[1:]...) // freelist[1:]
	case PoolPolicyRandom:
		if c.rand == nil {
			c.rand = rand.New(rand.NewSource(time.Now().Unix()))
		}
		idx := c.rand.Intn(len(freelist))
		conn := freelist[idx]
		return conn, append(freelist[:idx], freelist[idx+1:]...)
	}
	conn := freelist[len(freelist)-1]
	return conn, freelist[:len(freelist)-1]
}

func (c *Client) netTimeout() time.Duration {
	if c.Timeout != 0 {
		return c.Timeout
	}
	return DefaultTimeout
}

func (c *Client) maxIdleConns() int {
	if c.MaxIdleConns > 0 {
		return c.MaxIdleConns
	}
	return DefaultMaxIdleConns
}

// ConnectTimeoutError is the error type used when it takes
// too long to connect to the desired host. This level of
// detail can generally be ignored.
type ConnectTimeoutError struct {
	Addr net.Addr
}

func (cte *ConnectTimeoutError) Error() string {
	return "memcache: connect timeout to " + cte.Addr.String()
}

func (c *Client) dial(addr net.Addr) (net.Conn, error) {
	type connError struct {
		cn  net.Conn
		err error
	}

	nc, err := net.DialTimeout(addr.Network(), addr.String(), c.netTimeout())
	if err == nil {
		return nc, nil
	}

	if ne, ok := err.(net.Error); ok && ne.Timeout() {
		return nil, &ConnectTimeoutError{addr}
	}

	return nil, err
}

func (c *Client) getConn(ctx context.Context, addr net.Addr) (*conn, error) {
	// Check if the context is expired.
	select {
	default:
	case <-ctx.Done():
		return nil, ctx.Err()
	}

	cn, ok := c.getFreeConn(addr)
	if ok {
		cn.extendDeadline()
		return cn, nil
	}
	nc, err := c.dial(addr)
	if err != nil {
		return nil, err
	}
	cn = &conn{
		nc:   nc,
		addr: addr,
		rw:   bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc)),
		c:    c,
	}
	cn.extendDeadline()
	return cn, nil
}

func (c *Client) onItem(ctx context.Context, item *Item, fn func(*Client, *bufio.ReadWriter, *Item) error) error {
	addr, err := c.selector.PickServer(item.Key)
	if err != nil {
		return err
	}
	cn, err := c.getConn(ctx, addr)
	if err != nil {
		return err
	}
	defer cn.condRelease(&err)
	if err = fn(c, cn.rw, item); err != nil {
		return err
	}
	return nil
}

func (c *Client) FlushAll() error {
	return c.FlushAllWithContext(context.Background())
}

func (c *Client) FlushAllWithContext(ctx context.Context) error {
	return c.selector.Each(c.flushAllFromAddrWithContext(ctx))
}

// Get gets the item for the given key. ErrCacheMiss is returned for a
// memcache cache miss. The key must be at most 250 bytes in length.
func (c *Client) Get(key string) (item *Item, err error) {
	return c.GetWithContext(context.Background(), key)
}

// GetWithContext gets the item for the given key. ErrCacheMiss is returned
// for a memcache cache miss. The key must be at most 250 bytes in length.
func (c *Client) GetWithContext(ctx context.Context, key string) (item *Item, err error) {
	err = c.withKeyAddr(key, func(addr net.Addr) error {
		return c.getFromAddr(ctx, addr, []string{key}, func(it *Item) { item = it })
	})
	if err == nil && item == nil {
		err = ErrCacheMiss
	}
	return
}

// Touch updates the expiry for the given key. The seconds parameter is either
// a Unix timestamp or, if seconds is less than 1 month, the number of seconds
// into the future at which time the item will expire. Zero means the item has
// no expiration time. ErrCacheMiss is returned if the key is not in the cache.
// The key must be at most 250 bytes in length.
func (c *Client) Touch(key string, seconds int32) (err error) {
	return c.TouchWithContext(context.Background(), key, seconds)
}

// TouchWithContext updates the expiry for the given key. The seconds parameter
// is either a Unix timestamp or, if seconds is less than 1 month, the number
// of seconds into the future at which time the item will expire. Zero means the
// item has no expiration time. ErrCacheMiss is returned if the key is not in
// the cache. The key must be at most 250 bytes in length.
func (c *Client) TouchWithContext(ctx context.Context, key string, seconds int32) (err error) {
	return c.withKeyAddr(key, func(addr net.Addr) error {
		return c.touchFromAddr(ctx, addr, []string{key}, seconds)
	})
}

func (c *Client) withKeyAddr(key string, fn func(net.Addr) error) (err error) {
	if !legalKey(key) {
		return ErrMalformedKey
	}
	addr, err := c.selector.PickServer(key)
	if err != nil {
		return err
	}
	return fn(addr)
}

func (c *Client) withAddrRw(ctx context.Context, addr net.Addr, fn func(*bufio.ReadWriter) error) (err error) {
	cn, err := c.getConn(ctx, addr)
	if err != nil {
		return err
	}
	defer cn.condRelease(&err)
	return fn(cn.rw)
}

func (c *Client) withKeyRw(ctx context.Context, key string, fn func(*bufio.ReadWriter) error) error {
	return c.withKeyAddr(key, func(addr net.Addr) error {
		return c.withAddrRw(ctx, addr, fn)
	})
}

func (c *Client) getFromAddr(ctx context.Context, addr net.Addr, keys []string, cb func(*Item)) error {
	return c.withAddrRw(ctx, addr, func(rw *bufio.ReadWriter) error {
		if _, err := fmt.Fprintf(rw, "gets %s\r\n", strings.Join(keys, " ")); err != nil {
			return err
		}
		if err := rw.Flush(); err != nil {
			return err
		}
		if err := parseGetResponse(rw.Reader, cb); err != nil {
			return err
		}
		return nil
	})
}

// flushAllFromAddrWithContext send the flush_all command to the given addr
func (c *Client) flushAllFromAddrWithContext(ctx context.Context) func(addr net.Addr) error {
	return func(addr net.Addr) error {
		return c.withAddrRw(ctx, addr, func(rw *bufio.ReadWriter) error {
			if _, err := fmt.Fprintf(rw, "flush_all\r\n"); err != nil {
				return err
			}
			if err := rw.Flush(); err != nil {
				return err
			}
			line, err := rw.ReadSlice('\n')
			if err != nil {
				return err
			}
			switch {
			case bytes.Equal(line, resultOk):
				break
			default:
				return fmt.Errorf("memcache: unexpected response line from flush_all: %q", string(line))
			}
			return nil
		})
	}
}

// pingWithContext sends the version command to the given addr
func (c *Client) pingWithContext(ctx context.Context) func(addr net.Addr) error {
	return func(addr net.Addr) error {
		return c.withAddrRw(ctx, addr, func(rw *bufio.ReadWriter) error {
			if _, err := fmt.Fprintf(rw, "version\r\n"); err != nil {
				return err
			}
			if err := rw.Flush(); err != nil {
				return err
			}
			line, err := rw.ReadSlice('\n')
			if err != nil {
				return err
			}

			switch {
			case bytes.HasPrefix(line, versionPrefix):
				break
			default:
				return fmt.Errorf("memcache: unexpected response line from ping: %q", string(line))
			}
			return nil
		})
	}
}

func (c *Client) touchFromAddr(ctx context.Context, addr net.Addr, keys []string, expiration int32) error {
	return c.withAddrRw(ctx, addr, func(rw *bufio.ReadWriter) error {
		for _, key := range keys {
			if _, err := fmt.Fprintf(rw, "touch %s %d\r\n", key, expiration); err != nil {
				return err
			}
			if err := rw.Flush(); err != nil {
				return err
			}
			line, err := rw.ReadSlice('\n')
			if err != nil {
				return err
			}
			switch {
			case bytes.Equal(line, resultTouched):
				break
			case bytes.Equal(line, resultNotFound):
				return ErrCacheMiss
			default:
				return fmt.Errorf("memcache: unexpected response line from touch: %q", string(line))
			}
		}
		return nil
	})
}

// GetMulti is a batch version of Get. The returned map from keys to
// items may have fewer elements than the input slice, due to memcache
// cache misses. Each key must be at most 250 bytes in length.
// If no error is returned, the returned map will also be non-nil.
func (c *Client) GetMulti(keys []string) (map[string]*Item, error) {
	return c.GetMultiWithContext(context.Background(), keys)
}

// GetMultiWithContext is a batch version of Get. The returned map from
// keys to items may have fewer elements than the input slice, due to
// memcache cache misses. Each key must be at most 250 bytes in length.
// If no error is returned, the returned map will also be non-nil.
func (c *Client) GetMultiWithContext(ctx context.Context, keys []string) (map[string]*Item, error) {
	var lk sync.Mutex
	m := make(map[string]*Item)
	addItemToMap := func(it *Item) {
		lk.Lock()
		defer lk.Unlock()
		m[it.Key] = it
	}

	keyMap := make(map[net.Addr][]string)
	for _, key := range keys {
		if !legalKey(key) {
			return nil, ErrMalformedKey
		}
		addr, err := c.selector.PickServer(key)
		if err != nil {
			return nil, err
		}
		keyMap[addr] = append(keyMap[addr], key)
	}

	ch := make(chan error, buffered)
	for addr, keys := range keyMap {
		go func(addr net.Addr, keys []string) {
			ch <- c.getFromAddr(ctx, addr, keys, addItemToMap)
		}(addr, keys)
	}

	var err error
	for _ = range keyMap {
		if ge := <-ch; ge != nil {
			err = ge
		}
	}
	return m, err
}

// parseGetResponse reads a GET response from r and calls cb for each
// read and allocated Item
func parseGetResponse(r *bufio.Reader, cb func(*Item)) error {
	for {
		line, err := r.ReadSlice('\n')
		if err != nil {
			return err
		}
		if bytes.Equal(line, resultEnd) {
			return nil
		}
		it := new(Item)
		size, err := scanGetResponseLine(line, it)
		if err != nil {
			return err
		}
		it.Value = make([]byte, size+2)
		_, err = io.ReadFull(r, it.Value)
		if err != nil {
			it.Value = nil
			return err
		}
		if !bytes.HasSuffix(it.Value, crlf) {
			it.Value = nil
			return fmt.Errorf("memcache: corrupt get result read")
		}
		it.Value = it.Value[:size]
		cb(it)
	}
}

// scanGetResponseLine populates it and returns the declared size of the item.
// It does not read the bytes of the item.
func scanGetResponseLine(line []byte, it *Item) (size int, err error) {
	pattern := "VALUE %s %d %d %d\r\n"
	dest := []interface{}{&it.Key, &it.Flags, &size, &it.casid}
	if bytes.Count(line, space) == 3 {
		pattern = "VALUE %s %d %d\r\n"
		dest = dest[:3]
	}
	lineBuf := bytes.NewBuffer(line)
	n, err := fmt.Fscanf(lineBuf, pattern, dest...)
	if err != nil || n != len(dest) {
		return -1, fmt.Errorf("memcache: unexpected line in get response: %q", line)
	}
	return size, nil
}

// Set writes the given item, unconditionally.
func (c *Client) Set(item *Item) error {
	return c.SetWithContext(context.Background(), item)
}

// SetWithContext writes the given item, unconditionally.
func (c *Client) SetWithContext(ctx context.Context, item *Item) error {
	return c.onItem(ctx, item, (*Client).set)
}

func (c *Client) set(rw *bufio.ReadWriter, item *Item) error {
	return c.populateOne(rw, "set", item)
}

// Add writes the given item, if no value already exists for its
// key. ErrNotStored is returned if that condition is not met.
func (c *Client) Add(item *Item) error {
	return c.AddWithContext(context.Background(), item)
}

// AddWithContext writes the given item, if no value already exists for
// its key. ErrNotStored is returned if that condition is not met.
func (c *Client) AddWithContext(ctx context.Context, item *Item) error {
	return c.onItem(ctx, item, (*Client).add)
}

func (c *Client) add(rw *bufio.ReadWriter, item *Item) error {
	return c.populateOne(rw, "add", item)
}

// Replace writes the given item, but only if the server *does*
// already hold data for this key
func (c *Client) Replace(item *Item) error {
	return c.ReplaceWithContext(context.Background(), item)
}

// ReplaceWithContext writes the given item, but only if the server
// *does* already hold data for this key
func (c *Client) ReplaceWithContext(ctx context.Context, item *Item) error {
	return c.onItem(ctx, item, (*Client).replace)
}

func (c *Client) replace(rw *bufio.ReadWriter, item *Item) error {
	return c.populateOne(rw, "replace", item)
}

// CompareAndSwap writes the given item that was previously returned
// by Get, if the value was neither modified or evicted between the
// Get and the CompareAndSwap calls. The item's Key should not change
// between calls but all other item fields may differ. ErrCASConflict
// is returned if the value was modified in between the
// calls. ErrNotStored is returned if the value was evicted in between
// the calls.
func (c *Client) CompareAndSwap(item *Item) error {
	return c.CompareAndSwapWithContext(context.Background(), item)
}

// CompareAndSwapWithContext writes the given item that was previously
// returned by Get, if the value was neither modified or evicted between
// the Get and the CompareAndSwap calls. The item's Key should not change
// between calls but all other item fields may differ. ErrCASConflict
// is returned if the value was modified in between the
// calls. ErrNotStored is returned if the value was evicted in between
// the calls.
func (c *Client) CompareAndSwapWithContext(ctx context.Context, item *Item) error {
	return c.onItem(ctx, item, (*Client).cas)
}

func (c *Client) cas(rw *bufio.ReadWriter, item *Item) error {
	return c.populateOne(rw, "cas", item)
}

func (c *Client) populateOne(rw *bufio.ReadWriter, verb string, item *Item) error {
	if !legalKey(item.Key) {
		return ErrMalformedKey
	}
	var err error
	if verb == "cas" {
		_, err = fmt.Fprintf(rw, "%s %s %d %d %d %d\r\n",
			verb, item.Key, item.Flags, item.Expiration, len(item.Value), item.casid)
	} else {
		_, err = fmt.Fprintf(rw, "%s %s %d %d %d\r\n",
			verb, item.Key, item.Flags, item.Expiration, len(item.Value))
	}
	if err != nil {
		return err
	}
	if _, err = rw.Write(item.Value); err != nil {
		return err
	}
	if _, err := rw.Write(crlf); err != nil {
		return err
	}
	if err := rw.Flush(); err != nil {
		return err
	}
	line, err := rw.ReadSlice('\n')
	if err != nil {
		return err
	}
	switch {
	case bytes.Equal(line, resultStored):
		return nil
	case bytes.Equal(line, resultNotStored):
		return ErrNotStored
	case bytes.Equal(line, resultExists):
		return ErrCASConflict
	case bytes.Equal(line, resultNotFound):
		return ErrCacheMiss
	}
	return fmt.Errorf("memcache: unexpected response line from %q: %q", verb, string(line))
}

func writeReadLine(rw *bufio.ReadWriter, format string, args ...interface{}) ([]byte, error) {
	_, err := fmt.Fprintf(rw, format, args...)
	if err != nil {
		return nil, err
	}
	if err := rw.Flush(); err != nil {
		return nil, err
	}
	line, err := rw.ReadSlice('\n')
	return line, err
}

func writeExpectf(rw *bufio.ReadWriter, expect []byte, format string, args ...interface{}) error {
	line, err := writeReadLine(rw, format, args...)
	if err != nil {
		return err
	}
	switch {
	case bytes.Equal(line, resultOK):
		return nil
	case bytes.Equal(line, expect):
		return nil
	case bytes.Equal(line, resultNotStored):
		return ErrNotStored
	case bytes.Equal(line, resultExists):
		return ErrCASConflict
	case bytes.Equal(line, resultNotFound):
		return ErrCacheMiss
	}
	return fmt.Errorf("memcache: unexpected response line: %q", string(line))
}

// Delete deletes the item with the provided key. The error ErrCacheMiss is
// returned if the item didn't already exist in the cache.
func (c *Client) Delete(key string) error {
	return c.DeleteWithContext(context.Background(), key)
}

// DeleteWithContext deletes the item with the provided key. The error
// ErrCacheMiss is returned if the item didn't already exist in the cache.
func (c *Client) DeleteWithContext(ctx context.Context, key string) error {
	return c.withKeyRw(ctx, key, func(rw *bufio.ReadWriter) error {
		return writeExpectf(rw, resultDeleted, "delete %s\r\n", key)
	})
}

// DeleteAll deletes all items in the cache.
func (c *Client) DeleteAll() error {
	return c.DeleteAllWithContext(context.Background())
}

// DeleteAllWithContext deletes all items in the cache.
func (c *Client) DeleteAllWithContext(ctx context.Context) error {
	return c.withKeyRw(ctx, "", func(rw *bufio.ReadWriter) error {
		return writeExpectf(rw, resultDeleted, "flush_all\r\n")
	})
}

// Ping checks all instances if they are alive. Returns error if any
// of them is down.
func (c *Client) Ping() error {
	return c.PingWithContext(context.Background())
}

// Ping checks all instances if they are alive. Returns error if any
// of them is down.
func (c *Client) PingWithContext(ctx context.Context) error {
	return c.selector.Each(c.pingWithContext(ctx))
}

// Increment atomically increments key by delta. The return value is
// the new value after being incremented or an error. If the value
// didn't exist in memcached the error is ErrCacheMiss. The value in
// memcached must be an decimal number, or an error will be returned.
// On 64-bit overflow, the new value wraps around.
func (c *Client) Increment(key string, delta uint64) (newValue uint64, err error) {
	return c.IncrementWithContext(context.Background(), key, delta)
}

// IncrementWithContext atomically increments key by delta. The return
// value is the new value after being incremented or an error. If the
// value didn't exist in memcached the error is ErrCacheMiss. The value
// in memcached must be an decimal number, or an error will be returned.
// On 64-bit overflow, the new value wraps around.
func (c *Client) IncrementWithContext(ctx context.Context, key string, delta uint64) (newValue uint64, err error) {
	return c.incrDecr(ctx, "incr", key, delta)
}

// Decrement atomically decrements key by delta. The return value is
// the new value after being decremented or an error. If the value
// didn't exist in memcached the error is ErrCacheMiss. The value in
// memcached must be an decimal number, or an error will be returned.
// On underflow, the new value is capped at zero and does not wrap
// around.
func (c *Client) Decrement(key string, delta uint64) (newValue uint64, err error) {
	return c.DecrementWithContext(context.Background(), key, delta)
}

// DecrementWithContext atomically decrements key by delta. The return
// value is the new value after being decremented or an error. If the
// value didn't exist in memcached the error is ErrCacheMiss. The value
// in memcached must be an decimal number, or an error will be returned.
// On underflow, the new value is capped at zero and does not wrap
// around.
func (c *Client) DecrementWithContext(ctx context.Context, key string, delta uint64) (newValue uint64, err error) {
	return c.incrDecr(ctx, "decr", key, delta)
}

func (c *Client) incrDecr(ctx context.Context, verb, key string, delta uint64) (uint64, error) {
	var val uint64
	err := c.withKeyRw(ctx, key, func(rw *bufio.ReadWriter) error {
		line, err := writeReadLine(rw, "%s %s %d\r\n", verb, key, delta)
		if err != nil {
			return err
		}
		switch {
		case bytes.Equal(line, resultNotFound):
			return ErrCacheMiss
		case bytes.HasPrefix(line, resultClientErrorPrefix):
			errMsg := line[len(resultClientErrorPrefix) : len(line)-2]
			return errors.New("memcache: client error: " + string(errMsg))
		}
		val, err = strconv.ParseUint(string(line[:len(line)-2]), 10, 64)
		if err != nil {
			return err
		}
		return nil
	})
	return val, err
}