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batch_cursor.go
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batch_cursor.go
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// Copyright (C) MongoDB, Inc. 2022-present.
//
// 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
package driver
import (
"context"
"errors"
"fmt"
"strings"
"go.mongodb.org/mongo-driver/bson/bsontype"
"go.mongodb.org/mongo-driver/event"
"go.mongodb.org/mongo-driver/internal"
"go.mongodb.org/mongo-driver/mongo/description"
"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
"go.mongodb.org/mongo-driver/x/mongo/driver/session"
)
// BatchCursor is a batch implementation of a cursor. It returns documents in entire batches instead
// of one at a time. An individual document cursor can be built on top of this batch cursor.
type BatchCursor struct {
clientSession *session.Client
clock *session.ClusterClock
comment bsoncore.Value
database string
collection string
id int64
err error
server Server
serverDescription description.Server
errorProcessor ErrorProcessor // This will only be set when pinning to a connection.
connection PinnedConnection
batchSize int32
maxTimeMS int64
currentBatch *bsoncore.DocumentSequence
firstBatch bool
cmdMonitor *event.CommandMonitor
postBatchResumeToken bsoncore.Document
crypt Crypt
serverAPI *ServerAPIOptions
// legacy server (< 3.2) fields
limit int32
numReturned int32 // number of docs returned by server
}
// CursorResponse represents the response from a command the results in a cursor. A BatchCursor can
// be constructed from a CursorResponse.
type CursorResponse struct {
Server Server
ErrorProcessor ErrorProcessor // This will only be set when pinning to a connection.
Connection PinnedConnection
Desc description.Server
FirstBatch *bsoncore.DocumentSequence
Database string
Collection string
ID int64
postBatchResumeToken bsoncore.Document
}
// NewCursorResponse constructs a cursor response from the given response and server. This method
// can be used within the ProcessResponse method for an operation.
func NewCursorResponse(info ResponseInfo) (CursorResponse, error) {
response := info.ServerResponse
cur, ok := response.Lookup("cursor").DocumentOK()
if !ok {
return CursorResponse{}, fmt.Errorf("cursor should be an embedded document but is of BSON type %s", response.Lookup("cursor").Type)
}
elems, err := cur.Elements()
if err != nil {
return CursorResponse{}, err
}
curresp := CursorResponse{Server: info.Server, Desc: info.ConnectionDescription}
for _, elem := range elems {
switch elem.Key() {
case "firstBatch":
arr, ok := elem.Value().ArrayOK()
if !ok {
return CursorResponse{}, fmt.Errorf("firstBatch should be an array but is a BSON %s", elem.Value().Type)
}
curresp.FirstBatch = &bsoncore.DocumentSequence{Style: bsoncore.ArrayStyle, Data: arr}
case "ns":
ns, ok := elem.Value().StringValueOK()
if !ok {
return CursorResponse{}, fmt.Errorf("ns should be a string but is a BSON %s", elem.Value().Type)
}
index := strings.Index(ns, ".")
if index == -1 {
return CursorResponse{}, errors.New("ns field must contain a valid namespace, but is missing '.'")
}
curresp.Database = ns[:index]
curresp.Collection = ns[index+1:]
case "id":
curresp.ID, ok = elem.Value().Int64OK()
if !ok {
return CursorResponse{}, fmt.Errorf("id should be an int64 but it is a BSON %s", elem.Value().Type)
}
case "postBatchResumeToken":
curresp.postBatchResumeToken, ok = elem.Value().DocumentOK()
if !ok {
return CursorResponse{}, fmt.Errorf("post batch resume token should be a document but it is a BSON %s", elem.Value().Type)
}
}
}
// If the deployment is behind a load balancer and the cursor has a non-zero ID, pin the cursor to a connection and
// use the same connection to execute getMore and killCursors commands.
if curresp.Desc.LoadBalanced() && curresp.ID != 0 {
// Cache the server as an ErrorProcessor to use when constructing deployments for cursor commands.
ep, ok := curresp.Server.(ErrorProcessor)
if !ok {
return CursorResponse{}, fmt.Errorf("expected Server used to establish a cursor to implement ErrorProcessor, but got %T", curresp.Server)
}
curresp.ErrorProcessor = ep
refConn, ok := info.Connection.(PinnedConnection)
if !ok {
return CursorResponse{}, fmt.Errorf("expected Connection used to establish a cursor to implement PinnedConnection, but got %T", info.Connection)
}
if err := refConn.PinToCursor(); err != nil {
return CursorResponse{}, fmt.Errorf("error incrementing connection reference count when creating a cursor: %v", err)
}
curresp.Connection = refConn
}
return curresp, nil
}
// CursorOptions are extra options that are required to construct a BatchCursor.
type CursorOptions struct {
BatchSize int32
Comment bsoncore.Value
MaxTimeMS int64
Limit int32
CommandMonitor *event.CommandMonitor
Crypt Crypt
ServerAPI *ServerAPIOptions
}
// NewBatchCursor creates a new BatchCursor from the provided parameters.
func NewBatchCursor(cr CursorResponse, clientSession *session.Client, clock *session.ClusterClock, opts CursorOptions) (*BatchCursor, error) {
ds := cr.FirstBatch
bc := &BatchCursor{
clientSession: clientSession,
clock: clock,
comment: opts.Comment,
database: cr.Database,
collection: cr.Collection,
id: cr.ID,
server: cr.Server,
connection: cr.Connection,
errorProcessor: cr.ErrorProcessor,
batchSize: opts.BatchSize,
maxTimeMS: opts.MaxTimeMS,
cmdMonitor: opts.CommandMonitor,
firstBatch: true,
postBatchResumeToken: cr.postBatchResumeToken,
crypt: opts.Crypt,
serverAPI: opts.ServerAPI,
serverDescription: cr.Desc,
}
if ds != nil {
bc.numReturned = int32(ds.DocumentCount())
}
if cr.Desc.WireVersion == nil || cr.Desc.WireVersion.Max < 4 {
bc.limit = opts.Limit
// Take as many documents from the batch as needed.
if bc.limit != 0 && bc.limit < bc.numReturned {
for i := int32(0); i < bc.limit; i++ {
_, err := ds.Next()
if err != nil {
return nil, err
}
}
ds.Data = ds.Data[:ds.Pos]
ds.ResetIterator()
}
}
bc.currentBatch = ds
return bc, nil
}
// NewEmptyBatchCursor returns a batch cursor that is empty.
func NewEmptyBatchCursor() *BatchCursor {
return &BatchCursor{currentBatch: new(bsoncore.DocumentSequence)}
}
// NewBatchCursorFromDocuments returns a batch cursor with current batch set to a sequence-style
// DocumentSequence containing the provided documents.
func NewBatchCursorFromDocuments(documents []byte) *BatchCursor {
return &BatchCursor{
currentBatch: &bsoncore.DocumentSequence{
Data: documents,
Style: bsoncore.SequenceStyle,
},
// BatchCursors created with this function have no associated ID nor server, so no getMore
// calls will be made.
id: 0,
server: nil,
}
}
// ID returns the cursor ID for this batch cursor.
func (bc *BatchCursor) ID() int64 {
return bc.id
}
// Next indicates if there is another batch available. Returning false does not necessarily indicate
// that the cursor is closed. This method will return false when an empty batch is returned.
//
// If Next returns true, there is a valid batch of documents available. If Next returns false, there
// is not a valid batch of documents available.
func (bc *BatchCursor) Next(ctx context.Context) bool {
if ctx == nil {
ctx = context.Background()
}
if bc.firstBatch {
bc.firstBatch = false
return !bc.currentBatch.Empty()
}
if bc.id == 0 || bc.server == nil {
return false
}
bc.getMore(ctx)
return !bc.currentBatch.Empty()
}
// Batch will return a DocumentSequence for the current batch of documents. The returned
// DocumentSequence is only valid until the next call to Next or Close.
func (bc *BatchCursor) Batch() *bsoncore.DocumentSequence { return bc.currentBatch }
// Err returns the latest error encountered.
func (bc *BatchCursor) Err() error { return bc.err }
// Close closes this batch cursor.
func (bc *BatchCursor) Close(ctx context.Context) error {
if ctx == nil {
ctx = context.Background()
}
err := bc.KillCursor(ctx)
bc.id = 0
bc.currentBatch.Data = nil
bc.currentBatch.Style = 0
bc.currentBatch.ResetIterator()
connErr := bc.unpinConnection()
if err == nil {
err = connErr
}
return err
}
func (bc *BatchCursor) unpinConnection() error {
if bc.connection == nil {
return nil
}
err := bc.connection.UnpinFromCursor()
closeErr := bc.connection.Close()
if err == nil && closeErr != nil {
err = closeErr
}
bc.connection = nil
return err
}
// Server returns the server for this cursor.
func (bc *BatchCursor) Server() Server {
return bc.server
}
func (bc *BatchCursor) clearBatch() {
bc.currentBatch.Data = bc.currentBatch.Data[:0]
}
// KillCursor kills cursor on server without closing batch cursor
func (bc *BatchCursor) KillCursor(ctx context.Context) error {
if bc.server == nil || bc.id == 0 {
return nil
}
return Operation{
CommandFn: func(dst []byte, desc description.SelectedServer) ([]byte, error) {
dst = bsoncore.AppendStringElement(dst, "killCursors", bc.collection)
dst = bsoncore.BuildArrayElement(dst, "cursors", bsoncore.Value{Type: bsontype.Int64, Data: bsoncore.AppendInt64(nil, bc.id)})
return dst, nil
},
Database: bc.database,
Deployment: bc.getOperationDeployment(),
Client: bc.clientSession,
Clock: bc.clock,
Legacy: LegacyKillCursors,
CommandMonitor: bc.cmdMonitor,
ServerAPI: bc.serverAPI,
}.Execute(ctx)
}
// calcGetMoreBatchSize calculates the number of documents to return in the
// response of a "getMore" operation based on the given limit, batchSize, and
// number of documents already returned. Returns false if a non-trivial limit is
// lower than or equal to the number of documents already returned.
func calcGetMoreBatchSize(bc BatchCursor) (int32, bool) {
gmBatchSize := bc.batchSize
// Account for legacy operations that don't support setting a limit.
if bc.limit != 0 && bc.numReturned+bc.batchSize >= bc.limit {
gmBatchSize = bc.limit - bc.numReturned
if gmBatchSize <= 0 {
return gmBatchSize, false
}
}
return gmBatchSize, true
}
func (bc *BatchCursor) getMore(ctx context.Context) {
bc.clearBatch()
if bc.id == 0 {
return
}
numToReturn, ok := calcGetMoreBatchSize(*bc)
if !ok {
if err := bc.Close(ctx); err != nil {
bc.err = err
}
return
}
bc.err = Operation{
CommandFn: func(dst []byte, desc description.SelectedServer) ([]byte, error) {
dst = bsoncore.AppendInt64Element(dst, "getMore", bc.id)
dst = bsoncore.AppendStringElement(dst, "collection", bc.collection)
if numToReturn > 0 {
dst = bsoncore.AppendInt32Element(dst, "batchSize", numToReturn)
}
if bc.maxTimeMS > 0 {
dst = bsoncore.AppendInt64Element(dst, "maxTimeMS", bc.maxTimeMS)
}
// The getMore command does not support commenting pre-4.4.
if bc.comment.Type != bsontype.Type(0) && bc.serverDescription.WireVersion.Max >= 9 {
dst = bsoncore.AppendValueElement(dst, "comment", bc.comment)
}
return dst, nil
},
Database: bc.database,
Deployment: bc.getOperationDeployment(),
ProcessResponseFn: func(info ResponseInfo) error {
response := info.ServerResponse
id, ok := response.Lookup("cursor", "id").Int64OK()
if !ok {
return fmt.Errorf("cursor.id should be an int64 but is a BSON %s", response.Lookup("cursor", "id").Type)
}
bc.id = id
batch, ok := response.Lookup("cursor", "nextBatch").ArrayOK()
if !ok {
return fmt.Errorf("cursor.nextBatch should be an array but is a BSON %s", response.Lookup("cursor", "nextBatch").Type)
}
bc.currentBatch.Style = bsoncore.ArrayStyle
bc.currentBatch.Data = batch
bc.currentBatch.ResetIterator()
bc.numReturned += int32(bc.currentBatch.DocumentCount()) // Required for legacy operations which don't support limit.
pbrt, err := response.LookupErr("cursor", "postBatchResumeToken")
if err != nil {
// I don't really understand why we don't set bc.err here
return nil
}
pbrtDoc, ok := pbrt.DocumentOK()
if !ok {
bc.err = fmt.Errorf("expected BSON type for post batch resume token to be EmbeddedDocument but got %s", pbrt.Type)
return nil
}
bc.postBatchResumeToken = pbrtDoc
return nil
},
Client: bc.clientSession,
Clock: bc.clock,
Legacy: LegacyGetMore,
CommandMonitor: bc.cmdMonitor,
Crypt: bc.crypt,
ServerAPI: bc.serverAPI,
}.Execute(ctx)
// Once the cursor has been drained, we can unpin the connection if one is currently pinned.
if bc.id == 0 {
err := bc.unpinConnection()
if err != nil && bc.err == nil {
bc.err = err
}
}
// If we're in load balanced mode and the pinned connection encounters a network error, we should not use it for
// future commands. Per the spec, the connection will not be unpinned until the cursor is actually closed, but
// we set the cursor ID to 0 to ensure the Close() call will not execute a killCursors command.
if driverErr, ok := bc.err.(Error); ok && driverErr.NetworkError() && bc.connection != nil {
bc.id = 0
}
// Required for legacy operations which don't support limit.
if bc.limit != 0 && bc.numReturned >= bc.limit {
// call KillCursor instead of Close because Close will clear out the data for the current batch.
err := bc.KillCursor(ctx)
if err != nil && bc.err == nil {
bc.err = err
}
}
}
// PostBatchResumeToken returns the latest seen post batch resume token.
func (bc *BatchCursor) PostBatchResumeToken() bsoncore.Document {
return bc.postBatchResumeToken
}
// SetBatchSize sets the batchSize for future getMores.
func (bc *BatchCursor) SetBatchSize(size int32) {
bc.batchSize = size
}
func (bc *BatchCursor) getOperationDeployment() Deployment {
if bc.connection != nil {
return &loadBalancedCursorDeployment{
errorProcessor: bc.errorProcessor,
conn: bc.connection,
}
}
return SingleServerDeployment{bc.server}
}
// loadBalancedCursorDeployment is used as a Deployment for getMore and killCursors commands when pinning to a
// connection in load balanced mode. This type also functions as an ErrorProcessor to ensure that SDAM errors are
// handled for these commands in this mode.
type loadBalancedCursorDeployment struct {
errorProcessor ErrorProcessor
conn PinnedConnection
}
var _ Deployment = (*loadBalancedCursorDeployment)(nil)
var _ Server = (*loadBalancedCursorDeployment)(nil)
var _ ErrorProcessor = (*loadBalancedCursorDeployment)(nil)
func (lbcd *loadBalancedCursorDeployment) SelectServer(_ context.Context, _ description.ServerSelector) (Server, error) {
return lbcd, nil
}
func (lbcd *loadBalancedCursorDeployment) Kind() description.TopologyKind {
return description.LoadBalanced
}
func (lbcd *loadBalancedCursorDeployment) Connection(_ context.Context) (Connection, error) {
return lbcd.conn, nil
}
// RTTMonitor implements the driver.Server interface.
func (lbcd *loadBalancedCursorDeployment) RTTMonitor() RTTMonitor {
return &internal.ZeroRTTMonitor{}
}
func (lbcd *loadBalancedCursorDeployment) ProcessError(err error, conn Connection) ProcessErrorResult {
return lbcd.errorProcessor.ProcessError(err, conn)
}