/
cursor.go
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/
cursor.go
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// Copyright (C) MongoDB, Inc. 2017-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 mongo
import (
"context"
"errors"
"fmt"
"io"
"reflect"
"go.mongodb.org/mongo-driver/bson"
"go.mongodb.org/mongo-driver/bson/bsoncodec"
"go.mongodb.org/mongo-driver/bson/bsonrw"
"go.mongodb.org/mongo-driver/mongo/options"
"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
"go.mongodb.org/mongo-driver/x/mongo/driver"
"go.mongodb.org/mongo-driver/x/mongo/driver/session"
)
// Cursor is used to iterate over a stream of documents. Each document can be decoded into a Go type via the Decode
// method or accessed as raw BSON via the Current field. This type is not goroutine safe and must not be used
// concurrently by multiple goroutines.
type Cursor struct {
// Current contains the BSON bytes of the current change document. This property is only valid until the next call
// to Next or TryNext. If continued access is required, a copy must be made.
Current bson.Raw
bc batchCursor
batch *bsoncore.DocumentSequence
batchLength int
bsonOpts *options.BSONOptions
registry *bsoncodec.Registry
clientSession *session.Client
err error
}
func newCursor(
bc batchCursor,
bsonOpts *options.BSONOptions,
registry *bsoncodec.Registry,
) (*Cursor, error) {
return newCursorWithSession(bc, bsonOpts, registry, nil)
}
func newCursorWithSession(
bc batchCursor,
bsonOpts *options.BSONOptions,
registry *bsoncodec.Registry,
clientSession *session.Client,
) (*Cursor, error) {
if registry == nil {
registry = bson.DefaultRegistry
}
if bc == nil {
return nil, errors.New("batch cursor must not be nil")
}
c := &Cursor{
bc: bc,
bsonOpts: bsonOpts,
registry: registry,
clientSession: clientSession,
}
if bc.ID() == 0 {
c.closeImplicitSession()
}
// Initialize just the batchLength here so RemainingBatchLength will return an accurate result. The actual batch
// will be pulled up by the first Next/TryNext call.
c.batchLength = c.bc.Batch().DocumentCount()
return c, nil
}
func newEmptyCursor() *Cursor {
return &Cursor{bc: driver.NewEmptyBatchCursor()}
}
// NewCursorFromDocuments creates a new Cursor pre-loaded with the provided documents, error and registry. If no registry is provided,
// bson.DefaultRegistry will be used.
//
// The documents parameter must be a slice of documents. The slice may be nil or empty, but all elements must be non-nil.
func NewCursorFromDocuments(documents []interface{}, err error, registry *bsoncodec.Registry) (*Cursor, error) {
if registry == nil {
registry = bson.DefaultRegistry
}
// Convert documents slice to a sequence-style byte array.
var docsBytes []byte
for _, doc := range documents {
switch t := doc.(type) {
case nil:
return nil, ErrNilDocument
case []byte:
// Slight optimization so we'll just use MarshalBSON and not go through the codec machinery.
doc = bson.Raw(t)
}
var marshalErr error
docsBytes, marshalErr = bson.MarshalAppendWithRegistry(registry, docsBytes, doc)
if marshalErr != nil {
return nil, marshalErr
}
}
c := &Cursor{
bc: driver.NewBatchCursorFromDocuments(docsBytes),
registry: registry,
err: err,
}
// Initialize batch and batchLength here. The underlying batch cursor will be preloaded with the
// provided contents, and thus already has a batch before calls to Next/TryNext.
c.batch = c.bc.Batch()
c.batchLength = c.bc.Batch().DocumentCount()
return c, nil
}
// ID returns the ID of this cursor, or 0 if the cursor has been closed or exhausted.
func (c *Cursor) ID() int64 { return c.bc.ID() }
// Next gets the next document for this cursor. It returns true if there were no errors and the cursor has not been
// exhausted.
//
// Next blocks until a document is available or an error occurs. If the context expires, the cursor's error will
// be set to ctx.Err(). In case of an error, Next will return false.
//
// If Next returns false, subsequent calls will also return false.
func (c *Cursor) Next(ctx context.Context) bool {
return c.next(ctx, false)
}
// TryNext attempts to get the next document for this cursor. It returns true if there were no errors and the next
// document is available. This is only recommended for use with tailable cursors as a non-blocking alternative to
// Next. See https://www.mongodb.com/docs/manual/core/tailable-cursors/ for more information about tailable cursors.
//
// TryNext returns false if the cursor is exhausted, an error occurs when getting results from the server, the next
// document is not yet available, or ctx expires. If the context expires, the cursor's error will be set to ctx.Err().
//
// If TryNext returns false and an error occurred or the cursor has been exhausted (i.e. c.Err() != nil || c.ID() == 0),
// subsequent attempts will also return false. Otherwise, it is safe to call TryNext again until a document is
// available.
//
// This method requires driver version >= 1.2.0.
func (c *Cursor) TryNext(ctx context.Context) bool {
return c.next(ctx, true)
}
func (c *Cursor) next(ctx context.Context, nonBlocking bool) bool {
// return false right away if the cursor has already errored.
if c.err != nil {
return false
}
if ctx == nil {
ctx = context.Background()
}
doc, err := c.batch.Next()
switch err {
case nil:
// Consume the next document in the current batch.
c.batchLength--
c.Current = bson.Raw(doc)
return true
case io.EOF: // Need to do a getMore
default:
c.err = err
return false
}
// call the Next method in a loop until at least one document is returned in the next batch or
// the context times out.
for {
// If we don't have a next batch
if !c.bc.Next(ctx) {
// Do we have an error? If so we return false.
c.err = replaceErrors(c.bc.Err())
if c.err != nil {
return false
}
// Is the cursor ID zero?
if c.bc.ID() == 0 {
c.closeImplicitSession()
return false
}
// empty batch, but cursor is still valid.
// use nonBlocking to determine if we should continue or return control to the caller.
if nonBlocking {
return false
}
continue
}
// close the implicit session if this was the last getMore
if c.bc.ID() == 0 {
c.closeImplicitSession()
}
// Use the new batch to update the batch and batchLength fields. Consume the first document in the batch.
c.batch = c.bc.Batch()
c.batchLength = c.batch.DocumentCount()
doc, err = c.batch.Next()
switch err {
case nil:
c.batchLength--
c.Current = bson.Raw(doc)
return true
case io.EOF: // Empty batch so we continue
default:
c.err = err
return false
}
}
}
func getDecoder(
data []byte,
opts *options.BSONOptions,
reg *bsoncodec.Registry,
) (*bson.Decoder, error) {
dec, err := bson.NewDecoder(bsonrw.NewBSONDocumentReader(data))
if err != nil {
return nil, err
}
if opts != nil {
if opts.AllowTruncatingDoubles {
dec.AllowTruncatingDoubles()
}
if opts.BinaryAsSlice {
dec.BinaryAsSlice()
}
if opts.DefaultDocumentD {
dec.DefaultDocumentD()
}
if opts.DefaultDocumentM {
dec.DefaultDocumentM()
}
if opts.UseJSONStructTags {
dec.UseJSONStructTags()
}
if opts.UseLocalTimeZone {
dec.UseLocalTimeZone()
}
if opts.ZeroMaps {
dec.ZeroMaps()
}
if opts.ZeroStructs {
dec.ZeroStructs()
}
}
if reg != nil {
// TODO:(GODRIVER-2719): Remove error handling.
if err := dec.SetRegistry(reg); err != nil {
return nil, err
}
}
return dec, nil
}
// Decode will unmarshal the current document into val and return any errors from the unmarshalling process without any
// modification. If val is nil or is a typed nil, an error will be returned.
func (c *Cursor) Decode(val interface{}) error {
dec, err := getDecoder(c.Current, c.bsonOpts, c.registry)
if err != nil {
return fmt.Errorf("error configuring BSON decoder: %w", err)
}
return dec.Decode(val)
}
// Err returns the last error seen by the Cursor, or nil if no error has occurred.
func (c *Cursor) Err() error { return c.err }
// Close closes this cursor. Next and TryNext must not be called after Close has been called. Close is idempotent. After
// the first call, any subsequent calls will not change the state.
func (c *Cursor) Close(ctx context.Context) error {
defer c.closeImplicitSession()
return replaceErrors(c.bc.Close(ctx))
}
// All iterates the cursor and decodes each document into results. The results parameter must be a pointer to a slice.
// The slice pointed to by results will be completely overwritten. This method will close the cursor after retrieving
// all documents. If the cursor has been iterated, any previously iterated documents will not be included in results.
//
// This method requires driver version >= 1.1.0.
func (c *Cursor) All(ctx context.Context, results interface{}) error {
resultsVal := reflect.ValueOf(results)
if resultsVal.Kind() != reflect.Ptr {
return fmt.Errorf("results argument must be a pointer to a slice, but was a %s", resultsVal.Kind())
}
sliceVal := resultsVal.Elem()
if sliceVal.Kind() == reflect.Interface {
sliceVal = sliceVal.Elem()
}
if sliceVal.Kind() != reflect.Slice {
return fmt.Errorf("results argument must be a pointer to a slice, but was a pointer to %s", sliceVal.Kind())
}
elementType := sliceVal.Type().Elem()
var index int
var err error
// Defer a call to Close to try to clean up the cursor server-side when all
// documents have not been exhausted. Use context.Background() to ensure Close
// completes even if the context passed to All has errored.
defer c.Close(context.Background())
batch := c.batch // exhaust the current batch before iterating the batch cursor
for {
sliceVal, index, err = c.addFromBatch(sliceVal, elementType, batch, index)
if err != nil {
return err
}
if !c.bc.Next(ctx) {
break
}
batch = c.bc.Batch()
}
if err = replaceErrors(c.bc.Err()); err != nil {
return err
}
resultsVal.Elem().Set(sliceVal.Slice(0, index))
return nil
}
// RemainingBatchLength returns the number of documents left in the current batch. If this returns zero, the subsequent
// call to Next or TryNext will do a network request to fetch the next batch.
func (c *Cursor) RemainingBatchLength() int {
return c.batchLength
}
// addFromBatch adds all documents from batch to sliceVal starting at the given index. It returns the new slice value,
// the next empty index in the slice, and an error if one occurs.
func (c *Cursor) addFromBatch(sliceVal reflect.Value, elemType reflect.Type, batch *bsoncore.DocumentSequence,
index int) (reflect.Value, int, error) {
docs, err := batch.Documents()
if err != nil {
return sliceVal, index, err
}
for _, doc := range docs {
if sliceVal.Len() == index {
// slice is full
newElem := reflect.New(elemType)
sliceVal = reflect.Append(sliceVal, newElem.Elem())
sliceVal = sliceVal.Slice(0, sliceVal.Cap())
}
currElem := sliceVal.Index(index).Addr().Interface()
dec, err := getDecoder(doc, c.bsonOpts, c.registry)
if err != nil {
return sliceVal, index, fmt.Errorf("error configuring BSON decoder: %w", err)
}
err = dec.Decode(currElem)
if err != nil {
return sliceVal, index, err
}
index++
}
return sliceVal, index, nil
}
func (c *Cursor) closeImplicitSession() {
if c.clientSession != nil && c.clientSession.IsImplicit {
c.clientSession.EndSession()
}
}
// SetBatchSize sets the number of documents to fetch from the database with
// each iteration of the cursor's "Next" method. Note that some operations set
// an initial cursor batch size, so this setting only affects subsequent
// document batches fetched from the database.
func (c *Cursor) SetBatchSize(batchSize int32) {
c.bc.SetBatchSize(batchSize)
}
// BatchCursorFromCursor returns a driver.BatchCursor for the given Cursor. If there is no underlying
// driver.BatchCursor, nil is returned.
//
// Deprecated: This is an unstable function because the driver.BatchCursor type exists in the "x" package. Neither this
// function nor the driver.BatchCursor type should be used by applications and may be changed or removed in any release.
func BatchCursorFromCursor(c *Cursor) *driver.BatchCursor {
bc, _ := c.bc.(*driver.BatchCursor)
return bc
}