File: gridfs.go

package info (click to toggle)
golang-gopkg-mgo.v2 2016.08.01-5
  • links: PTS, VCS
  • area: main
  • in suites: bullseye, buster, sid, stretch-backports
  • size: 1,952 kB
  • sloc: ansic: 300; sh: 207; makefile: 7
file content (761 lines) | stat: -rw-r--r-- 21,207 bytes parent folder | download | duplicates (5)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
// 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 (
	"crypto/md5"
	"encoding/hex"
	"errors"
	"hash"
	"io"
	"os"
	"sync"
	"time"

	"gopkg.in/mgo.v2/bson"
)

type GridFS struct {
	Files  *Collection
	Chunks *Collection
}

type gfsFileMode int

const (
	gfsClosed  gfsFileMode = 0
	gfsReading gfsFileMode = 1
	gfsWriting gfsFileMode = 2
)

type GridFile struct {
	m    sync.Mutex
	c    sync.Cond
	gfs  *GridFS
	mode gfsFileMode
	err  error

	chunk  int
	offset int64

	wpending int
	wbuf     []byte
	wsum     hash.Hash

	rbuf   []byte
	rcache *gfsCachedChunk

	doc gfsFile
}

type gfsFile struct {
	Id          interface{} "_id"
	ChunkSize   int         "chunkSize"
	UploadDate  time.Time   "uploadDate"
	Length      int64       ",minsize"
	MD5         string
	Filename    string    ",omitempty"
	ContentType string    "contentType,omitempty"
	Metadata    *bson.Raw ",omitempty"
}

type gfsChunk struct {
	Id      interface{} "_id"
	FilesId interface{} "files_id"
	N       int
	Data    []byte
}

type gfsCachedChunk struct {
	wait sync.Mutex
	n    int
	data []byte
	err  error
}

func newGridFS(db *Database, prefix string) *GridFS {
	return &GridFS{db.C(prefix + ".files"), db.C(prefix + ".chunks")}
}

func (gfs *GridFS) newFile() *GridFile {
	file := &GridFile{gfs: gfs}
	file.c.L = &file.m
	//runtime.SetFinalizer(file, finalizeFile)
	return file
}

func finalizeFile(file *GridFile) {
	file.Close()
}

// Create creates a new file with the provided name in the GridFS.  If the file
// name already exists, a new version will be inserted with an up-to-date
// uploadDate that will cause it to be atomically visible to the Open and
// OpenId methods.  If the file name is not important, an empty name may be
// provided and the file Id used instead.
//
// It's important to Close files whether they are being written to
// or read from, and to check the err result to ensure the operation
// completed successfully.
//
// A simple example inserting a new file:
//
//     func check(err error) {
//         if err != nil {
//             panic(err.String())
//         }
//     }
//     file, err := db.GridFS("fs").Create("myfile.txt")
//     check(err)
//     n, err := file.Write([]byte("Hello world!"))
//     check(err)
//     err = file.Close()
//     check(err)
//     fmt.Printf("%d bytes written\n", n)
//
// The io.Writer interface is implemented by *GridFile and may be used to
// help on the file creation.  For example:
//
//     file, err := db.GridFS("fs").Create("myfile.txt")
//     check(err)
//     messages, err := os.Open("/var/log/messages")
//     check(err)
//     defer messages.Close()
//     err = io.Copy(file, messages)
//     check(err)
//     err = file.Close()
//     check(err)
//
func (gfs *GridFS) Create(name string) (file *GridFile, err error) {
	file = gfs.newFile()
	file.mode = gfsWriting
	file.wsum = md5.New()
	file.doc = gfsFile{Id: bson.NewObjectId(), ChunkSize: 255 * 1024, Filename: name}
	return
}

// OpenId returns the file with the provided id, for reading.
// If the file isn't found, err will be set to mgo.ErrNotFound.
//
// It's important to Close files whether they are being written to
// or read from, and to check the err result to ensure the operation
// completed successfully.
//
// The following example will print the first 8192 bytes from the file:
//
//     func check(err error) {
//         if err != nil {
//             panic(err.String())
//         }
//     }
//     file, err := db.GridFS("fs").OpenId(objid)
//     check(err)
//     b := make([]byte, 8192)
//     n, err := file.Read(b)
//     check(err)
//     fmt.Println(string(b))
//     check(err)
//     err = file.Close()
//     check(err)
//     fmt.Printf("%d bytes read\n", n)
//
// The io.Reader interface is implemented by *GridFile and may be used to
// deal with it.  As an example, the following snippet will dump the whole
// file into the standard output:
//
//     file, err := db.GridFS("fs").OpenId(objid)
//     check(err)
//     err = io.Copy(os.Stdout, file)
//     check(err)
//     err = file.Close()
//     check(err)
//
func (gfs *GridFS) OpenId(id interface{}) (file *GridFile, err error) {
	var doc gfsFile
	err = gfs.Files.Find(bson.M{"_id": id}).One(&doc)
	if err != nil {
		return
	}
	file = gfs.newFile()
	file.mode = gfsReading
	file.doc = doc
	return
}

// Open returns the most recently uploaded file with the provided
// name, for reading. If the file isn't found, err will be set
// to mgo.ErrNotFound.
//
// It's important to Close files whether they are being written to
// or read from, and to check the err result to ensure the operation
// completed successfully.
//
// The following example will print the first 8192 bytes from the file:
//
//     file, err := db.GridFS("fs").Open("myfile.txt")
//     check(err)
//     b := make([]byte, 8192)
//     n, err := file.Read(b)
//     check(err)
//     fmt.Println(string(b))
//     check(err)
//     err = file.Close()
//     check(err)
//     fmt.Printf("%d bytes read\n", n)
//
// The io.Reader interface is implemented by *GridFile and may be used to
// deal with it.  As an example, the following snippet will dump the whole
// file into the standard output:
//
//     file, err := db.GridFS("fs").Open("myfile.txt")
//     check(err)
//     err = io.Copy(os.Stdout, file)
//     check(err)
//     err = file.Close()
//     check(err)
//
func (gfs *GridFS) Open(name string) (file *GridFile, err error) {
	var doc gfsFile
	err = gfs.Files.Find(bson.M{"filename": name}).Sort("-uploadDate").One(&doc)
	if err != nil {
		return
	}
	file = gfs.newFile()
	file.mode = gfsReading
	file.doc = doc
	return
}

// OpenNext opens the next file from iter for reading, sets *file to it,
// and returns true on the success case. If no more documents are available
// on iter or an error occurred, *file is set to nil and the result is false.
// Errors will be available via iter.Err().
//
// The iter parameter must be an iterator on the GridFS files collection.
// Using the GridFS.Find method is an easy way to obtain such an iterator,
// but any iterator on the collection will work.
//
// If the provided *file is non-nil, OpenNext will close it before attempting
// to iterate to the next element. This means that in a loop one only
// has to worry about closing files when breaking out of the loop early
// (break, return, or panic).
//
// For example:
//
//     gfs := db.GridFS("fs")
//     query := gfs.Find(nil).Sort("filename")
//     iter := query.Iter()
//     var f *mgo.GridFile
//     for gfs.OpenNext(iter, &f) {
//         fmt.Printf("Filename: %s\n", f.Name())
//     }
//     if iter.Close() != nil {
//         panic(iter.Close())
//     }
//
func (gfs *GridFS) OpenNext(iter *Iter, file **GridFile) bool {
	if *file != nil {
		// Ignoring the error here shouldn't be a big deal
		// as we're reading the file and the loop iteration
		// for this file is finished.
		_ = (*file).Close()
	}
	var doc gfsFile
	if !iter.Next(&doc) {
		*file = nil
		return false
	}
	f := gfs.newFile()
	f.mode = gfsReading
	f.doc = doc
	*file = f
	return true
}

// Find runs query on GridFS's files collection and returns
// the resulting Query.
//
// This logic:
//
//     gfs := db.GridFS("fs")
//     iter := gfs.Find(nil).Iter()
//
// Is equivalent to:
//
//     files := db.C("fs" + ".files")
//     iter := files.Find(nil).Iter()
//
func (gfs *GridFS) Find(query interface{}) *Query {
	return gfs.Files.Find(query)
}

// RemoveId deletes the file with the provided id from the GridFS.
func (gfs *GridFS) RemoveId(id interface{}) error {
	err := gfs.Files.Remove(bson.M{"_id": id})
	if err != nil {
		return err
	}
	_, err = gfs.Chunks.RemoveAll(bson.D{{"files_id", id}})
	return err
}

type gfsDocId struct {
	Id interface{} "_id"
}

// Remove deletes all files with the provided name from the GridFS.
func (gfs *GridFS) Remove(name string) (err error) {
	iter := gfs.Files.Find(bson.M{"filename": name}).Select(bson.M{"_id": 1}).Iter()
	var doc gfsDocId
	for iter.Next(&doc) {
		if e := gfs.RemoveId(doc.Id); e != nil {
			err = e
		}
	}
	if err == nil {
		err = iter.Close()
	}
	return err
}

func (file *GridFile) assertMode(mode gfsFileMode) {
	switch file.mode {
	case mode:
		return
	case gfsWriting:
		panic("GridFile is open for writing")
	case gfsReading:
		panic("GridFile is open for reading")
	case gfsClosed:
		panic("GridFile is closed")
	default:
		panic("internal error: missing GridFile mode")
	}
}

// SetChunkSize sets size of saved chunks.  Once the file is written to, it
// will be split in blocks of that size and each block saved into an
// independent chunk document.  The default chunk size is 255kb.
//
// It is a runtime error to call this function once the file has started
// being written to.
func (file *GridFile) SetChunkSize(bytes int) {
	file.assertMode(gfsWriting)
	debugf("GridFile %p: setting chunk size to %d", file, bytes)
	file.m.Lock()
	file.doc.ChunkSize = bytes
	file.m.Unlock()
}

// Id returns the current file Id.
func (file *GridFile) Id() interface{} {
	return file.doc.Id
}

// SetId changes the current file Id.
//
// It is a runtime error to call this function once the file has started
// being written to, or when the file is not open for writing.
func (file *GridFile) SetId(id interface{}) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.Id = id
	file.m.Unlock()
}

// Name returns the optional file name.  An empty string will be returned
// in case it is unset.
func (file *GridFile) Name() string {
	return file.doc.Filename
}

// SetName changes the optional file name.  An empty string may be used to
// unset it.
//
// It is a runtime error to call this function when the file is not open
// for writing.
func (file *GridFile) SetName(name string) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.Filename = name
	file.m.Unlock()
}

// ContentType returns the optional file content type.  An empty string will be
// returned in case it is unset.
func (file *GridFile) ContentType() string {
	return file.doc.ContentType
}

// ContentType changes the optional file content type.  An empty string may be
// used to unset it.
//
// It is a runtime error to call this function when the file is not open
// for writing.
func (file *GridFile) SetContentType(ctype string) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.ContentType = ctype
	file.m.Unlock()
}

// GetMeta unmarshals the optional "metadata" field associated with the
// file into the result parameter. The meaning of keys under that field
// is user-defined. For example:
//
//     result := struct{ INode int }{}
//     err = file.GetMeta(&result)
//     if err != nil {
//         panic(err.String())
//     }
//     fmt.Printf("inode: %d\n", result.INode)
//
func (file *GridFile) GetMeta(result interface{}) (err error) {
	file.m.Lock()
	if file.doc.Metadata != nil {
		err = bson.Unmarshal(file.doc.Metadata.Data, result)
	}
	file.m.Unlock()
	return
}

// SetMeta changes the optional "metadata" field associated with the
// file. The meaning of keys under that field is user-defined.
// For example:
//
//     file.SetMeta(bson.M{"inode": inode})
//
// It is a runtime error to call this function when the file is not open
// for writing.
func (file *GridFile) SetMeta(metadata interface{}) {
	file.assertMode(gfsWriting)
	data, err := bson.Marshal(metadata)
	file.m.Lock()
	if err != nil && file.err == nil {
		file.err = err
	} else {
		file.doc.Metadata = &bson.Raw{Data: data}
	}
	file.m.Unlock()
}

// Size returns the file size in bytes.
func (file *GridFile) Size() (bytes int64) {
	file.m.Lock()
	bytes = file.doc.Length
	file.m.Unlock()
	return
}

// MD5 returns the file MD5 as a hex-encoded string.
func (file *GridFile) MD5() (md5 string) {
	return file.doc.MD5
}

// UploadDate returns the file upload time.
func (file *GridFile) UploadDate() time.Time {
	return file.doc.UploadDate
}

// SetUploadDate changes the file upload time.
//
// It is a runtime error to call this function when the file is not open
// for writing.
func (file *GridFile) SetUploadDate(t time.Time) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	file.doc.UploadDate = t
	file.m.Unlock()
}

// Close flushes any pending changes in case the file is being written
// to, waits for any background operations to finish, and closes the file.
//
// It's important to Close files whether they are being written to
// or read from, and to check the err result to ensure the operation
// completed successfully.
func (file *GridFile) Close() (err error) {
	file.m.Lock()
	defer file.m.Unlock()
	if file.mode == gfsWriting {
		if len(file.wbuf) > 0 && file.err == nil {
			file.insertChunk(file.wbuf)
			file.wbuf = file.wbuf[0:0]
		}
		file.completeWrite()
	} else if file.mode == gfsReading && file.rcache != nil {
		file.rcache.wait.Lock()
		file.rcache = nil
	}
	file.mode = gfsClosed
	debugf("GridFile %p: closed", file)
	return file.err
}

func (file *GridFile) completeWrite() {
	for file.wpending > 0 {
		debugf("GridFile %p: waiting for %d pending chunks to complete file write", file, file.wpending)
		file.c.Wait()
	}
	if file.err == nil {
		hexsum := hex.EncodeToString(file.wsum.Sum(nil))
		if file.doc.UploadDate.IsZero() {
			file.doc.UploadDate = bson.Now()
		}
		file.doc.MD5 = hexsum
		file.err = file.gfs.Files.Insert(file.doc)
	}
	if file.err != nil {
		file.gfs.Chunks.RemoveAll(bson.D{{"files_id", file.doc.Id}})
	}
	if file.err == nil {
		index := Index{
			Key:    []string{"files_id", "n"},
			Unique: true,
		}
		file.err = file.gfs.Chunks.EnsureIndex(index)
	}
}

// Abort cancels an in-progress write, preventing the file from being
// automically created and ensuring previously written chunks are
// removed when the file is closed.
//
// It is a runtime error to call Abort when the file was not opened
// for writing.
func (file *GridFile) Abort() {
	if file.mode != gfsWriting {
		panic("file.Abort must be called on file opened for writing")
	}
	file.err = errors.New("write aborted")
}

// Write writes the provided data to the file and returns the
// number of bytes written and an error in case something
// wrong happened.
//
// The file will internally cache the data so that all but the last
// chunk sent to the database have the size defined by SetChunkSize.
// This also means that errors may be deferred until a future call
// to Write or Close.
//
// The parameters and behavior of this function turn the file
// into an io.Writer.
func (file *GridFile) Write(data []byte) (n int, err error) {
	file.assertMode(gfsWriting)
	file.m.Lock()
	debugf("GridFile %p: writing %d bytes", file, len(data))
	defer file.m.Unlock()

	if file.err != nil {
		return 0, file.err
	}

	n = len(data)
	file.doc.Length += int64(n)
	chunkSize := file.doc.ChunkSize

	if len(file.wbuf)+len(data) < chunkSize {
		file.wbuf = append(file.wbuf, data...)
		return
	}

	// First, flush file.wbuf complementing with data.
	if len(file.wbuf) > 0 {
		missing := chunkSize - len(file.wbuf)
		if missing > len(data) {
			missing = len(data)
		}
		file.wbuf = append(file.wbuf, data[:missing]...)
		data = data[missing:]
		file.insertChunk(file.wbuf)
		file.wbuf = file.wbuf[0:0]
	}

	// Then, flush all chunks from data without copying.
	for len(data) > chunkSize {
		size := chunkSize
		if size > len(data) {
			size = len(data)
		}
		file.insertChunk(data[:size])
		data = data[size:]
	}

	// And append the rest for a future call.
	file.wbuf = append(file.wbuf, data...)

	return n, file.err
}

func (file *GridFile) insertChunk(data []byte) {
	n := file.chunk
	file.chunk++
	debugf("GridFile %p: adding to checksum: %q", file, string(data))
	file.wsum.Write(data)

	for file.doc.ChunkSize*file.wpending >= 1024*1024 {
		// Hold on.. we got a MB pending.
		file.c.Wait()
		if file.err != nil {
			return
		}
	}

	file.wpending++

	debugf("GridFile %p: inserting chunk %d with %d bytes", file, n, len(data))

	// We may not own the memory of data, so rather than
	// simply copying it, we'll marshal the document ahead of time.
	data, err := bson.Marshal(gfsChunk{bson.NewObjectId(), file.doc.Id, n, data})
	if err != nil {
		file.err = err
		return
	}

	go func() {
		err := file.gfs.Chunks.Insert(bson.Raw{Data: data})
		file.m.Lock()
		file.wpending--
		if err != nil && file.err == nil {
			file.err = err
		}
		file.c.Broadcast()
		file.m.Unlock()
	}()
}

// Seek sets the offset for the next Read or Write on file to
// offset, interpreted according to whence: 0 means relative to
// the origin of the file, 1 means relative to the current offset,
// and 2 means relative to the end. It returns the new offset and
// an error, if any.
func (file *GridFile) Seek(offset int64, whence int) (pos int64, err error) {
	file.m.Lock()
	debugf("GridFile %p: seeking for %s (whence=%d)", file, offset, whence)
	defer file.m.Unlock()
	switch whence {
	case os.SEEK_SET:
	case os.SEEK_CUR:
		offset += file.offset
	case os.SEEK_END:
		offset += file.doc.Length
	default:
		panic("unsupported whence value")
	}
	if offset > file.doc.Length {
		return file.offset, errors.New("seek past end of file")
	}
	if offset == file.doc.Length {
		// If we're seeking to the end of the file,
		// no need to read anything. This enables
		// a client to find the size of the file using only the
		// io.ReadSeeker interface with low overhead.
		file.offset = offset
		return file.offset, nil
	}
	chunk := int(offset / int64(file.doc.ChunkSize))
	if chunk+1 == file.chunk && offset >= file.offset {
		file.rbuf = file.rbuf[int(offset-file.offset):]
		file.offset = offset
		return file.offset, nil
	}
	file.offset = offset
	file.chunk = chunk
	file.rbuf = nil
	file.rbuf, err = file.getChunk()
	if err == nil {
		file.rbuf = file.rbuf[int(file.offset-int64(chunk)*int64(file.doc.ChunkSize)):]
	}
	return file.offset, err
}

// Read reads into b the next available data from the file and
// returns the number of bytes written and an error in case
// something wrong happened.  At the end of the file, n will
// be zero and err will be set to io.EOF.
//
// The parameters and behavior of this function turn the file
// into an io.Reader.
func (file *GridFile) Read(b []byte) (n int, err error) {
	file.assertMode(gfsReading)
	file.m.Lock()
	debugf("GridFile %p: reading at offset %d into buffer of length %d", file, file.offset, len(b))
	defer file.m.Unlock()
	if file.offset == file.doc.Length {
		return 0, io.EOF
	}
	for err == nil {
		i := copy(b, file.rbuf)
		n += i
		file.offset += int64(i)
		file.rbuf = file.rbuf[i:]
		if i == len(b) || file.offset == file.doc.Length {
			break
		}
		b = b[i:]
		file.rbuf, err = file.getChunk()
	}
	return n, err
}

func (file *GridFile) getChunk() (data []byte, err error) {
	cache := file.rcache
	file.rcache = nil
	if cache != nil && cache.n == file.chunk {
		debugf("GridFile %p: Getting chunk %d from cache", file, file.chunk)
		cache.wait.Lock()
		data, err = cache.data, cache.err
	} else {
		debugf("GridFile %p: Fetching chunk %d", file, file.chunk)
		var doc gfsChunk
		err = file.gfs.Chunks.Find(bson.D{{"files_id", file.doc.Id}, {"n", file.chunk}}).One(&doc)
		data = doc.Data
	}
	file.chunk++
	if int64(file.chunk)*int64(file.doc.ChunkSize) < file.doc.Length {
		// Read the next one in background.
		cache = &gfsCachedChunk{n: file.chunk}
		cache.wait.Lock()
		debugf("GridFile %p: Scheduling chunk %d for background caching", file, file.chunk)
		// Clone the session to avoid having it closed in between.
		chunks := file.gfs.Chunks
		session := chunks.Database.Session.Clone()
		go func(id interface{}, n int) {
			defer session.Close()
			chunks = chunks.With(session)
			var doc gfsChunk
			cache.err = chunks.Find(bson.D{{"files_id", id}, {"n", n}}).One(&doc)
			cache.data = doc.Data
			cache.wait.Unlock()
		}(file.doc.Id, file.chunk)
		file.rcache = cache
	}
	debugf("Returning err: %#v", err)
	return
}