grid_file.py 29.8 KB
Newer Older
convivae committed
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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842
# Copyright 2009-present MongoDB, 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.

"""Tools for representing files stored in GridFS."""
import datetime
import hashlib
import io
import math
import os

from bson.int64 import Int64
from bson.son import SON
from bson.binary import Binary
from bson.objectid import ObjectId
from bson.py3compat import text_type, StringIO
from gridfs.errors import CorruptGridFile, FileExists, NoFile
from pymongo import ASCENDING
from pymongo.collection import Collection
from pymongo.cursor import Cursor
from pymongo.errors import (ConfigurationError,
                            CursorNotFound,
                            DuplicateKeyError,
                            OperationFailure)
from pymongo.read_preferences import ReadPreference

try:
    _SEEK_SET = os.SEEK_SET
    _SEEK_CUR = os.SEEK_CUR
    _SEEK_END = os.SEEK_END
# before 2.5
except AttributeError:
    _SEEK_SET = 0
    _SEEK_CUR = 1
    _SEEK_END = 2

EMPTY = b""
NEWLN = b"\n"

"""Default chunk size, in bytes."""
# Slightly under a power of 2, to work well with server's record allocations.
DEFAULT_CHUNK_SIZE = 255 * 1024

_C_INDEX = SON([("files_id", ASCENDING), ("n", ASCENDING)])
_F_INDEX = SON([("filename", ASCENDING), ("uploadDate", ASCENDING)])


def _grid_in_property(field_name, docstring, read_only=False,
                      closed_only=False):
    """Create a GridIn property."""
    def getter(self):
        if closed_only and not self._closed:
            raise AttributeError("can only get %r on a closed file" %
                                 field_name)
        # Protect against PHP-237
        if field_name == 'length':
            return self._file.get(field_name, 0)
        return self._file.get(field_name, None)

    def setter(self, value):
        if self._closed:
            self._coll.files.update_one({"_id": self._file["_id"]},
                                        {"$set": {field_name: value}})
        self._file[field_name] = value

    if read_only:
        docstring += "\n\nThis attribute is read-only."
    elif closed_only:
        docstring = "%s\n\n%s" % (docstring, "This attribute is read-only and "
                                  "can only be read after :meth:`close` "
                                  "has been called.")

    if not read_only and not closed_only:
        return property(getter, setter, doc=docstring)
    return property(getter, doc=docstring)


def _grid_out_property(field_name, docstring):
    """Create a GridOut property."""
    def getter(self):
        self._ensure_file()

        # Protect against PHP-237
        if field_name == 'length':
            return self._file.get(field_name, 0)
        return self._file.get(field_name, None)

    docstring += "\n\nThis attribute is read-only."
    return property(getter, doc=docstring)


def _clear_entity_type_registry(entity, **kwargs):
    """Clear the given database/collection object's type registry."""
    codecopts = entity.codec_options.with_options(type_registry=None)
    return entity.with_options(codec_options=codecopts, **kwargs)


class GridIn(object):
    """Class to write data to GridFS.
    """
    def __init__(
            self, root_collection, session=None, disable_md5=False, **kwargs):
        """Write a file to GridFS

        Application developers should generally not need to
        instantiate this class directly - instead see the methods
        provided by :class:`~gridfs.GridFS`.

        Raises :class:`TypeError` if `root_collection` is not an
        instance of :class:`~pymongo.collection.Collection`.

        Any of the file level options specified in the `GridFS Spec
        <http://dochub.mongodb.org/core/gridfsspec>`_ may be passed as
        keyword arguments. Any additional keyword arguments will be
        set as additional fields on the file document. Valid keyword
        arguments include:

          - ``"_id"``: unique ID for this file (default:
            :class:`~bson.objectid.ObjectId`) - this ``"_id"`` must
            not have already been used for another file

          - ``"filename"``: human name for the file

          - ``"contentType"`` or ``"content_type"``: valid mime-type
            for the file

          - ``"chunkSize"`` or ``"chunk_size"``: size of each of the
            chunks, in bytes (default: 255 kb)

          - ``"encoding"``: encoding used for this file. In Python 2,
            any :class:`unicode` that is written to the file will be
            converted to a :class:`str`. In Python 3, any :class:`str`
            that is written to the file will be converted to
            :class:`bytes`.

        :Parameters:
          - `root_collection`: root collection to write to
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession` to use for all
            commands
          - `disable_md5` (optional): When True, an MD5 checksum will not be
            computed for the uploaded file. Useful in environments where
            MD5 cannot be used for regulatory or other reasons. Defaults to
            False.
          - `**kwargs` (optional): file level options (see above)

        .. versionchanged:: 3.6
           Added ``session`` parameter.

        .. versionchanged:: 3.0
           `root_collection` must use an acknowledged
           :attr:`~pymongo.collection.Collection.write_concern`
        """
        if not isinstance(root_collection, Collection):
            raise TypeError("root_collection must be an "
                            "instance of Collection")

        if not root_collection.write_concern.acknowledged:
            raise ConfigurationError('root_collection must use '
                                     'acknowledged write_concern')

        # Handle alternative naming
        if "content_type" in kwargs:
            kwargs["contentType"] = kwargs.pop("content_type")
        if "chunk_size" in kwargs:
            kwargs["chunkSize"] = kwargs.pop("chunk_size")

        coll = _clear_entity_type_registry(
            root_collection, read_preference=ReadPreference.PRIMARY)

        if not disable_md5:
            kwargs["md5"] = hashlib.md5()
        # Defaults
        kwargs["_id"] = kwargs.get("_id", ObjectId())
        kwargs["chunkSize"] = kwargs.get("chunkSize", DEFAULT_CHUNK_SIZE)
        object.__setattr__(self, "_session", session)
        object.__setattr__(self, "_coll", coll)
        object.__setattr__(self, "_chunks", coll.chunks)
        object.__setattr__(self, "_file", kwargs)
        object.__setattr__(self, "_buffer", StringIO())
        object.__setattr__(self, "_position", 0)
        object.__setattr__(self, "_chunk_number", 0)
        object.__setattr__(self, "_closed", False)
        object.__setattr__(self, "_ensured_index", False)

    def __create_index(self, collection, index_key, unique):
        doc = collection.find_one(projection={"_id": 1}, session=self._session)
        if doc is None:
            try:
                index_keys = [index_spec['key'] for index_spec in
                              collection.list_indexes(session=self._session)]
            except OperationFailure:
                index_keys = []
            if index_key not in index_keys:
                collection.create_index(
                    index_key.items(), unique=unique, session=self._session)

    def __ensure_indexes(self):
        if not object.__getattribute__(self, "_ensured_index"):
            self.__create_index(self._coll.files, _F_INDEX, False)
            self.__create_index(self._coll.chunks, _C_INDEX, True)
            object.__setattr__(self, "_ensured_index", True)

    def abort(self):
        """Remove all chunks/files that may have been uploaded and close.
        """
        self._coll.chunks.delete_many(
            {"files_id": self._file['_id']}, session=self._session)
        self._coll.files.delete_one(
            {"_id": self._file['_id']}, session=self._session)
        object.__setattr__(self, "_closed", True)

    @property
    def closed(self):
        """Is this file closed?
        """
        return self._closed

    _id = _grid_in_property("_id", "The ``'_id'`` value for this file.",
                            read_only=True)
    filename = _grid_in_property("filename", "Name of this file.")
    name = _grid_in_property("filename", "Alias for `filename`.")
    content_type = _grid_in_property("contentType", "Mime-type for this file.")
    length = _grid_in_property("length", "Length (in bytes) of this file.",
                               closed_only=True)
    chunk_size = _grid_in_property("chunkSize", "Chunk size for this file.",
                                   read_only=True)
    upload_date = _grid_in_property("uploadDate",
                                    "Date that this file was uploaded.",
                                    closed_only=True)
    md5 = _grid_in_property("md5", "MD5 of the contents of this file "
                            "if an md5 sum was created.",
                            closed_only=True)

    def __getattr__(self, name):
        if name in self._file:
            return self._file[name]
        raise AttributeError("GridIn object has no attribute '%s'" % name)

    def __setattr__(self, name, value):
        # For properties of this instance like _buffer, or descriptors set on
        # the class like filename, use regular __setattr__
        if name in self.__dict__ or name in self.__class__.__dict__:
            object.__setattr__(self, name, value)
        else:
            # All other attributes are part of the document in db.fs.files.
            # Store them to be sent to server on close() or if closed, send
            # them now.
            self._file[name] = value
            if self._closed:
                self._coll.files.update_one({"_id": self._file["_id"]},
                                            {"$set": {name: value}})

    def __flush_data(self, data):
        """Flush `data` to a chunk.
        """
        self.__ensure_indexes()
        if 'md5' in self._file:
            self._file['md5'].update(data)

        if not data:
            return
        assert(len(data) <= self.chunk_size)

        chunk = {"files_id": self._file["_id"],
                 "n": self._chunk_number,
                 "data": Binary(data)}

        try:
            self._chunks.insert_one(chunk, session=self._session)
        except DuplicateKeyError:
            self._raise_file_exists(self._file['_id'])
        self._chunk_number += 1
        self._position += len(data)

    def __flush_buffer(self):
        """Flush the buffer contents out to a chunk.
        """
        self.__flush_data(self._buffer.getvalue())
        self._buffer.close()
        self._buffer = StringIO()

    def __flush(self):
        """Flush the file to the database.
        """
        try:
            self.__flush_buffer()

            if "md5" in self._file:
                self._file["md5"] = self._file["md5"].hexdigest()
            # The GridFS spec says length SHOULD be an Int64.
            self._file["length"] = Int64(self._position)
            self._file["uploadDate"] = datetime.datetime.utcnow()

            return self._coll.files.insert_one(
                self._file, session=self._session)
        except DuplicateKeyError:
            self._raise_file_exists(self._id)

    def _raise_file_exists(self, file_id):
        """Raise a FileExists exception for the given file_id."""
        raise FileExists("file with _id %r already exists" % file_id)

    def close(self):
        """Flush the file and close it.

        A closed file cannot be written any more. Calling
        :meth:`close` more than once is allowed.
        """
        if not self._closed:
            self.__flush()
            object.__setattr__(self, "_closed", True)

    def read(self, size=-1):
        raise io.UnsupportedOperation('read')

    def readable(self):
        return False

    def seekable(self):
        return False

    def write(self, data):
        """Write data to the file. There is no return value.

        `data` can be either a string of bytes or a file-like object
        (implementing :meth:`read`). If the file has an
        :attr:`encoding` attribute, `data` can also be a
        :class:`unicode` (:class:`str` in python 3) instance, which
        will be encoded as :attr:`encoding` before being written.

        Due to buffering, the data may not actually be written to the
        database until the :meth:`close` method is called. Raises
        :class:`ValueError` if this file is already closed. Raises
        :class:`TypeError` if `data` is not an instance of
        :class:`str` (:class:`bytes` in python 3), a file-like object,
        or an instance of :class:`unicode` (:class:`str` in python 3).
        Unicode data is only allowed if the file has an :attr:`encoding`
        attribute.

        :Parameters:
          - `data`: string of bytes or file-like object to be written
            to the file
        """
        if self._closed:
            raise ValueError("cannot write to a closed file")

        try:
            # file-like
            read = data.read
        except AttributeError:
            # string
            if not isinstance(data, (text_type, bytes)):
                raise TypeError("can only write strings or file-like objects")
            if isinstance(data, text_type):
                try:
                    data = data.encode(self.encoding)
                except AttributeError:
                    raise TypeError("must specify an encoding for file in "
                                    "order to write %s" % (text_type.__name__,))
            read = StringIO(data).read

        if self._buffer.tell() > 0:
            # Make sure to flush only when _buffer is complete
            space = self.chunk_size - self._buffer.tell()
            if space:
                try:
                    to_write = read(space)
                except:
                    self.abort()
                    raise
                self._buffer.write(to_write)
                if len(to_write) < space:
                    return  # EOF or incomplete
            self.__flush_buffer()
        to_write = read(self.chunk_size)
        while to_write and len(to_write) == self.chunk_size:
            self.__flush_data(to_write)
            to_write = read(self.chunk_size)
        self._buffer.write(to_write)

    def writelines(self, sequence):
        """Write a sequence of strings to the file.

        Does not add seperators.
        """
        for line in sequence:
            self.write(line)

    def writeable(self):
        return True

    def __enter__(self):
        """Support for the context manager protocol.
        """
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        """Support for the context manager protocol.

        Close the file and allow exceptions to propagate.
        """
        self.close()

        # propagate exceptions
        return False


class GridOut(object):
    """Class to read data out of GridFS.
    """
    def __init__(self, root_collection, file_id=None, file_document=None,
                 session=None):
        """Read a file from GridFS

        Application developers should generally not need to
        instantiate this class directly - instead see the methods
        provided by :class:`~gridfs.GridFS`.

        Either `file_id` or `file_document` must be specified,
        `file_document` will be given priority if present. Raises
        :class:`TypeError` if `root_collection` is not an instance of
        :class:`~pymongo.collection.Collection`.

        :Parameters:
          - `root_collection`: root collection to read from
          - `file_id` (optional): value of ``"_id"`` for the file to read
          - `file_document` (optional): file document from
            `root_collection.files`
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession` to use for all
            commands

        .. versionchanged:: 3.8
           For better performance and to better follow the GridFS spec,
           :class:`GridOut` now uses a single cursor to read all the chunks in
           the file.

        .. versionchanged:: 3.6
           Added ``session`` parameter.

        .. versionchanged:: 3.0
           Creating a GridOut does not immediately retrieve the file metadata
           from the server. Metadata is fetched when first needed.
        """
        if not isinstance(root_collection, Collection):
            raise TypeError("root_collection must be an "
                            "instance of Collection")

        root_collection = _clear_entity_type_registry(root_collection)

        self.__chunks = root_collection.chunks
        self.__files = root_collection.files
        self.__file_id = file_id
        self.__buffer = EMPTY
        self.__chunk_iter = None
        self.__position = 0
        self._file = file_document
        self._session = session

    _id = _grid_out_property("_id", "The ``'_id'`` value for this file.")
    filename = _grid_out_property("filename", "Name of this file.")
    name = _grid_out_property("filename", "Alias for `filename`.")
    content_type = _grid_out_property("contentType", "Mime-type for this file.")
    length = _grid_out_property("length", "Length (in bytes) of this file.")
    chunk_size = _grid_out_property("chunkSize", "Chunk size for this file.")
    upload_date = _grid_out_property("uploadDate",
                                     "Date that this file was first uploaded.")
    aliases = _grid_out_property("aliases", "List of aliases for this file.")
    metadata = _grid_out_property("metadata", "Metadata attached to this file.")
    md5 = _grid_out_property("md5", "MD5 of the contents of this file "
                             "if an md5 sum was created.")

    def _ensure_file(self):
        if not self._file:
            self._file = self.__files.find_one({"_id": self.__file_id},
                                               session=self._session)
            if not self._file:
                raise NoFile("no file in gridfs collection %r with _id %r" %
                             (self.__files, self.__file_id))

    def __getattr__(self, name):
        self._ensure_file()
        if name in self._file:
            return self._file[name]
        raise AttributeError("GridOut object has no attribute '%s'" % name)

    def readable(self):
        return True

    def readchunk(self):
        """Reads a chunk at a time. If the current position is within a
        chunk the remainder of the chunk is returned.
        """
        received = len(self.__buffer)
        chunk_data = EMPTY
        chunk_size = int(self.chunk_size)

        if received > 0:
            chunk_data = self.__buffer
        elif self.__position < int(self.length):
            chunk_number = int((received + self.__position) / chunk_size)
            if self.__chunk_iter is None:
                self.__chunk_iter = _GridOutChunkIterator(
                    self, self.__chunks, self._session, chunk_number)

            chunk = self.__chunk_iter.next()
            chunk_data = chunk["data"][self.__position % chunk_size:]

            if not chunk_data:
                raise CorruptGridFile("truncated chunk")

        self.__position += len(chunk_data)
        self.__buffer = EMPTY
        return chunk_data

    def read(self, size=-1):
        """Read at most `size` bytes from the file (less if there
        isn't enough data).

        The bytes are returned as an instance of :class:`str` (:class:`bytes`
        in python 3). If `size` is negative or omitted all data is read.

        :Parameters:
          - `size` (optional): the number of bytes to read

        .. versionchanged:: 3.8
           This method now only checks for extra chunks after reading the
           entire file. Previously, this method would check for extra chunks
           on every call.
        """
        self._ensure_file()

        remainder = int(self.length) - self.__position
        if size < 0 or size > remainder:
            size = remainder

        if size == 0:
            return EMPTY

        received = 0
        data = StringIO()
        while received < size:
            chunk_data = self.readchunk()
            received += len(chunk_data)
            data.write(chunk_data)

        # Detect extra chunks after reading the entire file.
        if size == remainder and self.__chunk_iter:
            try:
                self.__chunk_iter.next()
            except StopIteration:
                pass

        self.__position -= received - size

        # Return 'size' bytes and store the rest.
        data.seek(size)
        self.__buffer = data.read()
        data.seek(0)
        return data.read(size)

    def readline(self, size=-1):
        """Read one line or up to `size` bytes from the file.

        :Parameters:
         - `size` (optional): the maximum number of bytes to read
        """
        remainder = int(self.length) - self.__position
        if size < 0 or size > remainder:
            size = remainder

        if size == 0:
            return EMPTY

        received = 0
        data = StringIO()
        while received < size:
            chunk_data = self.readchunk()
            pos = chunk_data.find(NEWLN, 0, size)
            if pos != -1:
                size = received + pos + 1

            received += len(chunk_data)
            data.write(chunk_data)
            if pos != -1:
                break

        self.__position -= received - size

        # Return 'size' bytes and store the rest.
        data.seek(size)
        self.__buffer = data.read()
        data.seek(0)
        return data.read(size)

    def tell(self):
        """Return the current position of this file.
        """
        return self.__position

    def seek(self, pos, whence=_SEEK_SET):
        """Set the current position of this file.

        :Parameters:
         - `pos`: the position (or offset if using relative
           positioning) to seek to
         - `whence` (optional): where to seek
           from. :attr:`os.SEEK_SET` (``0``) for absolute file
           positioning, :attr:`os.SEEK_CUR` (``1``) to seek relative
           to the current position, :attr:`os.SEEK_END` (``2``) to
           seek relative to the file's end.
        """
        if whence == _SEEK_SET:
            new_pos = pos
        elif whence == _SEEK_CUR:
            new_pos = self.__position + pos
        elif whence == _SEEK_END:
            new_pos = int(self.length) + pos
        else:
            raise IOError(22, "Invalid value for `whence`")

        if new_pos < 0:
            raise IOError(22, "Invalid value for `pos` - must be positive")

        # Optimization, continue using the same buffer and chunk iterator.
        if new_pos == self.__position:
            return

        self.__position = new_pos
        self.__buffer = EMPTY
        if self.__chunk_iter:
            self.__chunk_iter.close()
            self.__chunk_iter = None

    def seekable(self):
        return True

    def __iter__(self):
        """Return an iterator over all of this file's data.

        The iterator will return chunk-sized instances of
        :class:`str` (:class:`bytes` in python 3). This can be
        useful when serving files using a webserver that handles
        such an iterator efficiently.

        .. note::
           This is different from :py:class:`io.IOBase` which iterates over
           *lines* in the file. Use :meth:`GridOut.readline` to read line by
           line instead of chunk by chunk.

        .. versionchanged:: 3.8
           The iterator now raises :class:`CorruptGridFile` when encountering
           any truncated, missing, or extra chunk in a file. The previous
           behavior was to only raise :class:`CorruptGridFile` on a missing
           chunk.
        """
        return GridOutIterator(self, self.__chunks, self._session)

    def close(self):
        """Make GridOut more generically file-like."""
        if self.__chunk_iter:
            self.__chunk_iter.close()
            self.__chunk_iter = None

    def write(self, value):
        raise io.UnsupportedOperation('write')

    def __enter__(self):
        """Makes it possible to use :class:`GridOut` files
        with the context manager protocol.
        """
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        """Makes it possible to use :class:`GridOut` files
        with the context manager protocol.
        """
        self.close()
        return False


class _GridOutChunkIterator(object):
    """Iterates over a file's chunks using a single cursor.

    Raises CorruptGridFile when encountering any truncated, missing, or extra
    chunk in a file.
    """
    def __init__(self, grid_out, chunks, session, next_chunk):
        self._id = grid_out._id
        self._chunk_size = int(grid_out.chunk_size)
        self._length = int(grid_out.length)
        self._chunks = chunks
        self._session = session
        self._next_chunk = next_chunk
        self._num_chunks = math.ceil(float(self._length) / self._chunk_size)
        self._cursor = None

    def expected_chunk_length(self, chunk_n):
        if chunk_n < self._num_chunks - 1:
            return self._chunk_size
        return self._length - (self._chunk_size * (self._num_chunks - 1))

    def __iter__(self):
        return self

    def _create_cursor(self):
        filter = {"files_id": self._id}
        if self._next_chunk > 0:
            filter["n"] = {"$gte": self._next_chunk}
        self._cursor = self._chunks.find(filter, sort=[("n", 1)],
                                         session=self._session)

    def _next_with_retry(self):
        """Return the next chunk and retry once on CursorNotFound.

        We retry on CursorNotFound to maintain backwards compatibility in
        cases where two calls to read occur more than 10 minutes apart (the
        server's default cursor timeout).
        """
        if self._cursor is None:
            self._create_cursor()

        try:
            return self._cursor.next()
        except CursorNotFound:
            self._cursor.close()
            self._create_cursor()
            return self._cursor.next()

    def next(self):
        try:
            chunk = self._next_with_retry()
        except StopIteration:
            if self._next_chunk >= self._num_chunks:
                raise
            raise CorruptGridFile("no chunk #%d" % self._next_chunk)

        if chunk["n"] != self._next_chunk:
            self.close()
            raise CorruptGridFile(
                "Missing chunk: expected chunk #%d but found "
                "chunk with n=%d" % (self._next_chunk, chunk["n"]))

        if chunk["n"] >= self._num_chunks:
            # According to spec, ignore extra chunks if they are empty.
            if len(chunk["data"]):
                self.close()
                raise CorruptGridFile(
                    "Extra chunk found: expected %d chunks but found "
                    "chunk with n=%d" % (self._num_chunks, chunk["n"]))

        expected_length = self.expected_chunk_length(chunk["n"])
        if len(chunk["data"]) != expected_length:
            self.close()
            raise CorruptGridFile(
                "truncated chunk #%d: expected chunk length to be %d but "
                "found chunk with length %d" % (
                    chunk["n"], expected_length, len(chunk["data"])))

        self._next_chunk += 1
        return chunk

    __next__ = next

    def close(self):
        if self._cursor:
            self._cursor.close()
            self._cursor = None


class GridOutIterator(object):
    def __init__(self, grid_out, chunks, session):
        self.__chunk_iter = _GridOutChunkIterator(grid_out, chunks, session, 0)

    def __iter__(self):
        return self

    def next(self):
        chunk = self.__chunk_iter.next()
        return bytes(chunk["data"])

    __next__ = next


class GridOutCursor(Cursor):
    """A cursor / iterator for returning GridOut objects as the result
    of an arbitrary query against the GridFS files collection.
    """
    def __init__(self, collection, filter=None, skip=0, limit=0,
                 no_cursor_timeout=False, sort=None, batch_size=0,
                 session=None):
        """Create a new cursor, similar to the normal
        :class:`~pymongo.cursor.Cursor`.

        Should not be called directly by application developers - see
        the :class:`~gridfs.GridFS` method :meth:`~gridfs.GridFS.find` instead.

        .. versionadded 2.7

        .. mongodoc:: cursors
        """
        collection = _clear_entity_type_registry(collection)

        # Hold on to the base "fs" collection to create GridOut objects later.
        self.__root_collection = collection

        super(GridOutCursor, self).__init__(
            collection.files, filter, skip=skip, limit=limit,
            no_cursor_timeout=no_cursor_timeout, sort=sort,
            batch_size=batch_size, session=session)

    def next(self):
        """Get next GridOut object from cursor.
        """
        # Work around "super is not iterable" issue in Python 3.x
        next_file = super(GridOutCursor, self).next()
        return GridOut(self.__root_collection, file_document=next_file,
                       session=self.session)

    __next__ = next

    def add_option(self, *args, **kwargs):
        raise NotImplementedError("Method does not exist for GridOutCursor")

    def remove_option(self, *args, **kwargs):
        raise NotImplementedError("Method does not exist for GridOutCursor")

    def _clone_base(self, session):
        """Creates an empty GridOutCursor for information to be copied into.
        """
        return GridOutCursor(self.__root_collection, session=session)