18941: Add --threads option to arv-get
[arvados.git] / sdk / python / arvados / arvfile.py
index c52c7727aef048cfb1d3af21e18589f590a9ec26..fbf593d02619810b1647f78520217ac6a9b4bef7 100644 (file)
@@ -1,3 +1,7 @@
+# Copyright (C) The Arvados Authors. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
 from __future__ import absolute_import
 from __future__ import division
 from future import standard_library
@@ -24,7 +28,7 @@ from . import config
 from .errors import KeepWriteError, AssertionError, ArgumentError
 from .keep import KeepLocator
 from ._normalize_stream import normalize_stream
-from ._ranges import locators_and_ranges, replace_range, Range
+from ._ranges import locators_and_ranges, replace_range, Range, LocatorAndRange
 from .retry import retry_method
 
 MOD = "mod"
@@ -84,9 +88,6 @@ class _FileLikeObjectBase(object):
 class ArvadosFileReaderBase(_FileLikeObjectBase):
     def __init__(self, name, mode, num_retries=None):
         super(ArvadosFileReaderBase, self).__init__(name, mode)
-        self._binary = 'b' in mode
-        if sys.version_info >= (3, 0) and not self._binary:
-            raise NotImplementedError("text mode {!r} is not implemented".format(mode))
         self._filepos = 0
         self.num_retries = num_retries
         self._readline_cache = (None, None)
@@ -107,11 +108,23 @@ class ArvadosFileReaderBase(_FileLikeObjectBase):
             pos += self._filepos
         elif whence == os.SEEK_END:
             pos += self.size()
-        self._filepos = min(max(pos, 0), self.size())
+        if pos < 0:
+            raise IOError(errno.EINVAL, "Tried to seek to negative file offset.")
+        self._filepos = pos
+        return self._filepos
 
     def tell(self):
         return self._filepos
 
+    def readable(self):
+        return True
+
+    def writable(self):
+        return False
+
+    def seekable(self):
+        return True
+
     @_FileLikeObjectBase._before_close
     @retry_method
     def readall(self, size=2**20, num_retries=None):
@@ -183,13 +196,13 @@ class ArvadosFileReaderBase(_FileLikeObjectBase):
         return b''.join(data).decode().splitlines(True)
 
     def size(self):
-        raise NotImplementedError()
+        raise IOError(errno.ENOSYS, "Not implemented")
 
     def read(self, size, num_retries=None):
-        raise NotImplementedError()
+        raise IOError(errno.ENOSYS, "Not implemented")
 
     def readfrom(self, start, size, num_retries=None):
-        raise NotImplementedError()
+        raise IOError(errno.ENOSYS, "Not implemented")
 
 
 class StreamFileReader(ArvadosFileReaderBase):
@@ -288,6 +301,7 @@ class _BufferBlock(object):
     PENDING = 1
     COMMITTED = 2
     ERROR = 3
+    DELETED = 4
 
     def __init__(self, blockid, starting_capacity, owner):
         """
@@ -384,10 +398,54 @@ class _BufferBlock(object):
 
     @synchronized
     def clear(self):
+        self._state = _BufferBlock.DELETED
         self.owner = None
         self.buffer_block = None
         self.buffer_view = None
 
+    @synchronized
+    def repack_writes(self):
+        """Optimize buffer block by repacking segments in file sequence.
+
+        When the client makes random writes, they appear in the buffer block in
+        the sequence they were written rather than the sequence they appear in
+        the file.  This makes for inefficient, fragmented manifests.  Attempt
+        to optimize by repacking writes in file sequence.
+
+        """
+        if self._state != _BufferBlock.WRITABLE:
+            raise AssertionError("Cannot repack non-writable block")
+
+        segs = self.owner.segments()
+
+        # Collect the segments that reference the buffer block.
+        bufferblock_segs = [s for s in segs if s.locator == self.blockid]
+
+        # Collect total data referenced by segments (could be smaller than
+        # bufferblock size if a portion of the file was written and
+        # then overwritten).
+        write_total = sum([s.range_size for s in bufferblock_segs])
+
+        if write_total < self.size() or len(bufferblock_segs) > 1:
+            # If there's more than one segment referencing this block, it is
+            # due to out-of-order writes and will produce a fragmented
+            # manifest, so try to optimize by re-packing into a new buffer.
+            contents = self.buffer_view[0:self.write_pointer].tobytes()
+            new_bb = _BufferBlock(None, write_total, None)
+            for t in bufferblock_segs:
+                new_bb.append(contents[t.segment_offset:t.segment_offset+t.range_size])
+                t.segment_offset = new_bb.size() - t.range_size
+
+            self.buffer_block = new_bb.buffer_block
+            self.buffer_view = new_bb.buffer_view
+            self.write_pointer = new_bb.write_pointer
+            self._locator = None
+            new_bb.clear()
+            self.owner.set_segments(segs)
+
+    def __repr__(self):
+        return "<BufferBlock %s>" % (self.blockid)
+
 
 class NoopLock(object):
     def __enter__(self):
@@ -423,7 +481,7 @@ class _BlockManager(object):
     DEFAULT_PUT_THREADS = 2
     DEFAULT_GET_THREADS = 2
 
-    def __init__(self, keep, copies=None, put_threads=None):
+    def __init__(self, keep, copies=None, put_threads=None, num_retries=None, storage_classes_func=None, get_threads=None):
         """keep: KeepClient object to use"""
         self._keep = keep
         self._bufferblocks = collections.OrderedDict()
@@ -433,14 +491,14 @@ class _BlockManager(object):
         self._prefetch_threads = None
         self.lock = threading.Lock()
         self.prefetch_enabled = True
-        if put_threads:
-            self.num_put_threads = put_threads
-        else:
-            self.num_put_threads = _BlockManager.DEFAULT_PUT_THREADS
-        self.num_get_threads = _BlockManager.DEFAULT_GET_THREADS
+        self.num_put_threads = put_threads or _BlockManager.DEFAULT_PUT_THREADS
+        self.num_get_threads = get_threads or _BlockManager.DEFAULT_GET_THREADS
         self.copies = copies
+        self.storage_classes = storage_classes_func or (lambda: [])
         self._pending_write_size = 0
         self.threads_lock = threading.Lock()
+        self.padding_block = None
+        self.num_retries = num_retries
 
     @synchronized
     def alloc_bufferblock(self, blockid=None, starting_capacity=2**14, owner=None):
@@ -460,7 +518,7 @@ class _BlockManager(object):
 
     def _alloc_bufferblock(self, blockid=None, starting_capacity=2**14, owner=None):
         if blockid is None:
-            blockid = "%s" % uuid.uuid4()
+            blockid = str(uuid.uuid4())
         bufferblock = _BufferBlock(blockid, starting_capacity=starting_capacity, owner=owner)
         self._bufferblocks[bufferblock.blockid] = bufferblock
         return bufferblock
@@ -476,7 +534,7 @@ class _BlockManager(object):
           ArvadosFile that owns the new block
 
         """
-        new_blockid = "bufferblock%i" % len(self._bufferblocks)
+        new_blockid = str(uuid.uuid4())
         bufferblock = block.clone(new_blockid, owner)
         self._bufferblocks[bufferblock.blockid] = bufferblock
         return bufferblock
@@ -495,11 +553,10 @@ class _BlockManager(object):
                     return
 
                 if self.copies is None:
-                    loc = self._keep.put(bufferblock.buffer_view[0:bufferblock.write_pointer].tobytes())
+                    loc = self._keep.put(bufferblock.buffer_view[0:bufferblock.write_pointer].tobytes(), num_retries=self.num_retries, classes=self.storage_classes())
                 else:
-                    loc = self._keep.put(bufferblock.buffer_view[0:bufferblock.write_pointer].tobytes(), copies=self.copies)
+                    loc = self._keep.put(bufferblock.buffer_view[0:bufferblock.write_pointer].tobytes(), num_retries=self.num_retries, copies=self.copies, classes=self.storage_classes())
                 bufferblock.set_state(_BufferBlock.COMMITTED, loc)
-
             except Exception as e:
                 bufferblock.set_state(_BufferBlock.ERROR, e)
             finally:
@@ -513,7 +570,7 @@ class _BlockManager(object):
 
                 # If we don't limit the Queue size, the upload queue can quickly
                 # grow to take up gigabytes of RAM if the writing process is
-                # generating data more quickly than it can be send to the Keep
+                # generating data more quickly than it can be sent to the Keep
                 # servers.
                 #
                 # With two upload threads and a queue size of 2, this means up to 4
@@ -536,6 +593,9 @@ class _BlockManager(object):
                 b = self._prefetch_queue.get()
                 if b is None:
                     return
+                if self._keep.has_cache_slot(b):
+                    continue
+                _logger.debug("prefetching %s", b)
                 self._keep.get(b)
             except Exception:
                 _logger.exception("Exception doing block prefetch")
@@ -581,43 +641,59 @@ class _BlockManager(object):
     @synchronized
     def repack_small_blocks(self, force=False, sync=False, closed_file_size=0):
         """Packs small blocks together before uploading"""
+
         self._pending_write_size += closed_file_size
 
         # Check if there are enough small blocks for filling up one in full
-        if force or (self._pending_write_size >= config.KEEP_BLOCK_SIZE):
+        if not (force or (self._pending_write_size >= config.KEEP_BLOCK_SIZE)):
+            return
 
-            # Search blocks ready for getting packed together before being committed to Keep.
-            # A WRITABLE block always has an owner.
-            # A WRITABLE block with its owner.closed() implies that it's
-            # size is <= KEEP_BLOCK_SIZE/2.
-            try:
-                small_blocks = [b for b in listvalues(self._bufferblocks) if b.state() == _BufferBlock.WRITABLE and b.owner.closed()]
-            except AttributeError:
-                # Writable blocks without owner shouldn't exist.
-                raise UnownedBlockError()
+        # Search blocks ready for getting packed together before being
+        # committed to Keep.
+        # A WRITABLE block always has an owner.
+        # A WRITABLE block with its owner.closed() implies that its
+        # size is <= KEEP_BLOCK_SIZE/2.
+        try:
+            small_blocks = [b for b in listvalues(self._bufferblocks)
+                            if b.state() == _BufferBlock.WRITABLE and b.owner.closed()]
+        except AttributeError:
+            # Writable blocks without owner shouldn't exist.
+            raise UnownedBlockError()
+
+        if len(small_blocks) <= 1:
+            # Not enough small blocks for repacking
+            return
 
-            if len(small_blocks) <= 1:
-                # Not enough small blocks for repacking
-                return
+        for bb in small_blocks:
+            bb.repack_writes()
 
-            # Update the pending write size count with its true value, just in case
-            # some small file was opened, written and closed several times.
-            self._pending_write_size = sum([b.size() for b in small_blocks])
-            if self._pending_write_size < config.KEEP_BLOCK_SIZE and not force:
-                return
+        # Update the pending write size count with its true value, just in case
+        # some small file was opened, written and closed several times.
+        self._pending_write_size = sum([b.size() for b in small_blocks])
+
+        if self._pending_write_size < config.KEEP_BLOCK_SIZE and not force:
+            return
 
-            new_bb = self._alloc_bufferblock()
-            while len(small_blocks) > 0 and (new_bb.write_pointer + small_blocks[0].size()) <= config.KEEP_BLOCK_SIZE:
-                bb = small_blocks.pop(0)
-                arvfile = bb.owner
-                self._pending_write_size -= bb.size()
-                new_bb.append(bb.buffer_view[0:bb.write_pointer].tobytes())
-                arvfile.set_segments([Range(new_bb.blockid,
-                                            0,
-                                            bb.size(),
-                                            new_bb.write_pointer - bb.size())])
-                self._delete_bufferblock(bb.blockid)
-            self.commit_bufferblock(new_bb, sync=sync)
+        new_bb = self._alloc_bufferblock()
+        new_bb.owner = []
+        files = []
+        while len(small_blocks) > 0 and (new_bb.write_pointer + small_blocks[0].size()) <= config.KEEP_BLOCK_SIZE:
+            bb = small_blocks.pop(0)
+            new_bb.owner.append(bb.owner)
+            self._pending_write_size -= bb.size()
+            new_bb.append(bb.buffer_view[0:bb.write_pointer].tobytes())
+            files.append((bb, new_bb.write_pointer - bb.size()))
+
+        self.commit_bufferblock(new_bb, sync=sync)
+
+        for bb, new_bb_segment_offset in files:
+            newsegs = bb.owner.segments()
+            for s in newsegs:
+                if s.locator == bb.blockid:
+                    s.locator = new_bb.blockid
+                    s.segment_offset = new_bb_segment_offset+s.segment_offset
+            bb.owner.set_segments(newsegs)
+            self._delete_bufferblock(bb.blockid)
 
     def commit_bufferblock(self, block, sync):
         """Initiate a background upload of a bufferblock.
@@ -651,9 +727,9 @@ class _BlockManager(object):
         if sync:
             try:
                 if self.copies is None:
-                    loc = self._keep.put(block.buffer_view[0:block.write_pointer].tobytes())
+                    loc = self._keep.put(block.buffer_view[0:block.write_pointer].tobytes(), num_retries=self.num_retries, classes=self.storage_classes())
                 else:
-                    loc = self._keep.put(block.buffer_view[0:block.write_pointer].tobytes(), copies=self.copies)
+                    loc = self._keep.put(block.buffer_view[0:block.write_pointer].tobytes(), num_retries=self.num_retries, copies=self.copies, classes=self.storage_classes())
                 block.set_state(_BufferBlock.COMMITTED, loc)
             except Exception as e:
                 block.set_state(_BufferBlock.ERROR, e)
@@ -666,14 +742,31 @@ class _BlockManager(object):
     def get_bufferblock(self, locator):
         return self._bufferblocks.get(locator)
 
+    @synchronized
+    def get_padding_block(self):
+        """Get a bufferblock 64 MB in size consisting of all zeros, used as padding
+        when using truncate() to extend the size of a file.
+
+        For reference (and possible future optimization), the md5sum of the
+        padding block is: 7f614da9329cd3aebf59b91aadc30bf0+67108864
+
+        """
+
+        if self.padding_block is None:
+            self.padding_block = self._alloc_bufferblock(starting_capacity=config.KEEP_BLOCK_SIZE)
+            self.padding_block.write_pointer = config.KEEP_BLOCK_SIZE
+            self.commit_bufferblock(self.padding_block, False)
+        return self.padding_block
+
     @synchronized
     def delete_bufferblock(self, locator):
         self._delete_bufferblock(locator)
 
     def _delete_bufferblock(self, locator):
-        bb = self._bufferblocks[locator]
-        bb.clear()
-        del self._bufferblocks[locator]
+        if locator in self._bufferblocks:
+            bb = self._bufferblocks[locator]
+            bb.clear()
+            del self._bufferblocks[locator]
 
     def get_block_contents(self, locator, num_retries, cache_only=False):
         """Fetch a block.
@@ -708,7 +801,10 @@ class _BlockManager(object):
 
         for k,v in items:
             if v.state() != _BufferBlock.COMMITTED and v.owner:
-                v.owner.flush(sync=False)
+                # Ignore blocks with a list of owners, as if they're not in COMMITTED
+                # state, they're already being committed asynchronously.
+                if isinstance(v.owner, ArvadosFile):
+                    v.owner.flush(sync=False)
 
         with self.lock:
             if self._put_queue is not None:
@@ -725,7 +821,15 @@ class _BlockManager(object):
             # flush again with sync=True to remove committed bufferblocks from
             # the segments.
             if v.owner:
-                v.owner.flush(sync=True)
+                if isinstance(v.owner, ArvadosFile):
+                    v.owner.flush(sync=True)
+                elif isinstance(v.owner, list) and len(v.owner) > 0:
+                    # This bufferblock is referenced by many files as a result
+                    # of repacking small blocks, so don't delete it when flushing
+                    # its owners, just do it after flushing them all.
+                    for owner in v.owner:
+                        owner.flush(sync=True)
+                    self.delete_bufferblock(k)
 
     def block_prefetch(self, locator):
         """Initiate a background download of a block.
@@ -740,7 +844,7 @@ class _BlockManager(object):
         if not self.prefetch_enabled:
             return
 
-        if self._keep.get_from_cache(locator) is not None:
+        if self._keep.has_cache_slot(locator):
             return
 
         with self.lock:
@@ -748,6 +852,7 @@ class _BlockManager(object):
                 return
 
         self.start_get_threads()
+        # _logger.debug("pushing %s to prefetch", locator)
         self._prefetch_queue.put(locator)
 
 
@@ -762,6 +867,9 @@ class ArvadosFile(object):
 
     """
 
+    __slots__ = ('parent', 'name', '_writers', '_committed',
+                 '_segments', 'lock', '_current_bblock', 'fuse_entry')
+
     def __init__(self, parent, name, stream=[], segments=[]):
         """
         ArvadosFile constructor.
@@ -793,6 +901,38 @@ class ArvadosFile(object):
                 return True
         return False
 
+    @synchronized
+    def has_remote_blocks(self):
+        """Returns True if any of the segment's locators has a +R signature"""
+
+        for s in self._segments:
+            if '+R' in s.locator:
+                return True
+        return False
+
+    @synchronized
+    def _copy_remote_blocks(self, remote_blocks={}):
+        """Ask Keep to copy remote blocks and point to their local copies.
+
+        This is called from the parent Collection.
+
+        :remote_blocks:
+            Shared cache of remote to local block mappings. This is used to avoid
+            doing extra work when blocks are shared by more than one file in
+            different subdirectories.
+        """
+
+        for s in self._segments:
+            if '+R' in s.locator:
+                try:
+                    loc = remote_blocks[s.locator]
+                except KeyError:
+                    loc = self.parent._my_keep().refresh_signature(s.locator)
+                    remote_blocks[s.locator] = loc
+                s.locator = loc
+                self.parent.set_committed(False)
+        return remote_blocks
+
     @synchronized
     def segments(self):
         return copy.copy(self._segments)
@@ -913,11 +1053,11 @@ class ArvadosFile(object):
     @must_be_writable
     @synchronized
     def truncate(self, size):
-        """Shrink the size of the file.
+        """Shrink or expand the size of the file.
 
         If `size` is less than the size of the file, the file contents after
         `size` will be discarded.  If `size` is greater than the current size
-        of the file, an IOError will be raised.
+        of the file, it will be filled with zero bytes.
 
         """
         if size < self.size():
@@ -938,7 +1078,17 @@ class ArvadosFile(object):
             self._segments = new_segs
             self.set_committed(False)
         elif size > self.size():
-            raise IOError(errno.EINVAL, "truncate() does not support extending the file size")
+            padding = self.parent._my_block_manager().get_padding_block()
+            diff = size - self.size()
+            while diff > config.KEEP_BLOCK_SIZE:
+                self._segments.append(Range(padding.blockid, self.size(), config.KEEP_BLOCK_SIZE, 0))
+                diff -= config.KEEP_BLOCK_SIZE
+            if diff > 0:
+                self._segments.append(Range(padding.blockid, self.size(), diff, 0))
+            self.set_committed(False)
+        else:
+            # size == self.size()
+            pass
 
     def readfrom(self, offset, size, num_retries, exact=False):
         """Read up to `size` bytes from the file starting at `offset`.
@@ -953,7 +1103,7 @@ class ArvadosFile(object):
             if size == 0 or offset >= self.size():
                 return b''
             readsegs = locators_and_ranges(self._segments, offset, size)
-            prefetch = locators_and_ranges(self._segments, offset + size, config.KEEP_BLOCK_SIZE, limit=32)
+            prefetch = locators_and_ranges(self._segments, offset + size, config.KEEP_BLOCK_SIZE * self.parent._my_block_manager().num_get_threads, limit=32)
 
         locs = set()
         data = []
@@ -973,32 +1123,6 @@ class ArvadosFile(object):
 
         return b''.join(data)
 
-    def _repack_writes(self, num_retries):
-        """Test if the buffer block has more data than actual segments.
-
-        This happens when a buffered write over-writes a file range written in
-        a previous buffered write.  Re-pack the buffer block for efficiency
-        and to avoid leaking information.
-
-        """
-        segs = self._segments
-
-        # Sum up the segments to get the total bytes of the file referencing
-        # into the buffer block.
-        bufferblock_segs = [s for s in segs if s.locator == self._current_bblock.blockid]
-        write_total = sum([s.range_size for s in bufferblock_segs])
-
-        if write_total < self._current_bblock.size():
-            # There is more data in the buffer block than is actually accounted for by segments, so
-            # re-pack into a new buffer by copying over to a new buffer block.
-            contents = self.parent._my_block_manager().get_block_contents(self._current_bblock.blockid, num_retries)
-            new_bb = self.parent._my_block_manager().alloc_bufferblock(self._current_bblock.blockid, starting_capacity=write_total, owner=self)
-            for t in bufferblock_segs:
-                new_bb.append(contents[t.segment_offset:t.segment_offset+t.range_size])
-                t.segment_offset = new_bb.size() - t.range_size
-
-            self._current_bblock = new_bb
-
     @must_be_writable
     @synchronized
     def writeto(self, offset, data, num_retries):
@@ -1014,7 +1138,7 @@ class ArvadosFile(object):
             return
 
         if offset > self.size():
-            raise ArgumentError("Offset is past the end of the file")
+            self.truncate(offset)
 
         if len(data) > config.KEEP_BLOCK_SIZE:
             # Chunk it up into smaller writes
@@ -1031,7 +1155,7 @@ class ArvadosFile(object):
             self._current_bblock = self.parent._my_block_manager().alloc_bufferblock(owner=self)
 
         if (self._current_bblock.size() + len(data)) > config.KEEP_BLOCK_SIZE:
-            self._repack_writes(num_retries)
+            self._current_bblock.repack_writes()
             if (self._current_bblock.size() + len(data)) > config.KEEP_BLOCK_SIZE:
                 self.parent._my_block_manager().commit_bufferblock(self._current_bblock, sync=False)
                 self._current_bblock = self.parent._my_block_manager().alloc_bufferblock(owner=self)
@@ -1058,8 +1182,9 @@ class ArvadosFile(object):
 
         if self._current_bblock and self._current_bblock.state() != _BufferBlock.COMMITTED:
             if self._current_bblock.state() == _BufferBlock.WRITABLE:
-                self._repack_writes(num_retries)
-            self.parent._my_block_manager().commit_bufferblock(self._current_bblock, sync=sync)
+                self._current_bblock.repack_writes()
+            if self._current_bblock.state() != _BufferBlock.DELETED:
+                self.parent._my_block_manager().commit_bufferblock(self._current_bblock, sync=sync)
 
         if sync:
             to_delete = set()
@@ -1071,7 +1196,10 @@ class ArvadosFile(object):
                     to_delete.add(s.locator)
                     s.locator = bb.locator()
             for s in to_delete:
-               self.parent._my_block_manager().delete_bufferblock(s)
+                # Don't delete the bufferblock if it's owned by many files. It'll be
+                # deleted after all of its owners are flush()ed.
+                if self.parent._my_block_manager().get_bufferblock(s).owner is self:
+                    self.parent._my_block_manager().delete_bufferblock(s)
 
         self.parent.notify(MOD, self.parent, self.name, (self, self))
 
@@ -1108,17 +1236,17 @@ class ArvadosFile(object):
                       normalize=False, only_committed=False):
         buf = ""
         filestream = []
-        for segment in self.segments:
+        for segment in self._segments:
             loc = segment.locator
             if self.parent._my_block_manager().is_bufferblock(loc):
                 if only_committed:
                     continue
-                loc = self._bufferblocks[loc].calculate_locator()
+                loc = self.parent._my_block_manager().get_bufferblock(loc).locator()
             if portable_locators:
                 loc = KeepLocator(loc).stripped()
-            filestream.append(LocatorAndRange(loc, locator_block_size(loc),
+            filestream.append(LocatorAndRange(loc, KeepLocator(loc).size,
                                  segment.segment_offset, segment.range_size))
-        buf += ' '.join(normalize_stream(stream_name, {stream_name: filestream}))
+        buf += ' '.join(normalize_stream(stream_name, {self.name: filestream}))
         buf += "\n"
         return buf
 
@@ -1151,6 +1279,11 @@ class ArvadosFileReader(ArvadosFileReaderBase):
     def stream_name(self):
         return self.arvadosfile.parent.stream_name()
 
+    def readinto(self, b):
+        data = self.read(len(b))
+        b[:len(data)] = data
+        return len(data)
+
     @_FileLikeObjectBase._before_close
     @retry_method
     def read(self, size=None, num_retries=None):
@@ -1197,14 +1330,16 @@ class ArvadosFileWriter(ArvadosFileReader):
         super(ArvadosFileWriter, self).__init__(arvadosfile, mode=mode, num_retries=num_retries)
         self.arvadosfile.add_writer(self)
 
+    def writable(self):
+        return True
+
     @_FileLikeObjectBase._before_close
     @retry_method
     def write(self, data, num_retries=None):
         if self.mode[0] == "a":
-            self.arvadosfile.writeto(self.size(), data, num_retries)
-        else:
-            self.arvadosfile.writeto(self._filepos, data, num_retries)
-            self._filepos += len(data)
+            self._filepos = self.size()
+        self.arvadosfile.writeto(self._filepos, data, num_retries)
+        self._filepos += len(data)
         return len(data)
 
     @_FileLikeObjectBase._before_close
@@ -1218,8 +1353,6 @@ class ArvadosFileWriter(ArvadosFileReader):
         if size is None:
             size = self._filepos
         self.arvadosfile.truncate(size)
-        if self._filepos > self.size():
-            self._filepos = self.size()
 
     @_FileLikeObjectBase._before_close
     def flush(self):
@@ -1229,3 +1362,33 @@ class ArvadosFileWriter(ArvadosFileReader):
         if not self.closed:
             self.arvadosfile.remove_writer(self, flush)
             super(ArvadosFileWriter, self).close()
+
+
+class WrappableFile(object):
+    """An interface to an Arvados file that's compatible with io wrappers.
+
+    """
+    def __init__(self, f):
+        self.f = f
+        self.closed = False
+    def close(self):
+        self.closed = True
+        return self.f.close()
+    def flush(self):
+        return self.f.flush()
+    def read(self, *args, **kwargs):
+        return self.f.read(*args, **kwargs)
+    def readable(self):
+        return self.f.readable()
+    def readinto(self, *args, **kwargs):
+        return self.f.readinto(*args, **kwargs)
+    def seek(self, *args, **kwargs):
+        return self.f.seek(*args, **kwargs)
+    def seekable(self):
+        return self.f.seekable()
+    def tell(self):
+        return self.f.tell()
+    def writable(self):
+        return self.f.writable()
+    def write(self, *args, **kwargs):
+        return self.f.write(*args, **kwargs)