bpo-31993: Do not use memoryview when pickle large strings. (#5154)

PyMemoryView_FromMemory() created a memoryview referring to
the internal data of the string.  When the string is destroyed
the memoryview become referring to a freed memory.

Author: serhiy-storchaka

Lib/test/pickletester.py

@@ -2169,67 +2169,67 @@ def remove_frames(pickled, keep_frame=None):
     def test_framed_write_sizes_with_delayed_writer(self):
         class ChunkAccumulator:
             """Accumulate pickler output in a list of raw chunks."""
-
             def __init__(self):
                 self.chunks = []
-
             def write(self, chunk):
                 self.chunks.append(chunk)
-
             def concatenate_chunks(self):
-                # Some chunks can be memoryview instances, we need to convert
-                # them to bytes to be able to call join
-                return b"".join([c.tobytes() if hasattr(c, 'tobytes') else c
-                                 for c in self.chunks])
-
-        small_objects = [(str(i).encode('ascii'), i % 42, {'i': str(i)})
-                         for i in range(int(1e4))]
+                return b"".join(self.chunks)
 
         for proto in range(4, pickle.HIGHEST_PROTOCOL + 1):
+            objects = [(str(i).encode('ascii'), i % 42, {'i': str(i)})
+                       for i in range(int(1e4))]
+            # Add a large unique ASCII string
+            objects.append('0123456789abcdef' *
+                           (self.FRAME_SIZE_TARGET // 16 + 1))
+
             # Protocol 4 packs groups of small objects into frames and issues
             # calls to write only once or twice per frame:
             # The C pickler issues one call to write per-frame (header and
             # contents) while Python pickler issues two calls to write: one for
             # the frame header and one for the frame binary contents.
             writer = ChunkAccumulator()
-            self.pickler(writer, proto).dump(small_objects)
+            self.pickler(writer, proto).dump(objects)
 
             # Actually read the binary content of the chunks after the end
-            # of the call to dump: ant memoryview passed to write should not
+            # of the call to dump: any memoryview passed to write should not
             # be released otherwise this delayed access would not be possible.
             pickled = writer.concatenate_chunks()
             reconstructed = self.loads(pickled)
-            self.assertEqual(reconstructed, small_objects)
+            self.assertEqual(reconstructed, objects)
             self.assertGreater(len(writer.chunks), 1)
 
-            n_frames, remainder = divmod(len(pickled), self.FRAME_SIZE_TARGET)
-            if remainder > 0:
-                n_frames += 1
+            # memoryviews should own the memory.
+            del objects
+            support.gc_collect()
+            self.assertEqual(writer.concatenate_chunks(), pickled)
 
+            n_frames = (len(pickled) - 1) // self.FRAME_SIZE_TARGET + 1
             # There should be at least one call to write per frame
             self.assertGreaterEqual(len(writer.chunks), n_frames)
 
             # but not too many either: there can be one for the proto,
-            # one per-frame header and one per frame for the actual contents.
-            self.assertGreaterEqual(2 * n_frames + 1, len(writer.chunks))
+            # one per-frame header, one per frame for the actual contents,
+            # and two for the header.
+            self.assertLessEqual(len(writer.chunks), 2 * n_frames + 3)
 
-            chunk_sizes = [len(c) for c in writer.chunks[:-1]]
+            chunk_sizes = [len(c) for c in writer.chunks]
             large_sizes = [s for s in chunk_sizes
                            if s >= self.FRAME_SIZE_TARGET]
-            small_sizes = [s for s in chunk_sizes
-                           if s < self.FRAME_SIZE_TARGET]
+            medium_sizes = [s for s in chunk_sizes
+                           if 9 < s < self.FRAME_SIZE_TARGET]
+            small_sizes = [s for s in chunk_sizes if s <= 9]
 
             # Large chunks should not be too large:
             for chunk_size in large_sizes:
-                self.assertGreater(2 * self.FRAME_SIZE_TARGET, chunk_size)
-
-            last_chunk_size = len(writer.chunks[-1])
-            self.assertGreater(2 * self.FRAME_SIZE_TARGET, last_chunk_size)
-
-            # Small chunks (if any) should be very small
-            # (only proto and frame headers)
-            for chunk_size in small_sizes:
-                self.assertGreaterEqual(9, chunk_size)
+                self.assertLess(chunk_size, 2 * self.FRAME_SIZE_TARGET,
+                                chunk_sizes)
+            # There shouldn't bee too many small chunks: the protocol header,
+            # the frame headers and the large string headers are written
+            # in small chunks.
+            self.assertLessEqual(len(small_sizes),
+                                 len(large_sizes) + len(medium_sizes) + 3,
+                                 chunk_sizes)
 
     def test_nested_names(self):
         global Nested

Misc/NEWS.d/3.7.0a4.rst

@@ -693,9 +693,9 @@ ctypes._aix.find_library() Patch by: Michael Felt
 .. nonce: -OMNg8
 .. section: Library
 
-The picklers no longer allocate temporary memory when dumping large
-``bytes`` and ``str`` objects into a file object. Instead the data is
-directly streamed into the underlying file object.
+The pickler now uses less memory when serializing large bytes and str
+objects into a file.  Pickles created with protocol 4 will require less
+memory for unpickling large bytes and str objects.
 
 ..
 

Modules/_pickle.c

@@ -2184,8 +2184,9 @@ _Pickler_write_bytes(PicklerObject *self,
         /* Stream write the payload into the file without going through the
            output buffer. */
         if (payload == NULL) {
-            payload = mem = PyMemoryView_FromMemory((char *) data, data_size,
-                                                    PyBUF_READ);
+            /* TODO: It would be better to use a memoryview with a linked
+               original string if this is possible. */
+            payload = mem = PyBytes_FromStringAndSize(data, data_size);
             if (payload == NULL) {
                 return -1;
             }