While testing injection of memory allocation failures in the _bz2 extension module, I found a bug: PyBytesWriter_Discard() is called twice on a memory allocation failure.

Example:

import _testcapi
import bz2
data = bz2.compress(b'x' * 1024)
decompressor = bz2.BZ2Decompressor()
try:
    _testcapi.set_nomemory(3,4)
    try:
        decompressor.decompress(data)
    except MemoryError:
        pass
    else:
        raise Exception
finally:
    _testcapi.remove_mem_hooks()
print("exit")

The error occurs in decompress_buf():

• OutputBuffer_Finish() => _BlocksOutputBuffer_Finish() => PyBytesWriter_FinishWithSize() => _PyBytes_Resize(): _PyBytes_Resize() fails with a memory allocation failure, then PyBytesWriter_FinishWithSize() calls PyBytesWriter_Discard()
• OutputBuffer_OnError() => _BlocksOutputBuffer_OnError(): call PyBytesWriter_Discard() again

Attached PR clears the writer in _BlocksOutputBuffer_Finish(), so _BlocksOutputBuffer_OnError() does nothing insted of calling PyBytesWriter_Discard() again.

Linked PRs

• gh-148688: Fix _BlocksOutputBuffer_Finish() double free #148689