Fix BlobIO cleanup deadlock - #1491
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When reading less than 8 KiB from the blob's data (io.DEFAULT_BUFFER_SIZE), the close() method would hang. Namely, close() thread would wait for self._writer_closed but the writer thread that'd set this is blocked by waiting in Blob__write_to_queue/.../blob_filter_stream_write/queue.put. The queue remains "full" (maxsize=1) because of unread data and no progress happens. Fix this by switching to "busy" wait for _writer_closed thus we empty the queue from the main thread and writer thread can proceed to its termination. Fixes libgit2#1488 Assisted-by: sonnet-5
The reader won't properly synchronize threads and when a file larger than DEFAULT_CHUNK_SIZE (8 KiB) is partially read, the close() method would hand in deadlock. Note: test/test_blob.py::test_blob_partial_read will hang indefinitely without a fix.
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This is an attempt at fixing #1488.
It adds a busy wait in the BlobIO cleanup path, it stands the tests and fixes the reported deadlock. (This idea comes from LLM.)
It also adds a testcase with two caveats: there's new binary data in repo (bigrepo.zip) and the test before the fix hangs the whole pytest suite hence it's meant to be run either in isolation (
test/test_blob.py::test_blob_partial_read) or with the fix. I found no pre-existing test/data repo with large enough file (and thechunk_sizeis not runtime-changeable from tests).