The code in zlib module:

    self->zst.next_in = data->buf;  // set next_in
    ...
    ENTER_ZLIB(self);   // acquire thread lock

`self->zst` is a `z_stream` struct defined in zlib, used to record states of a compress/decompress stream:

    typedef struct z_stream_s {
        Bytef    *next_in;  /* next input byte */
        uInt     avail_in;  /* number of bytes available at next_in */
        uLong    total_in;  /* total number of input bytes read so far */

        Bytef    *next_out; /* next output byte will go here */
        uInt     avail_out; /* remaining free space at next_out */
        uLong    total_out; /* total number of bytes output so far */
        
        ... // Other states
    } z_stream;

Setting `next_in` before acquiring the thread lock may mix up compress/decompress state in other threads.

Moreover, modify `ENTER_ZLIB` macro, don't release the GIL when the thread lock can be acquired immediately. This behavior is the same as the bz2/lzma modules.

Thanks! ✨ 🍰 ✨

Thanks for review.

Sorry, I found an omission.

The previous PRs fixed the bug in these methods:

    zlib.Compress.compress()
    zlib.Decompress.decompress()

This method also has this bug, fix in PR29587 (main/3.10) and PR29588 (3.9-):

    zlib.Decompress.flush()
    
Attached file `test_flush.py` can reliably reproduce the bug.

This time I carefully checked bz2/lzma/zlib modules, it should be no problem anymore.

Gregory P. Smith should understand these codes, add him to nosy list.

New changeset 7edb6270a78c695e4c2ae2432797dc18105374fc by Ma Lin in branch 'main':
bpo-41735: Fix thread lock in zlib.Decompress.flush() may go wrong (GH-29587)
https://github.com/python/cpython/commit/7edb6270a78c695e4c2ae2432797dc18105374fc

New changeset 86c1265cdc64030c8921e0da5fcae2ac64299c26 by Ma Lin in branch '3.9':
[3.9] bpo-41735: Fix thread lock in zlib.Decompress.flush() may go wrong (GH-29588)
https://github.com/python/cpython/commit/86c1265cdc64030c8921e0da5fcae2ac64299c26

Thanks malin!

New changeset 57100c86baa8451a568348646834380cd425b858 by Miss Islington (bot) in branch '3.10':
[3.10] bpo-41735: Fix thread lock in zlib.Decompress.flush() may go wrong (GH-29587) (GH-29811)
https://github.com/python/cpython/commit/57100c86baa8451a568348646834380cd425b858

Thanks for review!