sf_error(NULL) race condition: LibsndfileError.code=0 under concurrent sf_open_virtual calls #479

Summary

When multiple threads concurrently call sf_open_virtual() on unsupported formats (e.g. MP4 bytes), the resulting LibsndfileError sometimes has code=0 instead of the correct error code (e.g. 1 for "Format not recognised"). This is because _open() at soundfile.py:L1264 calls sf_error(NULL) to retrieve the error code after a failed open, and sf_error(NULL) reads a global (non-thread-safe) error variable in libsndfile.

When thread A fails sf_open_virtual and thread B also fails concurrently, thread B's call to sf_error(NULL) may return 0 because thread A's subsequent operations (or libsndfile's internal cleanup) have already cleared the global error state.

This results in LibsndfileError with:

• code = 0
• error_string = "(Garbled error message from libsndfile)"

Reproduction

import soundfile
from io import BytesIO
from concurrent.futures import ThreadPoolExecutor, as_completed

# Any bytes that soundfile can't handle (e.g. an MP4 file)
with open("some_video.mp4", "rb") as f:
    mp4_data = f.read()

def try_open(i):
    try:
        soundfile.SoundFile(BytesIO(mp4_data))
        return i, "ok", None
    except soundfile.LibsndfileError as e:
        return i, e.code, e.error_string

with ThreadPoolExecutor(max_workers=8) as pool:
    futs = [pool.submit(try_open, i) for i in range(8)]
    for f in as_completed(futs):
        idx, code, msg = f.result()
        if code == 0:
            print(f"thread {idx}: code={code}, msg={msg!r}")
            # Expected code=1 ("Format not recognised"), got code=0

Running this repeatedly will produce code=0 on some threads.

Impact

Callers that branch on LibsndfileError.code to distinguish error types (e.g. "unrecognised format" vs "corrupt file") get incorrect codes under concurrency, leading to wrong error-handling paths.

Environment

• soundfile==0.13.1
• libsndfile 1.2.2
• Python 3.12, Linux x86_64

Possible fix

soundfile.py:_open() could use sf_error() on the (failed) file handle instead of sf_error(NULL), but since the handle is NULL on open failure, libsndfile doesn't support that. An alternative is to add a module-level lock around the sf_open_virtual + sf_error(NULL) sequence so the global error state can't be clobbered between the two calls.