zlib.decompressobj fails after documented-safe zdict mutation with a partial initial header

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python Library Reference, zlib.decompressobj(): “If zdict is a mutable object (such as a bytearray), you must not modify its contents between the call to decompressobj() and the first call to the decompressor’s decompress() method.”

Expected: Subsequent decompress() calls and flush() recover b'\x00\x00\x00\x00' using the original dictionary contents.

Actual: The subsequent decompress() call raises zlib.error: Error -3 while setting zdict: invalid input data.

import zlib

D = bytearray(b"\0")
P = b"\0" * 4
case = dict(dictionary=D[:], payload=P, mutation="replace each byte b with b ^ 255",
            first_chunk_length=1, wbits=15)
try:
    snapshot = bytes(D)
    c = zlib.compressobj(wbits=15, zdict=snapshot)
    stream = c.compress(P) + c.flush()
    reference = zlib.decompressobj(wbits=15, zdict=snapshot)
    expected = reference.decompress(stream) + reference.flush()
    if not D or expected != P or not reference.eof:
        print("REFUTATION REJECTED:", "input failed immutable-dictionary validation")
    else:
        try:
            d = zlib.decompressobj(wbits=15, zdict=D)
            actual = d.decompress(stream[:1])
            # Mutation occurs after the first decompress() call, as documented.
            D[:] = bytes(b ^ 255 for b in D)
            actual += d.decompress(stream[1:]) + d.flush()
        except Exception as e:
            actual = ("EXCEPTION", type(e).__name__, str(e))
        if actual != expected:
            print("REFUTATION CONFIRMED:", case, "actual:", actual, "expected:", expected)
        else:
            print("REFUTATION REJECTED:", "documented expectation was recovered")
except Exception as e:
    print("REFUTATION REJECTED:", "validation/setup failed:", type(e).__name__, str(e))

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library zlib:

REFUTATION CONFIRMED: {'dictionary': bytearray(b'\x00'), 'payload': b'\x00\x00\x00\x00', 'mutation': 'replace each byte b with b ^ 255', 'first_chunk_length': 1, 'wbits': 15} actual: ('EXCEPTION', 'error', 'Error -3 while setting zdict: invalid input data') expected: b'\x00\x00\x00\x00'

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/zlib-20261003-065622-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).