zlib._ZlibDecompressor(zdict=...) raises SystemError on a zlib-wrapped stream #160001

Bug description

zlib._ZlibDecompressor accepts a zdict argument, but for a zlib-wrapped stream (positive wbits) compressed with that dictionary, decompress fails with a SystemError instead of decompressing. zlib.decompressobj handles the same input:

>>> import zlib
>>> zd = b"hello world " * 10
>>> c = zlib.compressobj(zdict=zd)
>>> s = c.compress(b"hello world hello") + c.flush()
>>> zlib.decompressobj(zdict=zd).decompress(s)
b'hello world hello'
>>> zlib._ZlibDecompressor(zdict=zd).decompress(s)
Traceback (most recent call last):
  File "<python-input-5>", line 1, in <module>
    zlib._ZlibDecompressor(zdict=zd).decompress(s)
SystemError: <method 'decompress' of 'zlib._ZlibDecompressor' objects> returned NULL without setting an exception

Raw deflate works, so this is specific to the wrapped format:

>>> c = zlib.compressobj(wbits=-15, zdict=zd)
>>> s = c.compress(b"hello world hello") + c.flush()
>>> zlib._ZlibDecompressor(-15, zdict=zd).decompress(s)
b'hello world hello'

Two things combine here. The constructor only applies the dictionary up front for raw deflate (wbits < 0): Modules/zlibmodule.c lines 1775-1780. For a wrapped stream zlib instead signals Z_NEED_DICT after reading the header, and decompress_buf handles that by jumping to error without setting an exception: Modules/zlibmodule.c lines 1535-1541. zlib.Decompress handles the same return by calling inflateSetDictionary and continuing: Modules/zlibmodule.c lines 932-938, which is presumably what _ZlibDecompressor should do as well (and raise zlib.error when no zdict was given, as Decompress does).

_ZlibDecompressor is private and gzip only uses it with wbits=-15, so the impact is low; but a SystemError from a C method is always a bug, and the argument is accepted for this case, so we may as well fix it.

Found by the Hypothesis tests in #159960, recorded there as ZlibDecompressorZdictTests.test_zlib_wrapper under expectedFailure.

CPython versions tested on

CPython main (4e3d5ec), zlib 1.3.2

Operating systems tested on

Linux