[compiler-segfault]

zlib._ZlibDecompressor(zdict=...) only installs its dictionary in the
constructor, and the constructor only does that for raw deflate (wbits < 0).
For a zlib-wrapped stream zlib reports Z_NEED_DICT from inflate() after it
has read the header, and decompress_buf() handled that return code by
jumping to error without setting an exception. The method therefore returned
NULL with no exception set:

>>> import zlib
>>> zd = b"hello world " * 10
>>> c = zlib.compressobj(zdict=zd)
>>> s = c.compress(b"hello world hello") + c.flush()
>>> zlib._ZlibDecompressor(zdict=zd).decompress(s)
SystemError: <method 'decompress' of 'zlib._ZlibDecompressor' objects> returned NULL without setting an exception

This applies the dictionary from the decompression loop when one was supplied,
and raises zlib.error when the stream asks for a dictionary the caller never
provided -- which is what zlib.Decompress and zlib.decompressobj already
do for the same input.

The error path also covers the FDICT-only trigger from the comments: an 8-byte
zlib header with the FDICT flag set used to raise SystemError (and trip the
fatal assertion in ceval.c on a debug build) even when no zdict was passed,
because self->zdict == NULL was not distinguished from a dictionary that
still has to be installed.

Tests added to Lib/test/test_zlib.py:

• testDecompressZdictWrapped and testDecompressZdictWrappedChunks: the
  zdict is honored for zlib-wrapped streams, including input fed three bytes
  at a time so the Z_NEED_DICT return happens on a later call than the first.
• testDecompressZdictRaw: raw deflate continues to work.
• testDecompressFdictHeaderWithoutZdict: the FDICT header without a zdict
  raises zlib.error instead of SystemError.

./python -m test test_zlib passes. With the C change reverted,
testDecompressZdictWrapped, testDecompressZdictWrappedChunks and
testDecompressFdictHeaderWithoutZdict fail, while testDecompressZdictRaw
still passes.

[python-cla-bot]

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