GeezerVison

A Z_SYNC_FLUSH or Z_FULL_FLUSH with nothing compressed since the previous flush makes ISA-L write an empty block: 6 bytes at levels 1 to 3, and 117 bytes at level 0, where every block starts with a Huffman table. Each such flush also allocates a 16 KiB output buffer. zlib returns b"" in this case. IGzipFile.flush() calls flush(Z_SYNC_FLUSH), so a program that flushes a gzip file often without writing in between makes the file grow with every flush.

With this PR, Compress objects remember the mode of their last flush:

• A sync flush after a sync or full flush, or a full flush after a full flush, with no compress() call in between, returns b"".
• A full flush after a sync flush still writes a block, because it resets the history.
• compress() forgets the mode even for b"", as zlib's deflate() does, so compress(b"") followed by flush(Z_SYNC_FLUSH) still ends in 00 00 ff ff. permessage-deflate needs that for empty messages.
• Z_FINISH forgets it too, so flushes after finishing work as before.

One difference from zlib remains. For a full, a sync and a full flush with nothing in between, zlib writes an empty block on the third call, and this PR returns b"". Nothing was compressed since the first full flush, so the history is already reset.

I measured with pyperf in Docker on Linux, as the mean of 2 runs on Python 3.11 and 3.14:

One write followed by 100 IGzipFile.flush() calls now gives 29 bytes instead of 623 at levels 1 to 3 (the gzip module gives 30), and 253 bytes instead of 11836 at level 0. A flush after new data costs 8 more instructions out of about 55,000 for a compress() of 100 bytes and a flush(), which wall-clock time does not show.

The output changes only for back-to-back flushes, and the stream stays valid. The new tests cover repeated flushes at every level for raw, zlib and gzip streams, a flush before any data, a full flush after a sync flush, compress(b"") keeping the sync marker, a repeated sync flush after a zdict header, and IGzipFile.flush() twice in a row. They fail on develop. The test suite passes on this branch on Linux with CPython 3.11 and 3.14.

Checklist

• Pull request details were added to CHANGELOG.rst
• Documentation was updated (if needed)