zlib.Decompress.unconsumed_tail not cleared at end of stream (regression from gh-158169) #158398

The test test_decompress_unconsumed_tail_after_eof (added in gh-158169 / #158177) fails on main:

FAIL: test_decompress_unconsumed_tail_after_eof
AssertionError: b'0123456789' != b''

Reproducer:

import zlib

source = b'abcdefghijklmnopqrstuvwxyz'
remainder = b'0123456789'
dco = zlib.decompressobj()
data = dco.decompress(zlib.compress(source) + remainder, 1)
data += dco.decompress(dco.unconsumed_tail)

assert dco.eof
assert data == source
assert dco.unconsumed_tail == b''
assert dco.unused_data == remainder

Root cause:

Commit 8122ff4 (gh-158169) added this line to save_unconsumed_input() in Modules/zlibmodule.c:

    self->zst.next_in += left_size;

This advances next_in past the leftover input. The following block then computes:

    Py_ssize_t left_size = (Byte *)data->buf + data->len - self->zst.next_in;

Since next_in was already advanced, left_size is computed incorrectly, so unconsumed_tail is not cleared at end of stream.

Linked PRs

• gh-158398: Clear unconsumed_tail at end of stream #158408