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gh-160000: Fix LZMADecompressor raising on repeated empty decompress calls #160205

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gh-160000: Fix LZMADecompressor raising on repeated empty decompress calls #160205

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14 changes: 14 additions & 0 deletions Lib/test/test_lzma.py
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,20 @@ def test_decompressor_after_eof(self):
lzd.decompress(COMPRESSED_XZ)
self.assertRaises(EOFError, lzd.decompress, b"nyan")

def test_decompressor_empty_max_length_zero_repeated(self):
# gh-160000: repeated decompress(b'', max_length=0) calls must
# return b'' instead of raising LZMAError on the second call.
lzd = LZMADecompressor()
for _ in range(3):
self.assertEqual(lzd.decompress(b'', max_length=0), b'')
# Same after real input: one byte out, then empty calls,

@pganssle pganssle Oct 22, 2026

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I think this can be split into separate tests or parameterized in some way.

# then the rest of the stream still decompresses.
lzd = LZMADecompressor()
self.assertEqual(lzd.decompress(COMPRESSED_XZ, 1), INPUT[:1])
for _ in range(3):
self.assertEqual(lzd.decompress(b'', max_length=0), b'')
self.assertEqual(lzd.decompress(b''), INPUT[1:])

def test_decompressor_memlimit(self):
lzd = LZMADecompressor(memlimit=1024)
self.assertRaises(LZMAError, lzd.decompress, COMPRESSED_XZ)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-10-22-10-55-00.gh-issue-160000.a1b2c3.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix :class:`lzma.LZMADecompressor` raising :exc:`LZMAError` on repeated
``decompress(b'', max_length=0)`` calls.
Comment on lines +1 to +2

@emmatyping emmatyping Oct 23, 2026

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Suggested change
Fix :class:`lzma.LZMADecompressor` raising :exc:`LZMAError` on repeated
``decompress(b'', max_length=0)`` calls.
Fix :class:`lzma.LZMADecompressor` raising :exc:`LZMAError` on repeated
``decompress()`` calls with ``max_length=0``.

3 changes: 2 additions & 1 deletion Modules/_lzmamodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -959,7 +959,8 @@ decompress_buf(Decompressor *d, Py_ssize_t max_length)
lzret = lzma_code(lzs, LZMA_RUN);
Py_END_ALLOW_THREADS

if (lzret == LZMA_BUF_ERROR && lzs->avail_in == 0 && lzs->avail_out > 0) {
if (lzret == LZMA_BUF_ERROR &&
(lzs->avail_in == 0 || lzs->avail_out == 0)) {
lzret = LZMA_OK; /* That wasn't a real error */
Comment on lines +962 to 964

@emmatyping emmatyping Oct 23, 2026

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Rather than ignoring the error, we should add the input bytes to the input buffer, then return empty bytes without calling the library.

}
if (catch_lzma_error(state, lzret)) {
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