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

Author: bunnysayzz

Lib/test/test_lzma.py

@@ -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,
+        # 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)

Misc/NEWS.d/next/Library/2026-10-22-10-55-00.gh-issue-160000.a1b2c3.rst

@@ -0,0 +1,2 @@
+Fix :class:`lzma.LZMADecompressor` raising :exc:`LZMAError` on repeated
+``decompress(b'', max_length=0)`` calls.

Modules/_lzmamodule.c

@@ -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 */
         }
         if (catch_lzma_error(state, lzret)) {