[3.13] gh-156173: Fix zlib.Decompress.flush() silently returning corrupted output instead of raising zlib.error (GH-156176) (#156272)

(cherry picked from commit 5442687)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Lib/test/test_zlib.py

@@ -611,6 +611,20 @@ def test_decompress_eof_incomplete_stream(self):
         dco.flush()
         self.assertFalse(dco.eof)
 
+    def test_decompress_flush_corrupt_stream(self):
+        x = b'x\x9cK\xcb\xcf\x07\x00\x02\x82\x01E'  # 'foo'
+        corrupt = x[:-1] + b'\x00'
+        dco = zlib.decompressobj()
+        self.assertEqual(dco.decompress(corrupt, 1), b'f')
+        self.assertRaises(zlib.error, dco.flush)
+
+    def test_decompress_flush_twice(self):
+        x = b'x\x9cK\xcb\xcf\x07\x00\x02\x82\x01E'  # 'foo'
+        dco = zlib.decompressobj()
+        self.assertEqual(dco.decompress(x), b'foo')
+        self.assertEqual(dco.flush(), b'')
+        self.assertEqual(dco.flush(), b'')
+
     def test_decompress_unused_data(self):
         # Repeated calls to decompress() after EOF should accumulate data in
         # dco.unused_data, instead of just storing the arg to the last call.

Misc/NEWS.d/next/Library/2026-08-21-11-56-28.gh-issue-156173.mhZa8a.rst

@@ -0,0 +1,2 @@
+Calling :meth:`zlib.Decompress.flush` on invalid compressed data now
+raises :exc:`zlib.error` instead of being silently ignored.

Modules/zlibmodule.c

@@ -1277,6 +1277,13 @@ zlib_Decompress_flush_impl(compobject *self, PyTypeObject *cls,
 
     ENTER_ZLIB(self);
 
+    /* A previous flush() already reached the end of the stream and freed the
+       decompression state, so there is nothing left to process. */
+    if (!self->is_initialised) {
+        LEAVE_ZLIB(self);
+        return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+    }
+
     if (PyObject_GetBuffer(self->unconsumed_tail, &data, PyBUF_SIMPLE) == -1) {
         LEAVE_ZLIB(self);
         return NULL;
@@ -1334,6 +1341,10 @@ zlib_Decompress_flush_impl(compobject *self, PyTypeObject *cls,
             goto abort;
         }
     }
+    else if (err != Z_OK && err != Z_BUF_ERROR) {
+        zlib_error(state, self->zst, err, "while decompressing data");
+        goto abort;
+    }
 
     return_value = OutputBuffer_WindowFinish(&buffer, &window, self->zst.avail_out);
     if (return_value != NULL) {