[3.14] gh-158169: Clear zlib.Decompress.unconsumed_tail at end of stream (GH-158177)

(cherry picked from commit 8122ff4)

Co-authored-by: GruffElixir <samarthat9@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>

Author: GruffElixir

Lib/test/test_zlib.py

@@ -651,6 +651,18 @@ def test_decompress_unused_data(self):
                 self.assertEqual(dco.unconsumed_tail, b'')
                 self.assertEqual(dco.unused_data, remainder)
 
+    def test_decompress_unconsumed_tail_after_eof(self):
+        source = b'abcdefghijklmnopqrstuvwxyz'
+        remainder = b'0123456789'
+        dco = zlib.decompressobj()
+        data = dco.decompress(zlib.compress(source) + remainder, 1)
+        data += dco.decompress(dco.unconsumed_tail)
+
+        self.assertTrue(dco.eof)
+        self.assertEqual(data, source)
+        self.assertEqual(dco.unconsumed_tail, b'')
+        self.assertEqual(dco.unused_data, remainder)
+
     # issue27164
     def test_decompress_raw_with_dictionary(self):
         zdict = b'abcdefghijklmnopqrstuvwxyz'

Misc/NEWS.d/next/Library/2026-09-25-22-15-54.gh-issue-158169.ZlibTail.rst

@@ -0,0 +1,2 @@
+Fix :attr:`zlib.Decompress.unconsumed_tail` duplicating
+:attr:`~zlib.Decompress.unused_data` after end of stream when *max_length* was used.

Modules/zlibmodule.c

@@ -842,6 +842,7 @@ save_unconsumed_input(compobject *self, Py_buffer *data, int err)
             memcpy(PyBytes_AS_STRING(new_data) + old_size,
                       self->zst.next_in, left_size);
             Py_SETREF(self->unused_data, new_data);
+            self->zst.next_in += left_size;
             self->zst.avail_in = 0;
         }
     }