pythongh-129726: Break gzip.GzipFile reference loop

A reference loop was resulting in the `fileobj` held by the `GzipFile`
being closed before the `GzipFile`.

The issue started with pythongh-89550 in 3.12, but was hidden in most cases
until 3.13 when pythongh-62948 made it more visible.

Author: cmaloney

Lib/gzip.py

@@ -10,6 +10,7 @@
 import builtins
 import io
 import _compression
+import weakref
 
 __all__ = ["BadGzipFile", "GzipFile", "open", "compress", "decompress"]
 
@@ -226,7 +227,8 @@ def __init__(self, filename=None, mode=None,
                                              0)
             self._write_mtime = mtime
             self._buffer_size = _WRITE_BUFFER_SIZE
-            self._buffer = io.BufferedWriter(_WriteBufferStream(self),
+            write_wrap = _WriteBufferStream(weakref.proxy(self))
+            self._buffer = io.BufferedWriter(write_wrap,
                                              buffer_size=self._buffer_size)
         else:
             raise ValueError("Invalid mode: {!r}".format(mode))

Lib/test/test_gzip.py

@@ -3,12 +3,14 @@
 
 import array
 import functools
+import gc
 import io
 import os
 import struct
 import sys
 import unittest
 from subprocess import PIPE, Popen
+from test.support import catch_unraisable_exception
 from test.support import import_helper
 from test.support import os_helper
 from test.support import _4G, bigmemtest, requires_subprocess
@@ -859,6 +861,17 @@ def test_write_seek_write(self):
         self.assertEqual(gzip.decompress(data), message * 2)
 
 
+    def test_refloop_unraisable(self):
+        # Ensure a GzipFile referring to a temporary fileobj deletes cleanly.
+        # Previously an unraisable exception would occur on close because the
+        # fileobj would be closed before the GzipFile as the result of a
+        # reference loop. See issue gh-129726
+        with catch_unraisable_exception() as cm:
+            gzip.GzipFile(fileobj=io.BytesIO(), mode="w")
+            gc.collect()
+            self.assertIsNone(cm.unraisable)
+
+
 class TestOpen(BaseTest):
     def test_binary_modes(self):
         uncompressed = data1 * 50

Misc/NEWS.d/next/Library/2025-02-12-12-38-24.gh-issue-129726.jB0sxu.rst

@@ -0,0 +1,3 @@
+Fix :class:`gzip.GzipFile` raising an unraisable exception during garbage
+collection when referring to a temporary object by breaking the reference
+loop with :mod:`weakref`.