fix: preserve diff process stderr

puneetdixit200 · codex · puneetdixit200 · commit 59cc3bb1f5d4 · 2026-05-24T00:13:13.000+05:30
Co-authored-by: OpenAI Codex &lt;codex@openai.com&gt;
diff --git a/git/diff.py b/git/diff.py
@@ -602,7 +602,14 @@ def _index_from_patch_format(cls, repo: "Repo", proc: Union["Popen", "Git.AutoIn
 
         # FIXME: Here SLURPING raw, need to re-phrase header-regexes linewise.
         text_list: List[bytes] = []
-        handle_process_output(proc, text_list.append, None, finalize_process, decode_streams=False)
+        stderr_list: List[bytes] = []
+
+        def finalize_process_with_stderr(proc: Union["Popen", "Git.AutoInterrupt"]) -> None:
+            finalize_process(proc, stderr=b"".join(stderr_list))
+
+        handle_process_output(
+            proc, text_list.append, stderr_list.append, finalize_process_with_stderr, decode_streams=False
+        )
 
         # For now, we have to bake the stream.
         text = b"".join(text_list)
@@ -768,11 +775,16 @@ def _index_from_raw_format(cls, repo: "Repo", proc: "Popen") -> "DiffIndex[Diff]
         # :100644 100644 687099101... 37c5e30c8... M    .gitignore
 
         index: "DiffIndex" = DiffIndex()
+        stderr_list: List[bytes] = []
+
+        def finalize_process_with_stderr(proc: Union["Popen", "Git.AutoInterrupt"]) -> None:
+            finalize_process(proc, stderr=b"".join(stderr_list))
+
         handle_process_output(
             proc,
             lambda byt: cls._handle_diff_line(byt, repo, index),
-            None,
-            finalize_process,
+            stderr_list.append,
+            finalize_process_with_stderr,
             decode_streams=False,
         )
 
diff --git a/test/test_index.py b/test/test_index.py
@@ -585,6 +585,10 @@ def test_index_file_diff_null_tree_with_initial_index(self, rw_dir):
         self.assertEqual(len(patch), 1)
         self.assertIn(b"+# Initial file", patch[0].diff)
 
+        with self.assertRaises(GitCommandError) as exc_info:
+            index.diff(NULL_TREE, bogus_option=True)
+        self.assertIn("usage: git diff", exc_info.exception.stderr)
+
     def _count_existing(self, repo, files):
         """Return count of files that actually exist in the repository directory."""
         existing = 0
