Fix trailer subprocess lifetime

Agent-Logs-Url: https://github.com/gitpython-developers/GitPython/sessions/3cc0bd6d-d54d-4299-9a18-1576c2a91c12

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>

Author: Copilot

git/objects/commit.py

@@ -470,16 +470,18 @@ def _interpret_trailers(
         trailer_args: Sequence[str],
         encoding: str = default_encoding,
     ) -> str:
+        message_bytes = message if isinstance(message, bytes) else message.encode(encoding, errors="strict")
         cmd = [repo.git.GIT_PYTHON_GIT_EXECUTABLE, "interpret-trailers", *trailer_args]
         proc: Git.AutoInterrupt = repo.git.execute(  # type: ignore[call-overload]
             cmd,
             as_process=True,
             istream=PIPE,
         )
-        message_bytes = message if isinstance(message, bytes) else message.encode(encoding, errors="strict")
-        stdout_bytes, _ = proc.communicate(message_bytes)
-        finalize_process(proc)
-        return stdout_bytes.decode(encoding, errors="strict")
+        try:
+            stdout_bytes, _ = proc.communicate(message_bytes)
+            return stdout_bytes.decode(encoding, errors="strict")
+        finally:
+            finalize_process(proc)
 
     @property
     def trailers_dict(self) -> Dict[str, List[str]]:

test/test_commit.py

@@ -676,6 +676,17 @@ def test_trailers_list_with_non_utf8_message_bytes(self, rw_dir):
 
         assert bytes_commit.trailers_list == [("Reviewed-by", "André <andre@example.com>")]
 
+    def test_interpret_trailers_encodes_before_launching_process(self):
+        """Test that encoding failures happen before spawning interpret-trailers."""
+        repo = Mock()
+        repo.git = Mock()
+        repo.git.GIT_PYTHON_GIT_EXECUTABLE = "git"
+
+        with self.assertRaises(UnicodeEncodeError):
+            Commit._interpret_trailers(repo, "Euro: €", ["--parse"], encoding="ISO-8859-1")
+
+        repo.git.execute.assert_not_called()
+
     @with_rw_directory
     def test_index_commit_with_trailers(self, rw_dir):
         """Test that IndexFile.commit() supports adding trailers."""