Add trailer support for commit creation

Add a `trailers` parameter to `Commit.create_from_tree()` and
`IndexFile.commit()` that allows appending trailer key-value pairs
(e.g. Signed-off-by, Issue) to the commit message at creation time.

Trailers can be passed as either a dict or a list of (key, value) tuples,
the latter being useful when duplicate keys are needed. The implementation
uses `git interpret-trailers` for proper formatting, consistent with
the existing trailer parsing in `Commit.trailers_list`.

Closes #1998

Author: Krishnachaitanyakc

git/index/base.py

@@ -1133,6 +1133,7 @@ def commit(
         author_date: Union[datetime.datetime, str, None] = None,
         commit_date: Union[datetime.datetime, str, None] = None,
         skip_hooks: bool = False,
+        trailers: Union[None, "Dict[str, str]", "List[Tuple[str, str]]"] = None,
     ) -> Commit:
         """Commit the current default index file, creating a
         :class:`~git.objects.commit.Commit` object.
@@ -1169,6 +1170,7 @@ def commit(
             committer=committer,
             author_date=author_date,
             commit_date=commit_date,
+            trailers=trailers,
         )
         if not skip_hooks:
             run_commit_hook("post-commit", self)

git/objects/commit.py

@@ -570,6 +570,7 @@ def create_from_tree(
         committer: Union[None, Actor] = None,
         author_date: Union[None, str, datetime.datetime] = None,
         commit_date: Union[None, str, datetime.datetime] = None,
+        trailers: Union[None, Dict[str, str], List[Tuple[str, str]]] = None,
     ) -> "Commit":
         """Commit the given tree, creating a :class:`Commit` object.
 
@@ -609,6 +610,14 @@ def create_from_tree(
         :param commit_date:
             The timestamp for the committer field.
 
+        :param trailers:
+            Optional trailer key-value pairs to append to the commit message.
+            Can be a dictionary mapping trailer keys to values, or a list of
+            ``(key, value)`` tuples (useful when the same key appears multiple
+            times, e.g. multiple ``Signed-off-by`` trailers). Trailers are
+            appended using ``git interpret-trailers``.
+            See :manpage:`git-interpret-trailers(1)`.
+
         :return:
             :class:`Commit` object representing the new commit.
 
@@ -678,6 +687,27 @@ def create_from_tree(
             tree = repo.tree(tree)
         # END tree conversion
 
+        # APPLY TRAILERS
+        if trailers:
+            trailer_args: List[str] = []
+            if isinstance(trailers, dict):
+                for key, val in trailers.items():
+                    trailer_args.append("--trailer")
+                    trailer_args.append(f"{key}: {val}")
+            else:
+                for key, val in trailers:
+                    trailer_args.append("--trailer")
+                    trailer_args.append(f"{key}: {val}")
+
+            cmd = ["git", "interpret-trailers"] + trailer_args
+            proc: Git.AutoInterrupt = repo.git.execute(  # type: ignore[call-overload]
+                cmd,
+                as_process=True,
+                istream=PIPE,
+            )
+            message = proc.communicate(str(message).encode())[0].decode("utf8")
+        # END apply trailers
+
         # CREATE NEW COMMIT
         new_commit = cls(
             repo,

test/test_commit.py

@@ -566,3 +566,77 @@ def test_commit_co_authors(self):
             Actor("test_user_2", "another_user-email@github.com"),
             Actor("test_user_3", "test_user_3@github.com"),
         ]
+
+    @with_rw_directory
+    def test_create_from_tree_with_trailers_dict(self, rw_dir):
+        """Test that create_from_tree supports adding trailers via a dict."""
+        rw_repo = Repo.init(osp.join(rw_dir, "test_trailers_dict"))
+        path = osp.join(str(rw_repo.working_tree_dir), "hello.txt")
+        touch(path)
+        rw_repo.index.add([path])
+        tree = rw_repo.index.write_tree()
+
+        trailers = {"Issue": "123", "Signed-off-by": "Test User <test@test.com>"}
+        commit = Commit.create_from_tree(
+            rw_repo,
+            tree,
+            "Test commit with trailers",
+            head=True,
+            trailers=trailers,
+        )
+
+        assert "Issue: 123" in commit.message
+        assert "Signed-off-by: Test User <test@test.com>" in commit.message
+        assert commit.trailers_dict == {
+            "Issue": ["123"],
+            "Signed-off-by": ["Test User <test@test.com>"],
+        }
+
+    @with_rw_directory
+    def test_create_from_tree_with_trailers_list(self, rw_dir):
+        """Test that create_from_tree supports adding trailers via a list of tuples."""
+        rw_repo = Repo.init(osp.join(rw_dir, "test_trailers_list"))
+        path = osp.join(str(rw_repo.working_tree_dir), "hello.txt")
+        touch(path)
+        rw_repo.index.add([path])
+        tree = rw_repo.index.write_tree()
+
+        trailers = [
+            ("Signed-off-by", "Alice <alice@example.com>"),
+            ("Signed-off-by", "Bob <bob@example.com>"),
+            ("Issue", "456"),
+        ]
+        commit = Commit.create_from_tree(
+            rw_repo,
+            tree,
+            "Test commit with multiple trailers",
+            head=True,
+            trailers=trailers,
+        )
+
+        assert "Signed-off-by: Alice <alice@example.com>" in commit.message
+        assert "Signed-off-by: Bob <bob@example.com>" in commit.message
+        assert "Issue: 456" in commit.message
+        assert commit.trailers_dict == {
+            "Signed-off-by": ["Alice <alice@example.com>", "Bob <bob@example.com>"],
+            "Issue": ["456"],
+        }
+
+    @with_rw_directory
+    def test_index_commit_with_trailers(self, rw_dir):
+        """Test that IndexFile.commit() supports adding trailers."""
+        rw_repo = Repo.init(osp.join(rw_dir, "test_index_trailers"))
+        path = osp.join(str(rw_repo.working_tree_dir), "hello.txt")
+        touch(path)
+        rw_repo.index.add([path])
+
+        trailers = {"Reviewed-by": "Reviewer <reviewer@example.com>"}
+        commit = rw_repo.index.commit(
+            "Test index commit with trailers",
+            trailers=trailers,
+        )
+
+        assert "Reviewed-by: Reviewer <reviewer@example.com>" in commit.message
+        assert commit.trailers_dict == {
+            "Reviewed-by": ["Reviewer <reviewer@example.com>"],
+        }