feat: Wrap libgit2 git_message_trailers

git-log has a formatting function to extract trailers from the commit
message field. The libgit2 function `git_message_trailers` implements
this logic and seems to catch a lot of corner cases.

Rather than ask users to extract trailers from commit messages by
fiddling with the `message` attribute of the `Commit` object, wrap the
libgit2 implementation.

Author: thcipriani

docs/recipes/git-log.rst

@@ -34,6 +34,21 @@ Traverse commit history
     >>> for commit in repo.walk(last.id, pygit2.GIT_SORT_TIME):
     >>>     print(commit.message) # or some other operation
 
+======================================================================
+Show trailers from the last commit
+======================================================================
+
+.. code-block:: bash
+
+    $ git log --format='%(trailers:key=Bug)'
+
+.. code-block:: python
+
+    >>> last = repo[repo.head.target]
+    >>> for commit in repo.walk(last.id, pygit2.GIT_SORT_TIME):
+    >>>     print(commit.message_trailers.get('Bug'))
+
+
 ----------------------------------------------------------------------
 References
 ----------------------------------------------------------------------

src/commit.c

@@ -98,6 +98,46 @@ Commit_gpg_signature__get__(Commit *self)
 }
 
 
+PyDoc_STRVAR(Commit_message_trailers__doc__,
+    "Returns commit message trailers (e.g., Bug: 1234) as a dictionary."
+);
+
+PyObject *
+Commit_message_trailers__get__(Commit *self)
+{
+    git_message_trailer_array gmt_arr;
+    int i, trailer_count, err;
+    PyObject *dict;
+    PyObject *py_val;
+    const char *message = git_commit_message(self->commit);
+    const char *encoding = git_commit_message_encoding(self->commit);
+
+    err = git_message_trailers(&gmt_arr, message);
+    if (err < 0)
+        return Error_set(err);
+
+    dict = PyDict_New();
+    if (dict == NULL)
+        return NULL;
+
+    trailer_count = gmt_arr.count;
+    for (i=0; i < trailer_count; i++) {
+        py_val = to_unicode(gmt_arr.trailers[i].value, encoding, NULL);
+        err = PyDict_SetItemString(dict, gmt_arr.trailers[i].key, py_val);
+        if (err < 0)
+            goto error;
+
+    }
+
+    git_message_trailer_array_free(&gmt_arr);
+    return dict;
+
+error:
+    git_message_trailer_array_free(&gmt_arr);
+    Py_CLEAR(dict);
+    return NULL;
+}
+
 PyDoc_STRVAR(Commit_raw_message__doc__, "Message (bytes).");
 
 PyObject *
@@ -270,6 +310,7 @@ PyGetSetDef Commit_getseters[] = {
     GETTER(Commit, tree_id),
     GETTER(Commit, parents),
     GETTER(Commit, parent_ids),
+    GETTER(Commit, message_trailers),
     {NULL}
 };
 

src/commit.h

@@ -35,6 +35,7 @@
 PyObject* Commit_get_message_encoding(Commit *commit);
 PyObject* Commit_get_message(Commit *commit);
 PyObject* Commit_get_raw_message(Commit *commit);
+PyObject* Commit_get_message_trailers(Commit *commit);
 PyObject* Commit_get_commit_time(Commit *commit);
 PyObject* Commit_get_commit_time_offset(Commit *commit);
 PyObject* Commit_get_committer(Commit *self);

test/data/trailerrepo.tar

@@ -0,0 +1,48 @@
+# Copyright 2010-2021 The pygit2 contributors
+#
+# This file is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License, version 2,
+# as published by the Free Software Foundation.
+#
+# In addition to the permissions in the GNU General Public License,
+# the authors give you unlimited permission to link the compiled
+# version of this file into combinations with other programs,
+# and to distribute those combinations without any restriction
+# coming from the use of this file.  (The General Public License
+# restrictions do apply in other respects; for example, they cover
+# modification of the file, and distribution when not linked into
+# a combined executable.)
+#
+# This file is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING.  If not, write to
+# the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+# Boston, MA 02110-1301, USA.
+
+import pygit2
+import pytest
+
+from . import utils
+
+
+@pytest.fixture
+def repo(tmp_path):
+    with utils.TemporaryRepository('trailerrepo.tar', tmp_path) as path:
+        yield pygit2.Repository(path)
+
+
+def test_get_trailers_array(repo):
+    commit_hash = '010231b2fdaee6b21da4f06058cf6c6a3392dd12'
+    expected_trailers = {
+        'Bug': '1234',
+        'Signed-off-by': 'Tyler Cipriani <[email protected]>',
+    }
+    commit = repo.get(commit_hash)
+    trailers = commit.message_trailers
+
+    assert trailers['Bug'] == expected_trailers['Bug']
+    assert trailers['Signed-off-by'] == expected_trailers['Signed-off-by']