commit: Add support for getting GPG signature data

This change adds a new Commit.gpg_signature property which returns a
tuple containing the GPG signature for a given commit and the bytes that
the signature was computed over. This is accomplished by calling
libgit2's git_commit_extract_signature() function.

Author: mark-adams

src/commit.c

@@ -65,6 +65,38 @@ Commit_message__get__(Commit *commit)
     return to_unicode(message, encoding, "strict");
 }
 
+PyDoc_STRVAR(Commit_gpg_signature__doc__, "A tuple with the GPG signature and the signed payload.");
+
+PyObject *
+Commit_gpg_signature__get__(Commit *commit)
+{
+    git_buf gpg_signature = { NULL }, signed_data = { NULL };
+
+    const git_oid *oid = git_commit_id(commit->commit);
+    int err = git_commit_extract_signature(
+        &gpg_signature, &signed_data, commit->repo->repo, oid, NULL
+    );
+
+    if (err != GIT_OK){
+        git_buf_free(&gpg_signature);
+        git_buf_free(&signed_data);
+
+        if (err == GIT_ENOTFOUND){
+            Py_INCREF(Py_None); Py_INCREF(Py_None);
+            return Py_BuildValue("OO", Py_None, Py_None);
+        }
+
+        return Error_set(err);
+    }
+
+    PyObject *py_gpg_signature = PyBytes_FromString(gpg_signature.ptr);
+    PyObject *py_signed_data = PyBytes_FromString(signed_data.ptr);
+    git_buf_free(&gpg_signature);
+    git_buf_free(&signed_data);
+
+    return Py_BuildValue("OO", py_gpg_signature, py_signed_data);
+}
+
 
 PyDoc_STRVAR(Commit_raw_message__doc__, "Message (bytes).");
 
@@ -233,6 +265,7 @@ PyGetSetDef Commit_getseters[] = {
     GETTER(Commit, commit_time_offset),
     GETTER(Commit, committer),
     GETTER(Commit, author),
+    GETTER(Commit, gpg_signature),
     GETTER(Commit, tree),
     GETTER(Commit, tree_id),
     GETTER(Commit, parents),

test/data/gpgsigned.tar

@@ -160,5 +160,43 @@ def test_modify_commit(self):
         self.assertRaises(error_type, setattr, commit, 'parents', None)
 
 
+class GpgSignatureTestCase(utils.GpgSignedRepoTestCase):
+    signed_hash = 'a00b212d5455ad8c4c1779f778c7d2a81bb5da23'
+    unsigned_hash = 'a84938d1d885e80dae24b86b06621cec47ff6edd'
+
+    def test_get_gpg_signature_when_signed(self):
+        expected_signature = (
+            '-----BEGIN PGP SIGNATURE-----\n\n'
+            'iQFGBAABCgAwFiEEQZu9JtePgJbDk7VC0+mlK74z13oFAlpzXykSHG1hcmtAbWFy\n'
+            'a2FkYW1zLm1lAAoJENPppSu+M9d6FRoIAJXeQRRT1V47nnHITiel6426loYkeij7\n'
+            '66doGNIyll95H92SwH4LAjPyEEByIG1VsA6NztzUoNgnEvAXI0iAz3LyI7N16M4b\n'
+            'dPDkC72pp8tu280H5Qt5b2V5hmlKKSgtOS5iNhdU/FbWVS8MlHsqzQTZfoTdi6ch\n'
+            'KWUsjzudVd3F/H/AU+1Jsxt8Iz/oK4T/puUQLnJZKjKlljGP994FA3JIpnZpZmbG\n'
+            'FybYJEDXnng7uhx3Fz/Mo3KBJoQfAExTtaToY0n0hSjOe6GN9rEsRSMK3mWdysf2\n'
+            'wOdtYMMcT16hG5tAwnD/myZ4rIIpyZJ/9mjymdUsj6UKf7D+vJuqfsI=\n=IyYy\n'
+            '-----END PGP SIGNATURE-----'
+        ).encode('ascii')
+
+        expected_payload = (
+            'tree c36c20831e43e5984c672a714661870b67ab1d95\nauthor Mark Adams '
+            '<[email protected]> 1517510299 -0600\ncommitter Mark Adams <ma'
+            '[email protected]> 1517510441 -0600\n\nMaking a GPG signed commi'
+            't\n'
+        ).encode('ascii')
+
+        commit = self.repo.get(self.signed_hash)
+        signature, payload = commit.gpg_signature
+
+        assert signature == expected_signature
+        assert payload == expected_payload
+
+    def test_get_gpg_signature_when_unsigned(self):
+        commit = self.repo.get(self.unsigned_hash)
+        signature, payload = commit.gpg_signature
+
+        assert signature is None
+        assert payload is None
+
+
 if __name__ == '__main__':
     unittest.main()

test/test_commit.py

@@ -186,3 +186,8 @@ class SubmoduleRepoTestCase(AutoRepoTestCase):
 class BinaryFileRepoTestCase(AutoRepoTestCase):
 
     repo_spec = 'tar', 'binaryfilerepo'
+
+
+class GpgSignedRepoTestCase(AutoRepoTestCase):
+
+    repo_spec = 'tar', 'gpgsigned'