Introduce git_commit_extract_signature

This returns the GPG signature for a commit and its contents without the
signature block, allowing for the verification of the commit's
signature.

include/git2/commit.h

@@ -264,6 +264,18 @@ GIT_EXTERN(int) git_commit_nth_gen_ancestor(
 GIT_EXTERN(int) git_commit_header_field(git_buf *out, const git_commit *commit, const char *field);
 
 /**
+ * Extract the signature from a commit
+ *
+ * @param signature the signature block
+ * @param signed_data signed data; this is the commit contents minus the signature block
+ * @param repo the repository in which the commit exists
+ * @param commit_id the commit from which to extract the data
+ * @param field the name of the header field containing the signature
+ * block; pass `NULL` to extract the default 'gpgsig'
+ */
+GIT_EXTERN(int) git_commit_extract_signature(git_buf *signature, git_buf *signed_data, git_repository *repo, git_oid *commit_id, const char *field);
+
+/**
  * Create new commit in the repository from a list of `git_object` pointers
  *
  * The message will **not** be cleaned up automatically. You can do that

src/commit.c

@@ -616,3 +616,89 @@ oom:
 	giterr_set_oom();
 	return -1;
 }
+
+int git_commit_extract_signature(git_buf *signature, git_buf *signed_data, git_repository *repo, git_oid *commit_id, const char *field)
+{
+	git_odb_object *obj;
+	git_odb *odb;
+	const char *buf;
+	const char *h, *eol;
+	int error;
+
+	git_buf_sanitize(signature);
+	git_buf_sanitize(signed_data);
+
+	if (!field)
+		field = "gpgsig";
+
+	if ((error = git_repository_odb__weakptr(&odb, repo)) < 0)
+		return error;
+
+	if ((error = git_odb_read(&obj, odb, commit_id)) < 0)
+		return error;
+
+	buf = git_odb_object_data(obj);
+
+	while ((h = strchr(buf, '\n')) && h[1] != '\0' && h[1] != '\n') {
+		h++;
+		if (git__prefixcmp(buf, field)) {
+			if (git_buf_put(signed_data, buf, h - buf) < 0)
+				return -1;
+
+			buf = h;
+			continue;
+		}
+
+		h = buf;
+		h += strlen(field);
+		eol = strchr(h, '\n');
+		if (h[0] != ' ') {
+			buf = h;
+			continue;
+		}
+		if (!eol)
+			goto malformed;
+
+		h++; /* skip the SP */
+
+		git_buf_put(signature, h, eol - h);
+		if (git_buf_oom(signature))
+			goto oom;
+
+		/* If the next line starts with SP, it's multi-line, we must continue */
+		while (eol[1] == ' ') {
+			git_buf_putc(signature, '\n');
+			h = eol + 2;
+			eol = strchr(h, '\n');
+			if (!eol)
+				goto malformed;
+
+			git_buf_put(signature, h, eol - h);
+		}
+
+		if (git_buf_oom(signature))
+			goto oom;
+
+		git_odb_object_free(obj);
+		return git_buf_puts(signed_data, eol+1);
+	}
+
+	giterr_set(GITERR_INVALID, "this commit is not signed");
+	error = GIT_ENOTFOUND;
+	goto cleanup;
+
+malformed:
+	giterr_set(GITERR_OBJECT, "malformed header");
+	error = -1;
+	goto cleanup;
+oom:
+	giterr_set_oom();
+	error = -1;
+	goto cleanup;
+
+cleanup:
+	git_odb_object_free(obj);
+	git_buf_clear(signature);
+	git_buf_clear(signed_data);
+	return error;
+}

tests/commit/parse.c

@@ -456,3 +456,52 @@ cpxtDQQMGYFpXK/71stq\n\
 	git_buf_free(&buf);
 	git_commit__free(commit);
 }
+
+void test_commit_parse__extract_signature(void)
+{
+	git_odb *odb;
+	git_oid commit_id;
+	git_buf signature = GIT_BUF_INIT, signed_data = GIT_BUF_INIT;
+	const char *gpgsig = "-----BEGIN PGP SIGNATURE-----\n\
+Version: GnuPG v1.4.12 (Darwin)\n\
+\n\
+iQIcBAABAgAGBQJQ+FMIAAoJEH+LfPdZDSs1e3EQAJMjhqjWF+WkGLHju7pTw2al\n\
+o6IoMAhv0Z/LHlWhzBd9e7JeCnanRt12bAU7yvYp9+Z+z+dbwqLwDoFp8LVuigl8\n\
+JGLcnwiUW3rSvhjdCp9irdb4+bhKUnKUzSdsR2CK4/hC0N2i/HOvMYX+BRsvqweq\n\
+AsAkA6dAWh+gAfedrBUkCTGhlNYoetjdakWqlGL1TiKAefEZrtA1TpPkGn92vbLq\n\
+SphFRUY9hVn1ZBWrT3hEpvAIcZag3rTOiRVT1X1flj8B2vGCEr3RrcwOIZikpdaW\n\
+who/X3xh/DGbI2RbuxmmJpxxP/8dsVchRJJzBwG+yhwU/iN3MlV2c5D69tls/Dok\n\
+6VbyU4lm/ae0y3yR83D9dUlkycOnmmlBAHKIZ9qUts9X7mWJf0+yy2QxJVpjaTGG\n\
+cmnQKKPeNIhGJk2ENnnnzjEve7L7YJQF6itbx5VCOcsGh3Ocb3YR7DMdWjt7f8pu\n\
+c6j+q1rP7EpE2afUN/geSlp5i3x8aXZPDj67jImbVCE/Q1X9voCtyzGJH7MXR0N9\n\
+ZpRF8yzveRfMH8bwAJjSOGAFF5XkcR/RNY95o+J+QcgBLdX48h+ZdNmUf6jqlu3J\n\
+7KmTXXQcOVpN6dD3CmRFsbjq+x6RHwa8u1iGn+oIkX908r97ckfB/kHKH7ZdXIJc\n\
+cpxtDQQMGYFpXK/71stq\n\
+=ozeK\n\
+-----END PGP SIGNATURE-----";
+
+	const char *data =  "tree 6b79e22d69bf46e289df0345a14ca059dfc9bdf6\n\
+parent 34734e478d6cf50c27c9d69026d93974d052c454\n\
+author Ben Burkert <ben@benburkert.com> 1358451456 -0800\n\
+committer Ben Burkert <ben@benburkert.com> 1358451456 -0800\n\
+\n\
+a simple commit which works\n";
+
+
+	cl_git_pass(git_repository_odb__weakptr(&odb, g_repo));
+	cl_git_pass(git_odb_write(&commit_id, odb, passing_commit_cases[4], strlen(passing_commit_cases[4]), GIT_OBJ_COMMIT));
+
+	cl_git_pass(git_commit_extract_signature(&signature, &signed_data, g_repo, &commit_id, NULL));
+	cl_assert_equal_s(gpgsig, signature.ptr);
+	cl_assert_equal_s(data, signed_data.ptr);
+
+	git_buf_clear(&signature);
+	git_buf_clear(&signed_data);
+
+	cl_git_pass(git_commit_extract_signature(&signature, &signed_data, g_repo, &commit_id, "gpgsig"));
+	cl_assert_equal_s(gpgsig, signature.ptr);
+	cl_assert_equal_s(data, signed_data.ptr);
+
+	git_buf_free(&signature);
+	git_buf_free(&signed_data);
+}