commit: support custom user-specified headers

Some clients (eg GitButler) are storing additional information in custom
user-specified commit headers. We should make this a first-class
concept.

include/git2/commit.h

@@ -363,6 +363,11 @@ GIT_EXTERN(int) git_commit_create_from_stage(
 	const char *message,
 	const git_commit_create_options *opts);
 
+/** The field name and value for a custom commit header entry. */
+typedef struct {
+	const char *field;
+	const char *value;
+} git_commit_header;
 
 typedef struct {
 	unsigned int version;
@@ -380,6 +385,13 @@ typedef struct {
 
 	/** Encoding for the commit message; leave NULL for default. */
 	const char *message_encoding;
+
+	/**
+	 * Extra headers can be specified as an array of field name and
+	 * value pairs.
+	 */
+	const git_commit_header *extra_headers;
+	size_t extra_headers_len; /**< Number of extra headers */
 } git_commit_create_ext_options;
 
 /** Current version for the `git_commit_create_ext_options` structure */

src/libgit2/commit.c

@@ -42,6 +42,32 @@ void git_commit__free(void *_commit)
 	git__free(commit);
 }
 
+/**
+ * Append to 'out' properly marking continuations when there's a newline in 'content'
+ */
+static int format_header_field(git_str *out, const char *field, const char *content)
+{
+	const char *lf;
+
+	GIT_ASSERT_ARG(out);
+	GIT_ASSERT_ARG(field);
+	GIT_ASSERT_ARG(content);
+
+	git_str_puts(out, field);
+	git_str_putc(out, ' ');
+
+	while ((lf = strchr(content, '\n')) != NULL) {
+		git_str_put(out, content, lf - content);
+		git_str_puts(out, "\n ");
+		content = lf + 1;
+	}
+
+	git_str_puts(out, content);
+	git_str_putc(out, '\n');
+
+	return git_str_oom(out) ? -1 : 0;
+}
+
 static int git_commit__create_buffer_internal(
 	git_str *out,
 	const git_signature *author,
@@ -69,8 +95,20 @@ static int git_commit__create_buffer_internal(
 	git_signature__writebuf(out, "author ", author);
 	git_signature__writebuf(out, "committer ", committer);
 
-	if (opts && opts->message_encoding != NULL)
-		git_str_printf(out, "encoding %s\n", opts->message_encoding);
+	if (opts && opts->message_encoding != NULL) {
+		if (format_header_field(out, "encoding",
+			opts->message_encoding) < 0)
+			goto on_error;
+	}
+
+	if (opts && opts->extra_headers_len) {
+		for (i = 0; i < opts->extra_headers_len; i++) {
+			if (format_header_field(out,
+				opts->extra_headers[i].field,
+				opts->extra_headers[i].value) < 0)
+				goto on_error;
+		}
+	}
 
 	git_str_putc(out, '\n');
 
@@ -1026,38 +1064,11 @@ int git_commit__create_buffer(
 	return error;
 }
 
-/**
- * Append to 'out' properly marking continuations when there's a newline in 'content'
- */
-static int format_header_field(git_str *out, const char *field, const char *content)
-{
-	const char *lf;
-
-	GIT_ASSERT_ARG(out);
-	GIT_ASSERT_ARG(field);
-	GIT_ASSERT_ARG(content);
-
-	git_str_puts(out, field);
-	git_str_putc(out, ' ');
-
-	while ((lf = strchr(content, '\n')) != NULL) {
-		git_str_put(out, content, lf - content);
-		git_str_puts(out, "\n ");
-		content = lf + 1;
-	}
-
-	git_str_puts(out, content);
-	git_str_putc(out, '\n');
-
-	return git_str_oom(out) ? -1 : 0;
-}
-
 static const git_oid *commit_parent_from_commit(size_t n, void *payload)
 {
 	const git_commit *commit = (const git_commit *) payload;
 
 	return git_array_get(commit->parent_ids, n);
-
 }
 
 int git_commit_create_with_signature(

tests/libgit2/commit/write.c

@@ -422,3 +422,60 @@ a simple commit which works\n";
 	git_odb_object_free(obj);
 	git_odb_free(odb);
 }
+
+void test_commit_write__can_add_extra_headers(void)
+{
+	git_odb *odb;
+	git_signature *author, *committer;
+	git_oid tree_id, parent_id, commit_id;
+	git_commit *parent;
+	git_tree *tree;
+	git_odb_object *obj;
+	git_commit_create_ext_options opts = GIT_COMMIT_CREATE_EXT_OPTIONS_INIT;
+	const git_commit_header extra_headers[] = {
+		{ "line_one", "First extra header" },
+		{ "line_two", "Second extra header" }
+	};
+
+	const char *expected = "tree 1810dff58d8a660512d4832e740f692884338ccd\n\
+parent 8496071c1b46c854b31185ea97743be6a8774479\n\
+author Vicent Marti <vicent@github.com> 987654321 +0130\n\
+committer Vicent Marti <vicent@github.com> 123456789 +0100\n\
+line_one First extra header\n\
+line_two Second extra header\n\
+\n\
+This is a fun new commit.";
+
+	cl_git_pass(git_signature_new(
+		&author, committer_name, committer_email, 987654321, 90));
+	cl_git_pass(git_signature_new(
+		&committer, committer_name, committer_email, 123456789, 60));
+
+	/* this is a valid tree and parent */
+	git_oid_from_string(&tree_id, tree_id_str, GIT_OID_SHA1);
+	cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
+
+	git_oid_from_string(&parent_id, parent_id_str, GIT_OID_SHA1);
+	cl_git_pass(git_commit_lookup(&parent, g_repo, &parent_id));
+
+	opts.extra_headers = extra_headers;
+	opts.extra_headers_len = 2;
+
+	cl_git_pass(git_commit_create_ext(
+		&commit_id, g_repo, author, committer,
+		"This is a fun new commit.",
+		tree, 1, &parent, &opts));
+
+	cl_git_pass(git_repository_odb(&odb, g_repo));
+	cl_git_pass(git_odb_read(&obj, odb, &commit_id));
+
+	cl_assert_equal_s(expected, git_odb_object_data(obj));
+
+	git_odb_object_free(obj);
+	git_tree_free(tree);
+	git_commit_free(parent);
+	git_commit_free(commit);
+	git_signature_free(committer);
+	git_signature_free(author);
+	git_odb_free(odb);
+}