commit: add a function to create a commit on top of a refcmn/safe-commit

Add a way to safely create a commit which builds on top of the current
commit in a given reference.

3 files changed, 109 insertions, 9 deletions

diff --git a/include/git2/commit.h b/include/git2/commit.h
index 834330b5d..78deb6de0 100644
--- a/include/git2/commit.h
+++ b/include/git2/commit.h
@@ -351,6 +351,25 @@ GIT_EXTERN(int) git_commit_amend(
 	const char *message,
 	const git_tree *tree);
 
+/**
+ * Create a commit on top of the tip of a given branch
+ *
+ * All parameters have the same meaning as in `git_commit_create()`
+ * with the exception that `update_ref` is required and the new
+ * commit's single parent will be the value of that ref at the time of
+ * reading.
+ */
+int git_commit_append(
+	git_oid *id,
+	git_repository *repo,
+	const char *update_ref,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_tree *tree);
+
+
 /** @} */
 GIT_END_DECL
 #endif
diff --git a/src/commit.c b/src/commit.c
index de50e772e..6eb4f443b 100644
--- a/src/commit.c
+++ b/src/commit.c
@@ -16,6 +16,7 @@
 #include "commit.h"
 #include "signature.h"
 #include "message.h"
+#include "refs.h"
 
 #include <stdarg.h>
 
@@ -36,6 +37,25 @@ void git_commit__free(void *_commit)
 	git__free(commit);
 }
 
+static int commit_reflog_message(git_buf *buf, git_repository *repo, const git_oid *id)
+{
+	git_commit *c;
+	const char *shortmsg;
+
+	git_buf_clear(buf);
+
+	if (git_commit_lookup(&c, repo, id) < 0)
+		return -1;
+
+	shortmsg = git_commit_summary(c);
+	git_buf_printf(buf, "commit%s: %s",
+		       git_commit_parentcount(c) == 0 ? " (initial)" : "",
+		       shortmsg);
+	git_commit_free(c);
+
+	return 0;
+}
+
 int git_commit_create_from_callback(
 	git_oid *id,
 	git_repository *repo,
@@ -81,19 +101,11 @@ int git_commit_create_from_callback(
 
 	if (update_ref != NULL) {
 		int error;
-		git_commit *c;
-		const char *shortmsg;
 		git_buf reflog_msg = GIT_BUF_INIT;
 
-		if (git_commit_lookup(&c, repo, id) < 0)
+		if ((error = commit_reflog_message(&reflog_msg, repo, id)) < 0)
 			goto on_error;
 
-		shortmsg = git_commit_summary(c);
-		git_buf_printf(&reflog_msg, "commit%s: %s",
-				git_commit_parentcount(c) == 0 ? " (initial)" : "",
-				shortmsg);
-		git_commit_free(c);
-
 		error = git_reference__update_terminal(repo, update_ref, id,
 				committer, git_buf_cstr(&reflog_msg));
 
@@ -273,6 +285,48 @@ int git_commit_amend(
 		&tree_id, commit_parent_for_amend, (void *)commit_to_amend);
 }
 
+int git_commit_append(
+	git_oid *id,
+	git_repository *repo,
+	const char *update_ref,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_tree *tree)
+{
+	git_buf reflog_msg = GIT_BUF_INIT;
+	git_reference *ref, *ref2;
+	const git_oid *parent_id = NULL;
+	git_commit *parent;
+	int error;
+
+	assert(repo && update_ref && author && committer && message);
+
+	if ((error = git_reference_lookup_resolved(&ref, repo, update_ref, 5)) < 0)
+		return error;
+
+	parent_id = git_reference_target(ref);
+	if ((error = git_commit_lookup(&parent, repo, parent_id)) < 0)
+		goto out;
+
+	error = git_commit_create_v(id, repo, NULL, author, committer, message_encoding, message, tree, 1, parent_id);
+	if (error < 0)
+		goto out;
+
+	if ((error = commit_reflog_message(&reflog_msg, repo, parent_id)) < 0)
+		goto out;
+
+	error = git_reference_set_target(&ref2, ref, id, committer, git_buf_cstr(&reflog_msg));
+	git_buf_free(&reflog_msg);
+
+out:
+	git_commit_free(parent);
+	git_reference_free(ref);
+	git_reference_free(ref2);
+	return error;
+}
+
 int git_commit__parse(void *_commit, git_odb_object *odb_obj)
 {
 	git_commit *commit = _commit;
diff --git a/tests/commit/write.c b/tests/commit/write.c
index b1cdf4485..84cc99133 100644
--- a/tests/commit/write.c
+++ b/tests/commit/write.c
@@ -158,3 +158,30 @@ void test_commit_write__root(void)
 	git_signature_free(committer);
 	git_reflog_free(log);
 }
+
+void test_commit_write__append(void)
+{
+	git_tree *tree;
+	git_oid tree_id, commit_id;
+	git_signature *author, *committer;
+
+	git_oid_fromstr(&tree_id, tree_oid);
+	cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
+
+	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));
+
+	cl_git_pass(git_commit_append(
+		&commit_id, /* out id */
+		g_repo,
+		"HEAD",
+		author,
+		committer,
+		NULL,
+		root_commit_message,
+		tree));
+
+	git_tree_free(tree);
+	git_signature_free(committer);
+	git_signature_free(author);
+}