commit: introduce git_commit_amend_from_...

Similar to the `git_commit_create_from_...` APIs, a simple amend
function that uses smart defaults and amends HEAD from the staged
changes or a given tree.

include/git2/commit.h

@@ -612,6 +612,39 @@ GIT_EXTERN(int) git_commit_create_from_tree(
 	const char *message,
 	const git_commit_create_options *opts);
 
+/**
+ * Amends the HEAD commit in the repository using the staged changes;
+ * this is a near analog to `git commit --amend -m message`.
+ *
+ * @param id pointer to store the new commit's object id
+ * @param repo repository to commit changes in
+ * @param message the commit message
+ * @param opts options for creating the commit
+ * @return 0 on success or an error code
+ */
+GIT_EXTERN(int) git_commit_amend_from_stage(
+	git_oid *id,
+	git_repository *repo,
+	const char *message,
+	const git_commit_create_options *opts);
+
+/**
+ * Amends the HEAD commit in the repository using the given tree.
+ *
+ * @param id pointer to store the new commit's object id
+ * @param repo repository to commit changes in
+ * @param tree tree to point the commit to
+ * @param message the commit message
+ * @param opts options for creating the commit
+ * @return 0 on success or an error code
+ */
+GIT_EXTERN(int) git_commit_amend_from_tree(
+	git_oid *id,
+	git_repository *repo,
+	const git_tree *tree,
+	const char *message,
+	const git_commit_create_options *opts);
+
 /**
  * Amend an existing commit by replacing only non-NULL values.
  *

src/libgit2/commit.c

@@ -1330,6 +1330,100 @@ int git_commit_create_from_tree(
 	return create_from_tree(out, repo, tree, message, &opts);
 }
 
+int git_commit_amend_from_stage(
+	git_oid *out,
+	git_repository *repo,
+	const char *message,
+	const git_commit_create_options *opts)
+{
+	git_index *index = NULL;
+	git_oid tree_id;
+	git_tree *tree = NULL;
+	int error = 0;
+
+	GIT_ASSERT_ARG(out && repo);
+
+	if (git_repository_index(&index, repo) < 0 ||
+	    git_index_write_tree(&tree_id, index) < 0 ||
+	    git_tree_lookup(&tree, repo, &tree_id) < 0) {
+		error = -1;
+		goto done;
+	}
+
+	error = git_commit_amend_from_tree(out, repo, tree, message, opts);
+
+done:
+	git_tree_free(tree);
+	git_index_free(index);
+	return error;
+}
+
+int git_commit_amend_from_tree(
+	git_oid *out,
+	git_repository *repo,
+	const git_tree *tree,
+	const char *given_message,
+	const git_commit_create_options *given_opts)
+{
+	git_commit_create_options opts = GIT_COMMIT_CREATE_OPTIONS_INIT;
+	git_commit_create_ext_options ext_opts = GIT_COMMIT_CREATE_EXT_OPTIONS_INIT;
+	git_reference *head_ref = NULL;
+	git_commit *head_commit = NULL;
+	git_signature *new_committer = NULL;
+	const git_signature *author, *committer;
+	const char *message;
+	int error;
+
+	GIT_ASSERT_ARG(out && repo && tree);
+
+	if (given_opts)
+		memcpy(&opts, given_opts, sizeof(git_commit_create_options));
+
+	if ((error = git_repository_head(&head_ref, repo)) < 0 ||
+	    (error = git_reference_peel((git_object **)&head_commit, head_ref, GIT_OBJECT_COMMIT)) < 0)
+		goto done;
+
+	if (opts.author) {
+		author = opts.author;
+	} else {
+		author = git_commit_author(head_commit);
+	}
+
+	if (opts.committer) {
+		committer = opts.committer;
+	} else {
+		if ((error = git_signature_default(&new_committer, repo)) < 0)
+			goto done;
+
+		committer = new_committer;
+	}
+
+	if (given_message) {
+		message = given_message;
+		ext_opts.message_encoding = opts.message_encoding;
+	} else {
+		message = git_commit_message(head_commit);
+		ext_opts.message_encoding = git_commit_message_encoding(head_commit);
+	}
+
+	error = git_commit__create_internal(
+		out, repo, author, committer, message, git_tree_id(tree),
+		commit_parent_for_amend, (void *)head_commit, &ext_opts,
+		false);
+
+	if (!error)
+		error = git_reference__update_for_commit(
+			repo, head_ref, NULL, out, "commit");
+
+done:
+	git_signature_free(new_committer);
+	git_commit_free(head_commit);
+	git_reference_free(head_ref);
+
+	return error;
+}
+
+
 int git_commit_committer_with_mailmap(
 	git_signature **out, const git_commit *commit, const git_mailmap *mailmap)
 {

tests/libgit2/commit/amend.c

@@ -0,0 +1,122 @@
+#include "clar_libgit2.h"
+#include "repository.h"
+
+/* Fixture setup */
+static git_repository *g_repo;
+static git_signature *g_committer;
+
+void test_commit_amend__initialize(void)
+{
+	g_repo = cl_git_sandbox_init("testrepo2");
+	cl_git_pass(git_signature_new(&g_committer, "libgit2 user", "nobody@noreply.libgit2.org", 987654321, 90));
+}
+
+void test_commit_amend__cleanup(void)
+{
+	git_signature_free(g_committer);
+	cl_git_sandbox_cleanup();
+}
+
+
+void test_commit_amend__from_stage_simple(void)
+{
+	git_commit_create_options opts = GIT_COMMIT_CREATE_OPTIONS_INIT;
+	git_index *index;
+	git_oid commit_id;
+	git_tree *tree;
+
+	opts.committer = g_committer;
+
+	cl_git_rewritefile("testrepo2/newfile.txt", "This is a new file.\n");
+	cl_git_rewritefile("testrepo2/newfile2.txt", "This is a new file.\n");
+	cl_git_rewritefile("testrepo2/README", "hello, world.\n");
+	cl_git_rewritefile("testrepo2/new.txt", "hi there.\n");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "newfile2.txt"));
+	cl_git_pass(git_index_add_bypath(index, "README"));
+	cl_git_pass(git_index_write(index));
+
+	cl_git_pass(git_commit_amend_from_stage(&commit_id, g_repo, NULL, &opts));
+
+	cl_git_pass(git_repository_head_tree(&tree, g_repo));
+
+	cl_assert_equal_oidstr("63ec0b083fd14c22a68fd2b1794f26e4b396b6b3", &commit_id);
+	cl_assert_equal_oidstr("b27210772d0633870b4f486d04ed3eb5ebbef5e7", git_tree_id(tree));
+
+	git_index_free(index);
+	git_tree_free(tree);
+}
+
+void test_commit_amend__from_stage_newmessage(void)
+{
+	git_commit_create_options opts = GIT_COMMIT_CREATE_OPTIONS_INIT;
+	git_oid commit_id;
+	git_tree *tree;
+
+	opts.committer = g_committer;
+
+	cl_git_pass(git_commit_amend_from_stage(&commit_id, g_repo, "New message goes here.", &opts));
+
+	cl_git_pass(git_repository_head_tree(&tree, g_repo));
+
+	cl_assert_equal_oidstr("8b0e1cacc8380023705192466aaef8a15ddae7b3", &commit_id);
+	cl_assert_equal_oidstr("c4dc1555e4d4fa0e0c9c3fc46734c7c35b3ce90b", git_tree_id(tree));
+
+	git_tree_free(tree);
+}
+
+void test_commit_amend__from_stage_nochanges(void)
+{
+	git_commit_create_options opts = GIT_COMMIT_CREATE_OPTIONS_INIT;
+	git_oid commit_id;
+	git_tree *tree;
+
+	opts.committer = g_committer;
+
+	cl_git_pass(git_commit_amend_from_stage(&commit_id, g_repo, NULL, &opts));
+
+	cl_git_pass(git_repository_head_tree(&tree, g_repo));
+
+	cl_assert_equal_oidstr("da86907c6d505a92c5683bece08f23d68ac785bd", &commit_id);
+	cl_assert_equal_oidstr("c4dc1555e4d4fa0e0c9c3fc46734c7c35b3ce90b", git_tree_id(tree));
+
+	git_tree_free(tree);
+}
+
+void test_commit_amend__from_tree(void)
+{
+	git_commit_create_options opts = GIT_COMMIT_CREATE_OPTIONS_INIT;
+	git_index *index;
+	git_oid commit_id;
+	git_oid tree_id;
+	git_tree *tree, *lookedup;
+
+	opts.committer = g_committer;
+
+	cl_git_rewritefile("testrepo2/newfile.txt", "This is a new file.\n");
+	cl_git_rewritefile("testrepo2/newfile2.txt", "This is a new file.\n");
+	cl_git_rewritefile("testrepo2/README", "hello, world.\n");
+	cl_git_rewritefile("testrepo2/new.txt", "hi there.\n");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "newfile2.txt"));
+	cl_git_pass(git_index_add_bypath(index, "README"));
+	cl_git_pass(git_index_write(index));
+	cl_git_pass(git_index_write_tree(&tree_id, index));
+
+	cl_assert_equal_oidstr("b27210772d0633870b4f486d04ed3eb5ebbef5e7", &tree_id);
+
+	cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
+
+	cl_git_pass(git_commit_amend_from_tree(&commit_id, g_repo, tree, NULL, &opts));
+
+	cl_git_pass(git_repository_head_tree(&lookedup, g_repo));
+
+	cl_assert_equal_oidstr("63ec0b083fd14c22a68fd2b1794f26e4b396b6b3", &commit_id);
+	cl_assert_equal_oidstr("b27210772d0633870b4f486d04ed3eb5ebbef5e7", git_tree_id(lookedup));
+
+	git_index_free(index);
+	git_tree_free(tree);
+	git_tree_free(lookedup);
+}