Merge branch 'jc/merge-reduce-parents-early'

Octopus merge strategy did not reduce heads that are recorded in the
final commit correctly.

By Junio C Hamano (4) and Michał Kiedrowicz (1)
* jc/merge-reduce-parents-early:
  fmt-merge-msg: discard needless merge parents
  builtin/merge.c: reduce parents early
  builtin/merge.c: collect other parents early
  builtin/merge.c: remove "remoteheads" global variable
  merge tests: octopus with redundant parents

5 files changed, 275 insertions, 70 deletions

diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 1bc6b8b8c..a517f1794 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -55,7 +55,48 @@ static void init_src_data(struct src_data *data)
 static struct string_list srcs = STRING_LIST_INIT_DUP;
 static struct string_list origins = STRING_LIST_INIT_DUP;
 
-static int handle_line(char *line)
+struct merge_parents {
+	int alloc, nr;
+	struct merge_parent {
+		unsigned char given[20];
+		unsigned char commit[20];
+		unsigned char used;
+	} *item;
+};
+
+/*
+ * I know, I know, this is inefficient, but you won't be pulling and merging
+ * hundreds of heads at a time anyway.
+ */
+static struct merge_parent *find_merge_parent(struct merge_parents *table,
+					      unsigned char *given,
+					      unsigned char *commit)
+{
+	int i;
+	for (i = 0; i < table->nr; i++) {
+		if (given && hashcmp(table->item[i].given, given))
+			continue;
+		if (commit && hashcmp(table->item[i].commit, commit))
+			continue;
+		return &table->item[i];
+	}
+	return NULL;
+}
+
+static void add_merge_parent(struct merge_parents *table,
+			     unsigned char *given,
+			     unsigned char *commit)
+{
+	if (table->nr && find_merge_parent(table, given, commit))
+		return;
+	ALLOC_GROW(table->item, table->nr + 1, table->alloc);
+	hashcpy(table->item[table->nr].given, given);
+	hashcpy(table->item[table->nr].commit, commit);
+	table->item[table->nr].used = 0;
+	table->nr++;
+}
+
+static int handle_line(char *line, struct merge_parents *merge_parents)
 {
 	int i, len = strlen(line);
 	struct origin_data *origin_data;
@@ -63,6 +104,7 @@ static int handle_line(char *line)
 	struct src_data *src_data;
 	struct string_list_item *item;
 	int pulling_head = 0;
+	unsigned char sha1[20];
 
 	if (len < 43 || line[40] != '\t')
 		return 1;
@@ -73,14 +115,15 @@ static int handle_line(char *line)
 	if (line[41] != '\t')
 		return 2;
 
-	line[40] = 0;
-	origin_data = xcalloc(1, sizeof(struct origin_data));
-	i = get_sha1(line, origin_data->sha1);
-	line[40] = '\t';
-	if (i) {
-		free(origin_data);
+	i = get_sha1_hex(line, sha1);
+	if (i)
 		return 3;
-	}
+
+	if (!find_merge_parent(merge_parents, sha1, NULL))
+		return 0; /* subsumed by other parents */
+
+	origin_data = xcalloc(1, sizeof(struct origin_data));
+	hashcpy(origin_data->sha1, sha1);
 
 	if (line[len - 1] == '\n')
 		line[len - 1] = 0;
@@ -472,6 +515,67 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 	strbuf_release(&tagbuf);
 }
 
+static void find_merge_parents(struct merge_parents *result,
+			       struct strbuf *in, unsigned char *head)
+{
+	struct commit_list *parents, *next;
+	struct commit *head_commit;
+	int pos = 0, i, j;
+
+	parents = NULL;
+	while (pos < in->len) {
+		int len;
+		char *p = in->buf + pos;
+		char *newline = strchr(p, '\n');
+		unsigned char sha1[20];
+		struct commit *parent;
+		struct object *obj;
+
+		len = newline ? newline - p : strlen(p);
+		pos += len + !!newline;
+
+		if (len < 43 ||
+		    get_sha1_hex(p, sha1) ||
+		    p[40] != '\t' ||
+		    p[41] != '\t')
+			continue; /* skip not-for-merge */
+		/*
+		 * Do not use get_merge_parent() here; we do not have
+		 * "name" here and we do not want to contaminate its
+		 * util field yet.
+		 */
+		obj = parse_object(sha1);
+		parent = (struct commit *)peel_to_type(NULL, 0, obj, OBJ_COMMIT);
+		if (!parent)
+			continue;
+		commit_list_insert(parent, &parents);
+		add_merge_parent(result, obj->sha1, parent->object.sha1);
+	}
+	head_commit = lookup_commit(head);
+	if (head_commit)
+		commit_list_insert(head_commit, &parents);
+	parents = reduce_heads(parents);
+
+	while (parents) {
+		for (i = 0; i < result->nr; i++)
+			if (!hashcmp(result->item[i].commit,
+				     parents->item->object.sha1))
+				result->item[i].used = 1;
+		next = parents->next;
+		free(parents);
+		parents = next;
+	}
+
+	for (i = j = 0; i < result->nr; i++) {
+		if (result->item[i].used) {
+			if (i != j)
+				result->item[j] = result->item[i];
+			j++;
+		}
+	}
+	result->nr = j;
+}
+
 int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		  struct fmt_merge_msg_opts *opts)
 {
@@ -479,6 +583,9 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 	unsigned char head_sha1[20];
 	const char *current_branch;
 	void *current_branch_to_free;
+	struct merge_parents merge_parents;
+
+	memset(&merge_parents, 0, sizeof(merge_parents));
 
 	/* get current branch */
 	current_branch = current_branch_to_free =
@@ -488,6 +595,8 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 	if (!prefixcmp(current_branch, "refs/heads/"))
 		current_branch += 11;
 
+	find_merge_parents(&merge_parents, in, head_sha1);
+
 	/* get a line */
 	while (pos < in->len) {
 		int len;
@@ -498,7 +607,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		pos += len + !!newline;
 		i++;
 		p[len] = 0;
-		if (handle_line(p))
+		if (handle_line(p, &merge_parents))
 			die ("Error in line %d: %.*s", i, len, p);
 	}
 
@@ -529,6 +638,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 
 	strbuf_complete_line(out);
 	free(current_branch_to_free);
+	free(merge_parents.item);
 	return 0;
 }
 
diff --git a/builtin/merge.c b/builtin/merge.c
index 08e01e8a6..470fc57c5 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -52,7 +52,6 @@ static int fast_forward_only, option_edit = -1;
 static int allow_trivial = 1, have_message;
 static int overwrite_ignore = 1;
 static struct strbuf merge_msg = STRBUF_INIT;
-static struct commit_list *remoteheads;
 static struct strategy **use_strategies;
 static size_t use_strategies_nr, use_strategies_alloc;
 static const char **xopts;
@@ -318,7 +317,7 @@ static void finish_up_to_date(const char *msg)
 	drop_save();
 }
 
-static void squash_message(struct commit *commit)
+static void squash_message(struct commit *commit, struct commit_list *remoteheads)
 {
 	struct rev_info rev;
 	struct strbuf out = STRBUF_INIT;
@@ -366,6 +365,7 @@ static void squash_message(struct commit *commit)
 }
 
 static void finish(struct commit *head_commit,
+		   struct commit_list *remoteheads,
 		   const unsigned char *new_head, const char *msg)
 {
 	struct strbuf reflog_message = STRBUF_INIT;
@@ -380,7 +380,7 @@ static void finish(struct commit *head_commit,
 			getenv("GIT_REFLOG_ACTION"), msg);
 	}
 	if (squash) {
-		squash_message(head_commit);
+		squash_message(head_commit, remoteheads);
 	} else {
 		if (verbosity >= 0 && !merge_msg.len)
 			printf(_("No merge message -- not updating HEAD\n"));
@@ -683,6 +683,7 @@ int try_merge_command(const char *strategy, size_t xopts_nr,
 }
 
 static int try_merge_strategy(const char *strategy, struct commit_list *common,
+			      struct commit_list *remoteheads,
 			      struct commit *head, const char *head_arg)
 {
 	int index_fd;
@@ -876,14 +877,14 @@ static void read_merge_msg(struct strbuf *msg)
 		die_errno(_("Could not read from '%s'"), filename);
 }
 
-static void write_merge_state(void);
-static void abort_commit(const char *err_msg)
+static void write_merge_state(struct commit_list *);
+static void abort_commit(struct commit_list *remoteheads, const char *err_msg)
 {
 	if (err_msg)
 		error("%s", err_msg);
 	fprintf(stderr,
 		_("Not committing merge; use 'git commit' to complete the merge.\n"));
-	write_merge_state();
+	write_merge_state(remoteheads);
 	exit(1);
 }
 
@@ -894,7 +895,7 @@ N_("Please enter a commit message to explain why this merge is necessary,\n"
    "Lines starting with '#' will be ignored, and an empty message aborts\n"
    "the commit.\n");
 
-static void prepare_to_commit(void)
+static void prepare_to_commit(struct commit_list *remoteheads)
 {
 	struct strbuf msg = STRBUF_INIT;
 	const char *comment = _(merge_editor_comment);
@@ -907,18 +908,18 @@ static void prepare_to_commit(void)
 		 git_path("MERGE_MSG"), "merge", NULL, NULL);
 	if (0 < option_edit) {
 		if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
-			abort_commit(NULL);
+			abort_commit(remoteheads, NULL);
 	}
 	read_merge_msg(&msg);
 	stripspace(&msg, 0 < option_edit);
 	if (!msg.len)
-		abort_commit(_("Empty commit message."));
+		abort_commit(remoteheads, _("Empty commit message."));
 	strbuf_release(&merge_msg);
 	strbuf_addbuf(&merge_msg, &msg);
 	strbuf_release(&msg);
 }
 
-static int merge_trivial(struct commit *head)
+static int merge_trivial(struct commit *head, struct commit_list *remoteheads)
 {
 	unsigned char result_tree[20], result_commit[20];
 	struct commit_list *parent = xmalloc(sizeof(*parent));
@@ -929,45 +930,37 @@ static int merge_trivial(struct commit *head)
 	parent->next = xmalloc(sizeof(*parent->next));
 	parent->next->item = remoteheads->item;
 	parent->next->next = NULL;
-	prepare_to_commit();
+	prepare_to_commit(remoteheads);
 	if (commit_tree(&merge_msg, result_tree, parent, result_commit, NULL,
 			sign_commit))
 		die(_("failed to write commit object"));
-	finish(head, result_commit, "In-index merge");
+	finish(head, remoteheads, result_commit, "In-index merge");
 	drop_save();
 	return 0;
 }
 
 static int finish_automerge(struct commit *head,
+			    int head_subsumed,
 			    struct commit_list *common,
+			    struct commit_list *remoteheads,
 			    unsigned char *result_tree,
 			    const char *wt_strategy)
 {
-	struct commit_list *parents = NULL, *j;
+	struct commit_list *parents = NULL;
 	struct strbuf buf = STRBUF_INIT;
 	unsigned char result_commit[20];
 
 	free_commit_list(common);
-	if (allow_fast_forward) {
-		parents = remoteheads;
+	parents = remoteheads;
+	if (!head_subsumed || !allow_fast_forward)
 		commit_list_insert(head, &parents);
-		parents = reduce_heads(parents);
-	} else {
-		struct commit_list **pptr = &parents;
-
-		pptr = &commit_list_insert(head,
-				pptr)->next;
-		for (j = remoteheads; j; j = j->next)
-			pptr = &commit_list_insert(j->item, pptr)->next;
-	}
 	strbuf_addch(&merge_msg, '\n');
-	prepare_to_commit();
-	free_commit_list(remoteheads);
+	prepare_to_commit(remoteheads);
 	if (commit_tree(&merge_msg, result_tree, parents, result_commit,
 			NULL, sign_commit))
 		die(_("failed to write commit object"));
 	strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
-	finish(head, result_commit, buf.buf);
+	finish(head, remoteheads, result_commit, buf.buf);
 	strbuf_release(&buf);
 	drop_save();
 	return 0;
@@ -1072,7 +1065,7 @@ static int setup_with_upstream(const char ***argv)
 	return i;
 }
 
-static void write_merge_state(void)
+static void write_merge_state(struct commit_list *remoteheads)
 {
 	const char *filename;
 	int fd;
@@ -1137,6 +1130,39 @@ static int default_edit_option(void)
 		st_stdin.st_mode == st_stdout.st_mode);
 }
 
+static struct commit_list *collect_parents(struct commit *head_commit,
+					   int *head_subsumed,
+					   int argc, const char **argv)
+{
+	int i;
+	struct commit_list *remoteheads = NULL, *parents, *next;
+	struct commit_list **remotes = &remoteheads;
+
+	if (head_commit)
+		remotes = &commit_list_insert(head_commit, remotes)->next;
+	for (i = 0; i < argc; i++) {
+		struct commit *commit = get_merge_parent(argv[i]);
+		if (!commit)
+			die(_("%s - not something we can merge"), argv[i]);
+		remotes = &commit_list_insert(commit, remotes)->next;
+	}
+	*remotes = NULL;
+
+	parents = reduce_heads(remoteheads);
+
+	*head_subsumed = 1; /* we will flip this to 0 when we find it */
+	for (remoteheads = NULL, remotes = &remoteheads;
+	     parents;
+	     parents = next) {
+		struct commit *commit = parents->item;
+		next = parents->next;
+		if (commit == head_commit)
+			*head_subsumed = 0;
+		else
+			remotes = &commit_list_insert(commit, remotes)->next;
+	}
+	return remoteheads;
+}
 
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
@@ -1146,11 +1172,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	struct commit *head_commit;
 	struct strbuf buf = STRBUF_INIT;
 	const char *head_arg;
-	int flag, i, ret = 0;
+	int flag, i, ret = 0, head_subsumed;
 	int best_cnt = -1, merge_was_ok = 0, automerge_was_ok = 0;
 	struct commit_list *common = NULL;
 	const char *best_strategy = NULL, *wt_strategy = NULL;
-	struct commit_list **remotes = &remoteheads;
+	struct commit_list *remoteheads, *p;
 	void *branch_to_free;
 
 	if (argc == 2 && !strcmp(argv[1], "-h"))
@@ -1255,6 +1281,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		head_arg = argv[1];
 		argv += 2;
 		argc -= 2;
+		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
 	} else if (!head_commit) {
 		struct commit *remote_head;
 		/*
@@ -1270,7 +1297,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		if (!allow_fast_forward)
 			die(_("Non-fast-forward commit does not make sense into "
 			    "an empty head"));
-		remote_head = get_merge_parent(argv[0]);
+		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
+		remote_head = remoteheads->item;
 		if (!remote_head)
 			die(_("%s - not something we can merge"), argv[0]);
 		read_empty(remote_head->object.sha1, 0);
@@ -1288,8 +1316,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * the standard merge summary message to be appended
 		 * to the given message.
 		 */
-		for (i = 0; i < argc; i++)
-			merge_name(argv[i], &merge_names);
+		remoteheads = collect_parents(head_commit, &head_subsumed, argc, argv);
+		for (p = remoteheads; p; p = p->next)
+			merge_name(merge_remote_util(p->item)->name, &merge_names);
 
 		if (!have_message || shortlog_len) {
 			struct fmt_merge_msg_opts opts;
@@ -1308,19 +1337,16 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			builtin_merge_options);
 
 	strbuf_addstr(&buf, "merge");
-	for (i = 0; i < argc; i++)
-		strbuf_addf(&buf, " %s", argv[i]);
+	for (p = remoteheads; p; p = p->next)
+		strbuf_addf(&buf, " %s", merge_remote_util(p->item)->name);
 	setenv("GIT_REFLOG_ACTION", buf.buf, 0);
 	strbuf_reset(&buf);
 
-	for (i = 0; i < argc; i++) {
-		struct commit *commit = get_merge_parent(argv[i]);
-		if (!commit)
-			die(_("%s - not something we can merge"), argv[i]);
-		remotes = &commit_list_insert(commit, remotes)->next;
+	for (p = remoteheads; p; p = p->next) {
+		struct commit *commit = p->item;
 		strbuf_addf(&buf, "GITHEAD_%s",
 			    sha1_to_hex(commit->object.sha1));
-		setenv(buf.buf, argv[i], 1);
+		setenv(buf.buf, merge_remote_util(commit)->name, 1);
 		strbuf_reset(&buf);
 		if (!fast_forward_only &&
 		    merge_remote_util(commit) &&
@@ -1333,7 +1359,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		option_edit = default_edit_option();
 
 	if (!use_strategies) {
-		if (!remoteheads->next)
+		if (!remoteheads)
+			; /* already up-to-date */
+		else if (!remoteheads->next)
 			add_strategies(pull_twohead, DEFAULT_TWOHEAD);
 		else
 			add_strategies(pull_octopus, DEFAULT_OCTOPUS);
@@ -1346,7 +1374,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			allow_trivial = 0;
 	}
 
-	if (!remoteheads->next)
+	if (!remoteheads)
+		; /* already up-to-date */
+	else if (!remoteheads->next)
 		common = get_merge_bases(head_commit, remoteheads->item, 1);
 	else {
 		struct commit_list *list = remoteheads;
@@ -1358,10 +1388,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	update_ref("updating ORIG_HEAD", "ORIG_HEAD", head_commit->object.sha1,
 		   NULL, 0, DIE_ON_ERR);
 
-	if (!common)
+	if (remoteheads && !common)
 		; /* No common ancestors found. We need a real merge. */
-	else if (!remoteheads->next && !common->next &&
-			common->item == remoteheads->item) {
+	else if (!remoteheads ||
+		 (!remoteheads->next && !common->next &&
+		  common->item == remoteheads->item)) {
 		/*
 		 * If head can reach all the merge then we are up to date.
 		 * but first the most common case of merging one remote.
@@ -1399,7 +1430,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			goto done;
 		}
 
-		finish(head_commit, commit->object.sha1, msg.buf);
+		finish(head_commit, remoteheads, commit->object.sha1, msg.buf);
 		drop_save();
 		goto done;
 	} else if (!remoteheads->next && common->next)
@@ -1421,7 +1452,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			if (!read_tree_trivial(common->item->object.sha1,
 					       head_commit->object.sha1,
 					       remoteheads->item->object.sha1)) {
-				ret = merge_trivial(head_commit);
+				ret = merge_trivial(head_commit, remoteheads);
 				goto done;
 			}
 			printf(_("Nope.\n"));
@@ -1492,7 +1523,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		wt_strategy = use_strategies[i]->name;
 
 		ret = try_merge_strategy(use_strategies[i]->name,
-					 common, head_commit, head_arg);
+					 common, remoteheads,
+					 head_commit, head_arg);
 		if (!option_commit && !ret) {
 			merge_was_ok = 1;
 			/*
@@ -1534,8 +1566,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	 * auto resolved the merge cleanly.
 	 */
 	if (automerge_was_ok) {
-		ret = finish_automerge(head_commit, common, result_tree,
-				       wt_strategy);
+		ret = finish_automerge(head_commit, head_subsumed,
+				       common, remoteheads,
+				       result_tree, wt_strategy);
 		goto done;
 	}
 
@@ -1560,13 +1593,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		restore_state(head_commit->object.sha1, stash);
 		printf(_("Using the %s to prepare resolving by hand.\n"),
 			best_strategy);
-		try_merge_strategy(best_strategy, common, head_commit, head_arg);
+		try_merge_strategy(best_strategy, common, remoteheads,
+				   head_commit, head_arg);
 	}
 
 	if (squash)
-		finish(head_commit, NULL, NULL);
+		finish(head_commit, remoteheads, NULL, NULL);
 	else
-		write_merge_state();
+		write_merge_state(remoteheads);
 
 	if (merge_was_ok)
 		fprintf(stderr, _("Automatic merge went well; "
diff --git a/t/t6028-merge-up-to-date.sh b/t/t6028-merge-up-to-date.sh
index a91644e3b..c518e9c30 100755
--- a/t/t6028-merge-up-to-date.sh
+++ b/t/t6028-merge-up-to-date.sh
@@ -16,7 +16,12 @@ test_expect_success setup '
 	test_tick &&
 	git commit -m second &&
 	git tag c1 &&
-	git branch test
+	git branch test &&
+	echo third >file &&
+	git add file &&
+	test_tick &&
+	git commit -m third &&
+	git tag c2
 '
 
 test_expect_success 'merge -s recursive up-to-date' '
@@ -74,4 +79,14 @@ test_expect_success 'merge -s subtree up-to-date' '
 
 '
 
+test_expect_success 'merge fast-forward octopus' '
+
+	git reset --hard c0 &&
+	test_tick &&
+	git merge c1 c2
+	expect=$(git rev-parse c2) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+'
+
 test_done
diff --git a/t/t7602-merge-octopus-many.sh b/t/t7602-merge-octopus-many.sh
index bce0bd37c..3b72c097e 100755
--- a/t/t7602-merge-octopus-many.sh
+++ b/t/t7602-merge-octopus-many.sh
@@ -70,16 +70,14 @@ test_expect_success 'merge output uses pretty names' '
 '
 
 cat >expected <<\EOF
-Already up-to-date with c4
-Trying simple merge with c5
-Merge made by the 'octopus' strategy.
+Merge made by the 'recursive' strategy.
  c5.c |    1 +
  1 file changed, 1 insertion(+)
  create mode 100644 c5.c
 EOF
 
-test_expect_success 'merge up-to-date output uses pretty names' '
-	git merge c4 c5 >actual &&
+test_expect_success 'merge reduces irrelevant remote heads' '
+	GIT_MERGE_VERBOSITY=0 git merge c4 c5 >actual &&
 	test_i18ncmp expected actual
 '
 
diff --git a/t/t7603-merge-reduce-heads.sh b/t/t7603-merge-reduce-heads.sh
index 7e17eb490..98948955a 100755
--- a/t/t7603-merge-reduce-heads.sh
+++ b/t/t7603-merge-reduce-heads.sh
@@ -57,7 +57,36 @@ test_expect_success 'merge c1 with c2, c3, c4, c5' '
 	test -f c2.c &&
 	test -f c3.c &&
 	test -f c4.c &&
-	test -f c5.c
+	test -f c5.c &&
+	git show --format=%s -s >actual &&
+	! grep c1 actual &&
+	grep c2 actual &&
+	grep c3 actual &&
+	! grep c4 actual &&
+	grep c5 actual
+'
+
+test_expect_success 'pull c2, c3, c4, c5 into c1' '
+	git reset --hard c1 &&
+	git pull . c2 c3 c4 c5 &&
+	test "$(git rev-parse c1)" != "$(git rev-parse HEAD)" &&
+	test "$(git rev-parse c1)" = "$(git rev-parse HEAD^1)" &&
+	test "$(git rev-parse c2)" = "$(git rev-parse HEAD^2)" &&
+	test "$(git rev-parse c3)" = "$(git rev-parse HEAD^3)" &&
+	test "$(git rev-parse c5)" = "$(git rev-parse HEAD^4)" &&
+	git diff --exit-code &&
+	test -f c0.c &&
+	test -f c1.c &&
+	test -f c2.c &&
+	test -f c3.c &&
+	test -f c4.c &&
+	test -f c5.c &&
+	git show --format=%s -s >actual &&
+	! grep c1 actual &&
+	grep c2 actual &&
+	grep c3 actual &&
+	! grep c4 actual &&
+	grep c5 actual
 '
 
 test_expect_success 'setup' '
@@ -113,4 +142,23 @@ test_expect_success 'verify merge result' '
 	test $(git rev-parse HEAD^1) = $(git rev-parse E2) &&
 	test $(git rev-parse HEAD^2) = $(git rev-parse I2)
 '
+
+test_expect_success 'fast-forward to redundant refs' '
+	git reset --hard c0 &&
+	git merge c4 c5
+'
+
+test_expect_success 'verify merge result' '
+	test $(git rev-parse HEAD) = $(git rev-parse c5)
+'
+
+test_expect_success 'merge up-to-date redundant refs' '
+	git reset --hard c5 &&
+	git merge c0 c4
+'
+
+test_expect_success 'verify merge result' '
+	test $(git rev-parse HEAD) = $(git rev-parse c5)
+'
+
 test_done