replay: offer an option to linearize the commit topology

One of the stated goals of git-replay(1) is to allow implementing the
git-rebase(1) functionality on the server side.

The default mode of git-rebase(1) is to act as if `--no-rebase-merges`
was given. This mode drops merge commits instead of replaying them, and
linearizes the commit history into a sequence of the
regular (single-parent) commits.

Add option `--linearize` to git-replay(1) to do the same.

Co-authored-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-replay.adoc

@@ -88,6 +88,11 @@ incompatible with `--contained` (which is a modifier for `--onto` only).
 +
 The default mode can be configured via the `replay.refAction` configuration variable.
 
+--linearize::
+	In this mode, `git replay` imitates `git rebase --no-rebase-merges`,
+	i.e. it cherry-picks only non-merge commits, each one on top of the
+	previous one.
+
 <revision-range>::
 	Range of commits to replay; see "Specifying Ranges" in
 	linkgit:git-rev-parse[1]. In `--advance=<branch>` or

builtin/replay.c

@@ -111,6 +111,8 @@ int cmd_replay(int argc,
 			     N_("mode"),
 			     N_("control ref update behavior (update|print)"),
 			     PARSE_OPT_NONEG),
+		OPT_BOOL(0, "linearize", &opts.linearize,
+			 N_("ignore merge commits instead of replaying them")),
 		OPT_END()
 	};
 
@@ -132,6 +134,8 @@ int cmd_replay(int argc,
 				  opts.contained, "--contained");
 	die_for_incompatible_opt2(!!opts.ref, "--ref",
 				  !!opts.contained, "--contained");
+	die_for_incompatible_opt2(!!opts.revert, "--revert",
+				  opts.linearize, "--linearize");
 
 	/* Parse ref action mode from command line or config */
 	ref_mode = get_ref_action_mode(repo, ref_action);

replay.c

@@ -277,12 +277,16 @@ static struct commit *pick_regular_commit(struct repository *repo,
 					  struct commit *onto,
 					  struct merge_options *merge_opt,
 					  struct merge_result *result,
+					  struct commit *replayed_base,
 					  bool reverse,
 					  enum replay_empty_commit_action empty)
 {
-	struct commit *base, *replayed_base;
+	struct commit *base;
 	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
 
+	if (replayed_base && reverse)
+		BUG("Linearizing commits is not supported when replaying in reverse");
+
 	if (pickme->parents) {
 		base = pickme->parents->item;
 		base_tree = repo_get_commit_tree(repo, base);
@@ -291,7 +295,8 @@ static struct commit *pick_regular_commit(struct repository *repo,
 		base_tree = lookup_tree(repo, repo->hash_algo->empty_tree);
 	}
 
-	replayed_base = get_mapped_commit(replayed_commits, base, onto);
+	if (!replayed_base)
+		replayed_base = get_mapped_commit(replayed_commits, base, onto);
 	replayed_base_tree = repo_get_commit_tree(repo, replayed_base);
 	pickme_tree = repo_get_commit_tree(repo, pickme);
 
@@ -430,12 +435,23 @@ int replay_revisions(struct rev_info *revs,
 	while ((commit = get_revision(revs))) {
 		const struct name_decoration *decoration;
 
-		if (commit->parents && commit->parents->next)
-			die(_("replaying merge commits is not supported yet!"));
+		if (commit->parents && commit->parents->next) {
+			if (!opts->linearize)
+				die(_("replaying merge commits is not supported yet!"));
+			/*
+			 * When linearizing, a merge commit itself is not picked,
+			 * but refs that point to it might need updating.
+			 */
+		} else {
+			struct commit *to_pick = reverse ? last_commit : onto;
+			last_commit =
+				pick_regular_commit(revs->repo, commit,
+						    replayed_commits, to_pick,
+						    &merge_opt, &result,
+						    opts->linearize ? last_commit : NULL,
+						    reverse, opts->empty);
+		}
 
-		last_commit = pick_regular_commit(revs->repo, commit, replayed_commits,
-						  reverse ? last_commit : onto,
-						  &merge_opt, &result, reverse, opts->empty);
 		if (!last_commit)
 			break;
 

replay.h

@@ -62,6 +62,11 @@ struct replay_revisions_options {
 	 * Defaults to REPLAY_EMPTY_COMMIT_DROP.
 	 */
 	enum replay_empty_commit_action empty;
+
+	/*
+	 * Whether to linearize the commits (i.e. drop merge commits).
+	 */
+	int linearize;
 };
 
 /* This struct is used as an out-parameter by `replay_revisions()`. */

t/t3650-replay-basics.sh

@@ -565,4 +565,30 @@ test_expect_success '--onto with --ref rejects multiple revision ranges' '
 	test_grep "cannot be used with multiple revision ranges" err
 '
 
+test_expect_success 'replay merge commit fails' '
+	echo "fatal: replaying merge commits is not supported yet!" >expect &&
+	test_must_fail git replay --ref-action=print --onto main I..P 2>actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'replay to rebase merge commit with --linearize' '
+	git replay --ref-action=print --linearize --onto main I..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines O N J M L B A >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'replay to rebase merge commit with --linearize down to root commit' '
+	git replay --ref-action=print --linearize --onto main A..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines O N J I M L B A >expect &&
+	test_cmp expect actual
+'
+
 test_done