[PATCH v9 3/3] replay: offer an option to linearize the commit topology

From

Toon Claes <toon@iotcl.com>

Date

Aug 31, 2026, 13:13 UTC

Message-ID

<20260831-toon-git-replay-drop-merges-v9-3-61c4232c6f36@iotcl.com>

In-Reply-To

<20260831-toon-git-replay-drop-merges-v9-0-61c4232c6f36@iotcl.com>

git-replay(1) accepts multiple branches, for example:

    $ git replay --onto main topic1 topic2

Based-on-patches-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Toon Claes <toon@iotcl.com>
---
 Documentation/git-replay.adoc |  17 ++++++-
 builtin/replay.c              |   6 ++-
 replay.c                      |  60 +++++++++++++++--------
 replay.h                      |   5 ++
 t/t3650-replay-basics.sh      | 109 +++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 174 insertions(+), 23 deletions(-)

diff --git a/Documentation/git-replay.adoc b/Documentation/git-replay.adoc
index ea4d14badd..58b4c0c470 100644
--- a/Documentation/git-replay.adoc
+++ b/Documentation/git-replay.adoc
@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 (EXPERIMENTAL!) 'git replay' ([--contained] --onto=<newbase> | --advance=<branch> | --revert=<branch>)
-			     [--ref=<ref>] [--ref-action=<mode>] <revision-range>
+			     [--ref=<ref>] [--ref-action=<mode>] [--linearize] <revision-range>
 
 DESCRIPTION
 -----------
@@ -91,6 +91,21 @@ Expanded description list compared to 'replay.refAction'.
 +
 The default mode can be configured via the `replay.refAction` configuration variable.
 
+--linearize::
+	In this mode, each replayed commit is stacked on top of the
+	previously replayed one, so all replayed commits are flattened into
+	a single linear history.
++
+When a merge commit is encountered, all commits in the range reachable
+from the merge commit are replayed into the linear history, and the
+merge commit itself is dropped. A ref that pointed to a merge commit is
+updated to the merge's last replayed ancestor. (This matches the
+behavior of git-rebase(1)'s `--no-rebase-merges` option.)
++
+`--linearize` cannot be combined with multiple branches or with
+`--contained`. To linearize several branches, replay them in separate
+`git replay` invocations.
+
 <revision-range>::
 	Range of commits to replay; see "Specifying Ranges" in
 	linkgit:git-rev-parse[1]. In `--advance=<branch>` or
diff --git a/builtin/replay.c b/builtin/replay.c
index 39e3a86f6c..d39626a37d 100644
--- a/builtin/replay.c
+++ b/builtin/replay.c
@@ -85,7 +85,7 @@ int cmd_replay(int argc,
 	const char *const replay_usage[] = {
 		N_("(EXPERIMENTAL!) git replay "
 		   "([--contained] --onto=<newbase> | --advance=<branch> | --revert=<branch>)\n"
-		   "[--ref=<ref>] [--ref-action=<mode>] <revision-range>"),
+		   "[--ref=<ref>] [--ref-action=<mode>] [--linearize] <revision-range>"),
 		NULL
 	};
 	struct option replay_options[] = {
@@ -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_("drop merge commits, replaying only non-merge commits")),
 		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.linearize, "--linearize",
+				  !!opts.contained, "--contained");
 
 	/* Parse ref action mode from command line or config */
 	ref_mode = get_ref_action_mode(repo, ref_action);
diff --git a/replay.c b/replay.c
index 7e35f40d37..6565e4d215 100644
--- a/replay.c
+++ b/replay.c
@@ -404,6 +404,12 @@ int replay_revisions(struct rev_info *revs,
 	set_up_replay_mode(revs->repo, &revs->cmdline, opts->onto,
 			   &detached_head, &advance, &revert, &onto, &update_refs);
 
+	if (opts->linearize &&
+	    update_refs && strset_get_size(update_refs) > 1) {
+		ret = error(_("'--linearize' cannot be used with multiple branches"));
+		goto out;
+	}
+
 	if (opts->ref) {
 		struct object_id oid;
 
@@ -437,26 +443,40 @@ int replay_revisions(struct rev_info *revs,
 	while ((commit = get_revision(revs))) {
 		const struct name_decoration *decoration;
 
-		/*
-		 * Decide where to replay this commit on.
-		 * If the parent commit was replayed already, the replayed result
-		 * can be found in `replayed_commits`. Otherwise fall back to `onto`.
-		 * When reverting, commits are replayed in reverse order and thus
-		 * its parent isn't replayed yet. Therefore revert commits are
-		 * always replayed onto `last_commit`.
-		 */
-		struct commit *parent = commit->parents ? commit->parents->item : NULL;
-		struct commit *base = get_mapped_commit(replayed_commits, parent, onto);
-
-		if (mode == REPLAY_MODE_REVERT)
-			base = last_commit;
-
-		if (commit->parents && commit->parents->next)
-			die(_("replaying merge commits is not supported yet!"));
-
-		last_commit = pick_regular_commit(revs->repo, commit, base,
-						  &merge_opt, &result,
-						  mode, opts->empty);
+		if (commit->parents && commit->parents->next) {
+			if (!opts->linearize)
+				die(_("replaying merge commits is not supported yet!"));
+			/*
+			 * Drop the merge commit: do not pick it, leave
+			 * `last_commit` unchanged, and fall through to the
+			 * rest of the loop. As a result:
+			 * - refs pointing to the merge commit will be updated
+			 *   to `last_commit`.
+			 * - the next replayed commit uses `last_commit` as its
+			 *   `base`.
+			 */
+		} else {
+			/*
+			 * Decide where to replay this commit onto.
+			 * If the parent commit was replayed already, the replayed result
+			 * can be found in `replayed_commits`. Otherwise fall back to `onto`.
+			 * When reverting, commits are replayed in reverse order and thus
+			 * its parent isn't replayed yet. Therefore revert commits are
+			 * always replayed onto `last_commit`.
+			 * Also when opts->linearize is true, set the base to
+			 * `last_commit` to create a single linear history.
+			 */
+			struct commit *parent = commit->parents ? commit->parents->item : NULL;
+			struct commit *base = get_mapped_commit(replayed_commits, parent, onto);
+
+			if (opts->linearize || mode == REPLAY_MODE_REVERT)
+				base = last_commit;
+
+			last_commit = pick_regular_commit(revs->repo, commit, base,
+							  &merge_opt, &result,
+							  mode, opts->empty);
+		}
+
 		if (!last_commit)
 			break;
 
diff --git a/replay.h b/replay.h
index 491db145e3..2c71afbfde 100644
--- a/replay.h
+++ b/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()`. */
diff --git a/t/t3650-replay-basics.sh b/t/t3650-replay-basics.sh
index 3353bc4a4d..d3409b9bb1 100755
--- a/t/t3650-replay-basics.sh
+++ b/t/t3650-replay-basics.sh
@@ -52,8 +52,19 @@ test_expect_success 'setup' '
 	test_merge P O --no-ff &&
 	git switch main &&
 
+	git switch --orphan unrelated &&
+	test_commit unrelated-root &&
+
 	git switch -c conflict B &&
-	test_commit C.conflict C.t conflict
+	test_commit C.conflict C.t conflict &&
+	git branch -D unrelated &&
+
+	git switch -c divergent-x main &&
+	test_commit X &&
+	git switch -c divergent-y main &&
+	test_commit Y &&
+	git switch divergent-x &&
+	test_merge Z divergent-y --no-ff
 '
 
 test_expect_success 'setup bare' '
@@ -565,4 +576,100 @@ test_expect_success '--onto with --ref rejects multiple revision ranges' '
 	test_grep "cannot be used with multiple revision ranges" err
 '
 
+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 the root commit' '
+	git replay --ref-action=print --linearize \
+		--onto unrelated-root 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 B A unrelated-root >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'replay to cherry-pick merge commit with --linearize' '
+	git replay --ref-action=print --linearize \
+		--advance 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 &&
+
+	printf "update refs/heads/main " >expect &&
+	printf "%s " $(cut -f 3 -d " " result) >>expect &&
+	git rev-parse main >>expect &&
+	test_cmp expect result
+'
+
+test_expect_success 'replay --linearize produces the same patches' '
+	git replay --ref-action=print --linearize \
+		--onto main I..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+	tip=$(cut -f 3 -d " " result) &&
+
+	# range-diff does not care about the dropped merge,
+	# so the original commits (I..topic-with-merge)
+	# and the replayed chain (main..tip) must produce identical patches.
+	git range-diff I..topic-with-merge main..$tip >out &&
+	test_file_not_empty out &&
+	test_grep ! -v "=" out &&
+
+	git log --oneline main..$tip >out &&
+	test_line_count = 3 out
+'
+
+test_expect_success '--linearize rejects multiple branches' '
+	test_must_fail git replay --ref-action=print --linearize \
+		--onto main ^B topic2 topic3 topic4 2>err &&
+	test_grep "cannot be used with multiple branches" err
+'
+
+test_expect_success 'replay with --linearize of a divergent merge keeps both sides' '
+	git replay --ref-action=print --linearize \
+		--onto main main..divergent-x >result &&
+	test_line_count = 1 result &&
+	tip=$(cut -f 3 -d " " result) &&
+
+	# The merge Z is dropped, but both X and Y are linearized onto main;
+	# neither side is lost.
+	git log --format=%s main..$tip >actual &&
+	test_write_lines Y X >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--linearize and --contained cannot be used together' '
+	test_must_fail git replay --ref-action=print --linearize --contained \
+		--onto main ^B topic-with-merge 2>err &&
+	test_grep "cannot be used together" err
+'
+
+test_expect_success 'replay --revert with --linearize reverts a range containing a merge' '
+	git replay --ref-action=print --revert=divergent-x --linearize \
+		main..divergent-x >result &&
+	test_line_count = 1 result &&
+	tip=$(cut -f 3 -d " " result) &&
+
+	git log --format=%s $tip >actual &&
+	test_write_lines \
+		"Revert \"X\"" "Revert \"Y\"" Z Y X M L B A >expect &&
+	test_cmp expect actual &&
+
+	test_must_fail git cat-file -e $tip:X.t &&
+	test_must_fail git cat-file -e $tip:Y.t
+'
+
 test_done

-- 
2.55.0.679.g6767b8d81c