Merge branch 'sa/replay-revert'

"git replay" (experimental) learns, in addition to "pick" and
"replay", a new operating mode "revert".

* sa/replay-revert:
  replay: add --revert mode to reverse commit changes
  sequencer: extract revert message formatting into shared function

Author: gitster

Documentation/git-replay.adoc

@@ -9,7 +9,7 @@ git-replay - EXPERIMENTAL: Replay commits on a new base, works with bare repos t
 SYNOPSIS
 --------
 [verse]
-(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch>) [--ref-action[=<mode>]] <revision-range>
+(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch> | --revert <branch>) [--ref-action[=<mode>]] <revision-range>
 
 DESCRIPTION
 -----------
@@ -42,6 +42,25 @@ The history is replayed on top of the <branch> and <branch> is updated to
 point at the tip of the resulting history. This is different from `--onto`,
 which uses the target only as a starting point without updating it.
 
+--revert <branch>::
+	Starting point at which to create the reverted commits; must be a
+	branch name.
++
+When `--revert` is specified, the commits in the revision range are reverted
+(their changes are undone) and the reverted commits are created on top of
+<branch>. The <branch> is then updated to point at the new commits. This is
+the same as running `git revert <revision-range>` but does not update the
+working tree.
++
+The commit messages follow `git revert` conventions: they are prefixed with
+"Revert" and include "This reverts commit <hash>." When reverting a commit
+whose message starts with "Revert", the new message uses "Reapply" instead.
+Unlike cherry-pick which preserves the original author, revert commits use
+the current user as the author, matching the behavior of `git revert`.
++
+This option is mutually exclusive with `--onto` and `--advance`. It is also
+incompatible with `--contained` (which is a modifier for `--onto` only).
+
 --contained::
 	Update all branches that point at commits in
 	<revision-range>. Requires `--onto`.
@@ -60,10 +79,11 @@ The default mode can be configured via the `replay.refAction` configuration vari
 
 <revision-range>::
 	Range of commits to replay; see "Specifying Ranges" in
-	linkgit:git-rev-parse[1]. In `--advance <branch>` mode, the
-	range should have a single tip, so that it's clear to which tip the
-	advanced <branch> should point. Any commits in the range whose
-	changes are already present in the branch the commits are being
+	linkgit:git-rev-parse[1]. In `--advance <branch>` or
+	`--revert <branch>` mode, the range should have a single tip,
+	so that it's clear to which tip the advanced or reverted
+	<branch> should point. Any commits in the range whose changes
+	are already present in the branch the commits are being
 	replayed onto will be dropped.
 
 :git-replay: 1
@@ -84,9 +104,10 @@ When using `--ref-action=print`, the output is usable as input to
 	update refs/heads/branch3 ${NEW_branch3_HASH} ${OLD_branch3_HASH}
 
 where the number of refs updated depends on the arguments passed and
-the shape of the history being replayed.  When using `--advance`, the
-number of refs updated is always one, but for `--onto`, it can be one
-or more (rebasing multiple branches simultaneously is supported).
+the shape of the history being replayed.  When using `--advance` or
+`--revert`, the number of refs updated is always one, but for `--onto`,
+it can be one or more (rebasing multiple branches simultaneously is
+supported).
 
 There is no stderr output on conflicts; see the <<exit-status,EXIT
 STATUS>> section below.
@@ -152,6 +173,21 @@ all commits they have since `base`, playing them on top of
 `origin/main`. These three branches may have commits on top of `base`
 that they have in common, but that does not need to be the case.
 
+To revert commits on a branch:
+
+------------
+$ git replay --revert main topic~2..topic
+------------
+
+This reverts the last two commits from `topic`, creating revert commits on
+top of `main`, and updates `main` to point at the result. This is useful when
+commits from `topic` were previously merged or cherry-picked into `main` and
+need to be undone.
+
+NOTE: For reverting an entire merge request as a single commit (rather than
+commit-by-commit), consider using `git merge-tree --merge-base $TIP HEAD $BASE`
+which can avoid unnecessary merge conflicts.
+
 GIT
 ---
 Part of the linkgit:git[1] suite

builtin/replay.c

@@ -79,11 +79,12 @@ int cmd_replay(int argc,
 	struct ref_transaction *transaction = NULL;
 	struct strbuf transaction_err = STRBUF_INIT;
 	struct strbuf reflog_msg = STRBUF_INIT;
+	int desired_reverse;
 	int ret = 0;
 
 	const char *const replay_usage[] = {
 		N_("(EXPERIMENTAL!) git replay "
-		   "([--contained] --onto <newbase> | --advance <branch>) "
+		   "([--contained] --onto <newbase> | --advance <branch> | --revert <branch>) "
 		   "[--ref-action[=<mode>]] <revision-range>"),
 		NULL
 	};
@@ -96,6 +97,9 @@ int cmd_replay(int argc,
 			   N_("replay onto given commit")),
 		OPT_BOOL(0, "contained", &opts.contained,
 			 N_("update all branches that point at commits in <revision-range>")),
+		OPT_STRING(0, "revert", &opts.revert,
+			   N_("branch"),
+			   N_("revert commits onto given branch")),
 		OPT_STRING(0, "ref-action", &ref_action,
 			   N_("mode"),
 			   N_("control ref update behavior (update|print)")),
@@ -105,19 +109,31 @@ int cmd_replay(int argc,
 	argc = parse_options(argc, argv, prefix, replay_options, replay_usage,
 			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN_OPT);
 
-	if (!opts.onto && !opts.advance) {
-		error(_("option --onto or --advance is mandatory"));
+	/* Exactly one mode must be specified */
+	if (!opts.onto && !opts.advance && !opts.revert) {
+		error(_("exactly one of --onto, --advance, or --revert is required"));
 		usage_with_options(replay_usage, replay_options);
 	}
 
+	die_for_incompatible_opt3(!!opts.onto, "--onto",
+				  !!opts.advance, "--advance",
+				  !!opts.revert, "--revert");
 	die_for_incompatible_opt2(!!opts.advance, "--advance",
 				  opts.contained, "--contained");
-	die_for_incompatible_opt2(!!opts.advance, "--advance",
-				  !!opts.onto, "--onto");
+	die_for_incompatible_opt2(!!opts.revert, "--revert",
+				  opts.contained, "--contained");
 
 	/* Parse ref action mode from command line or config */
 	ref_mode = get_ref_action_mode(repo, ref_action);
 
+	/*
+	 * Cherry-pick/rebase need oldest-first ordering so that each
+	 * replayed commit can build on its already-replayed parent.
+	 * Revert needs newest-first ordering (like git revert) to
+	 * reduce conflicts by peeling off changes from the top.
+	 */
+	desired_reverse = !opts.revert;
+
 	repo_init_revisions(repo, &revs, prefix);
 
 	/*
@@ -129,7 +145,7 @@ int cmd_replay(int argc,
 	 * some options changing these values if we think they could
 	 * be useful.
 	 */
-	revs.reverse = 1;
+	revs.reverse = desired_reverse;
 	revs.sort_order = REV_SORT_IN_GRAPH_ORDER;
 	revs.topo_order = 1;
 	revs.simplify_history = 0;
@@ -144,11 +160,11 @@ int cmd_replay(int argc,
 	 * Detect and warn if we override some user specified rev
 	 * walking options.
 	 */
-	if (revs.reverse != 1) {
+	if (revs.reverse != desired_reverse) {
 		warning(_("some rev walking options will be overridden as "
 			  "'%s' bit in 'struct rev_info' will be forced"),
 			"reverse");
-		revs.reverse = 1;
+		revs.reverse = desired_reverse;
 	}
 	if (revs.sort_order != REV_SORT_IN_GRAPH_ORDER) {
 		warning(_("some rev walking options will be overridden as "
@@ -174,7 +190,9 @@ int cmd_replay(int argc,
 		goto cleanup;
 
 	/* Build reflog message */
-	if (opts.advance) {
+	if (opts.revert) {
+		strbuf_addf(&reflog_msg, "replay --revert %s", opts.revert);
+	} else if (opts.advance) {
 		strbuf_addf(&reflog_msg, "replay --advance %s", opts.advance);
 	} else {
 		struct object_id oid;