replay: add --advance or 'cherry-pick' mode

There is already a 'rebase' mode with `--onto`. Let's add an 'advance' or
'cherry-pick' mode with `--advance`. This new mode will make the target
branch advance as we replay commits onto it.

The replayed commits should have a single tip, so that it's clear where
the target branch should be advanced. If they have more than one tip,
this new mode will error out.

Co-authored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-replay.txt

@@ -9,7 +9,7 @@ git-replay - EXPERIMENTAL: Replay commits on a new base, works with bare repos t
 SYNOPSIS
 --------
 [verse]
-(EXPERIMENTAL!) 'git replay' --onto <newbase> <revision-range>...
+(EXPERIMENTAL!) 'git replay' (--onto <newbase> | --advance <branch>) <revision-range>...
 
 DESCRIPTION
 -----------
@@ -29,14 +29,25 @@ OPTIONS
 	Starting point at which to create the new commits.  May be any
 	valid commit, and not just an existing branch name.
 +
-The update-ref command(s) in the output will update the branch(es) in
-the revision range to point at the new commits, similar to the way how
-`git rebase --update-refs` updates multiple branches in the affected
-range.
+When `--onto` is specified, the update-ref command(s) in the output will
+update the branch(es) in the revision range to point at the new
+commits, similar to the way how `git rebase --update-refs` updates
+multiple branches in the affected range.
+
+--advance <branch>::
+	Starting point at which to create the new commits; must be a
+	branch name.
++
+When `--advance` is specified, the update-ref command(s) in the output
+will update the branch passed as an argument to `--advance` to point at
+the new commits (in other words, this mimics a cherry-pick operation).
 
 <revision-range>::
-	Range of commits to replay; see "Specifying Ranges" in
-	linkgit:git-rev-parse and the "Commit Limiting" options below.
+	Range of commits to replay. More than one <revision-range> can
+	be passed, but in `--advance <branch>` mode, they should have
+	a single tip, so that it's clear where <branch> should point
+	to. See "Specifying Ranges" in linkgit:git-rev-parse and the
+	"Commit Limiting" options below.
 
 include::rev-list-options.txt[]
 
@@ -51,7 +62,9 @@ input to `git update-ref --stdin`.  It is of the form:
 	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.
+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).
 
 EXIT STATUS
 -----------
@@ -71,6 +84,18 @@ $ git replay --onto target origin/main..mybranch
 update refs/heads/mybranch ${NEW_mybranch_HASH} ${OLD_mybranch_HASH}
 ------------
 
+To cherry-pick the commits from mybranch onto target:
+
+------------
+$ git replay --advance target origin/main..mybranch
+update refs/heads/target ${NEW_target_HASH} ${OLD_target_HASH}
+------------
+
+Note that the first two examples replay the exact same commits and on
+top of the exact same new base, they only differ in that the first
+provides instructions to make mybranch point at the new commits and
+the second provides instructions to make target point at them.
+
 When calling `git replay`, one does not need to specify a range of
 commits to replay using the syntax `A..B`; any range expression will
 do:

builtin/replay.c

@@ -14,6 +14,7 @@
 #include "parse-options.h"
 #include "refs.h"
 #include "revision.h"
+#include "strmap.h"
 #include <oidset.h>
 #include <tree.h>
 
@@ -82,6 +83,146 @@ static struct commit *create_commit(struct tree *tree,
 	return (struct commit *)obj;
 }
 
+struct ref_info {
+	struct commit *onto;
+	struct strset positive_refs;
+	struct strset negative_refs;
+	int positive_refexprs;
+	int negative_refexprs;
+};
+
+static void get_ref_information(struct rev_cmdline_info *cmd_info,
+				struct ref_info *ref_info)
+{
+	int i;
+
+	ref_info->onto = NULL;
+	strset_init(&ref_info->positive_refs);
+	strset_init(&ref_info->negative_refs);
+	ref_info->positive_refexprs = 0;
+	ref_info->negative_refexprs = 0;
+
+	/*
+	 * When the user specifies e.g.
+	 *   git replay origin/main..mybranch
+	 *   git replay ^origin/next mybranch1 mybranch2
+	 * we want to be able to determine where to replay the commits.  In
+	 * these examples, the branches are probably based on an old version
+	 * of either origin/main or origin/next, so we want to replay on the
+	 * newest version of that branch.  In contrast we would want to error
+	 * out if they ran
+	 *   git replay ^origin/master ^origin/next mybranch
+	 *   git replay mybranch~2..mybranch
+	 * the first of those because there's no unique base to choose, and
+	 * the second because they'd likely just be replaying commits on top
+	 * of the same commit and not making any difference.
+	 */
+	for (i = 0; i < cmd_info->nr; i++) {
+		struct rev_cmdline_entry *e = cmd_info->rev + i;
+		struct object_id oid;
+		const char *refexpr = e->name;
+		char *fullname = NULL;
+		int can_uniquely_dwim = 1;
+
+		if (*refexpr == '^')
+			refexpr++;
+		if (repo_dwim_ref(the_repository, refexpr, strlen(refexpr), &oid, &fullname, 0) != 1)
+			can_uniquely_dwim = 0;
+
+		if (e->flags & BOTTOM) {
+			if (can_uniquely_dwim)
+				strset_add(&ref_info->negative_refs, fullname);
+			if (!ref_info->negative_refexprs)
+				ref_info->onto = lookup_commit_reference_gently(the_repository,
+										&e->item->oid, 1);
+			ref_info->negative_refexprs++;
+		} else {
+			if (can_uniquely_dwim)
+				strset_add(&ref_info->positive_refs, fullname);
+			ref_info->positive_refexprs++;
+		}
+
+		free(fullname);
+	}
+}
+
+static void determine_replay_mode(struct rev_cmdline_info *cmd_info,
+				  const char *onto_name,
+				  const char **advance_name,
+				  struct commit **onto,
+				  struct strset **update_refs)
+{
+	struct ref_info rinfo;
+
+	get_ref_information(cmd_info, &rinfo);
+	if (!rinfo.positive_refexprs)
+		die(_("need some commits to replay"));
+	if (onto_name && *advance_name)
+		die(_("--onto and --advance are incompatible"));
+	else if (onto_name) {
+		*onto = peel_committish(onto_name);
+		if (rinfo.positive_refexprs <
+		    strset_get_size(&rinfo.positive_refs))
+			die(_("all positive revisions given must be references"));
+	} else if (*advance_name) {
+		struct object_id oid;
+		char *fullname = NULL;
+
+		*onto = peel_committish(*advance_name);
+		if (repo_dwim_ref(the_repository, *advance_name, strlen(*advance_name),
+			     &oid, &fullname, 0) == 1) {
+			*advance_name = fullname;
+		} else {
+			die(_("argument to --advance must be a reference"));
+		}
+		if (rinfo.positive_refexprs > 1)
+			die(_("cannot advance target with multiple sources because ordering would be ill-defined"));
+	} else {
+		int positive_refs_complete = (
+			rinfo.positive_refexprs ==
+			strset_get_size(&rinfo.positive_refs));
+		int negative_refs_complete = (
+			rinfo.negative_refexprs ==
+			strset_get_size(&rinfo.negative_refs));
+		/*
+		 * We need either positive_refs_complete or
+		 * negative_refs_complete, but not both.
+		 */
+		if (rinfo.negative_refexprs > 0 &&
+		    positive_refs_complete == negative_refs_complete)
+			die(_("cannot implicitly determine whether this is an --advance or --onto operation"));
+		if (negative_refs_complete) {
+			struct hashmap_iter iter;
+			struct strmap_entry *entry;
+
+			if (rinfo.negative_refexprs == 0)
+				die(_("all positive revisions given must be references"));
+			else if (rinfo.negative_refexprs > 1)
+				die(_("cannot implicitly determine whether this is an --advance or --onto operation"));
+			else if (rinfo.positive_refexprs > 1)
+				die(_("cannot advance target with multiple source branches because ordering would be ill-defined"));
+
+			/* Only one entry, but we have to loop to get it */
+			strset_for_each_entry(&rinfo.negative_refs,
+					      &iter, entry) {
+				*advance_name = entry->key;
+			}
+		} else { /* positive_refs_complete */
+			if (rinfo.negative_refexprs > 1)
+				die(_("cannot implicitly determine correct base for --onto"));
+			if (rinfo.negative_refexprs == 1)
+				*onto = rinfo.onto;
+		}
+	}
+	if (!*advance_name) {
+		*update_refs = xcalloc(1, sizeof(**update_refs));
+		**update_refs = rinfo.positive_refs;
+		memset(&rinfo.positive_refs, 0, sizeof(**update_refs));
+	}
+	strset_clear(&rinfo.negative_refs);
+	strset_clear(&rinfo.positive_refs);
+}
+
 static struct commit *pick_regular_commit(struct commit *pickme,
 					  struct commit *last_commit,
 					  struct merge_options *merge_opt,
@@ -114,20 +255,26 @@ static struct commit *pick_regular_commit(struct commit *pickme,
 
 int cmd_replay(int argc, const char **argv, const char *prefix)
 {
-	struct commit *onto;
+	const char *advance_name = NULL;
+	struct commit *onto = NULL;
 	const char *onto_name = NULL;
-	struct commit *last_commit = NULL;
+
 	struct rev_info revs;
+	struct commit *last_commit = NULL;
 	struct commit *commit;
 	struct merge_options merge_opt;
 	struct merge_result result;
+	struct strset *update_refs = NULL;
 	int ret = 0;
 
 	const char * const replay_usage[] = {
-		N_("(EXPERIMENTAL!) git replay --onto <newbase> <revision-range>..."),
+		N_("(EXPERIMENTAL!) git replay (--onto <newbase> | --advance <branch>) <revision-range>..."),
 		NULL
 	};
 	struct option replay_options[] = {
+		OPT_STRING(0, "advance", &advance_name,
+			   N_("branch"),
+			   N_("make replay advance given branch")),
 		OPT_STRING(0, "onto", &onto_name,
 			   N_("revision"),
 			   N_("replay onto given commit")),
@@ -137,13 +284,11 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, replay_options, replay_usage,
 			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN_OPT);
 
-	if (!onto_name) {
-		error(_("option --onto is mandatory"));
+	if (!onto_name && !advance_name) {
+		error(_("option --onto or --advance is mandatory"));
 		usage_with_options(replay_usage, replay_options);
 	}
 
-	onto = peel_committish(onto_name);
-
 	repo_init_revisions(the_repository, &revs, prefix);
 
 	/*
@@ -195,6 +340,12 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 		revs.simplify_history = 0;
 	}
 
+	determine_replay_mode(&revs.cmdline, onto_name, &advance_name,
+			      &onto, &update_refs);
+
+	if (!onto) /* FIXME: Should handle replaying down to root commit */
+		die("Replaying down to root commit is not supported yet!");
+
 	if (prepare_revision_walk(&revs) < 0) {
 		ret = error(_("error preparing revisions"));
 		goto cleanup;
@@ -203,6 +354,7 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 	init_merge_options(&merge_opt, the_repository);
 	memset(&result, 0, sizeof(result));
 	merge_opt.show_rename_progress = 0;
+
 	result.tree = repo_get_commit_tree(the_repository, onto);
 	last_commit = onto;
 	while ((commit = get_revision(&revs))) {
@@ -217,12 +369,15 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 		if (!last_commit)
 			break;
 
+		/* Update any necessary branches */
+		if (advance_name)
+			continue;
 		decoration = get_name_decoration(&commit->object);
 		if (!decoration)
 			continue;
-
 		while (decoration) {
-			if (decoration->type == DECORATION_REF_LOCAL) {
+			if (decoration->type == DECORATION_REF_LOCAL &&
+			    strset_contains(update_refs, decoration->name)) {
 				printf("update %s %s %s\n",
 				       decoration->name,
 				       oid_to_hex(&last_commit->object.oid),
@@ -232,10 +387,22 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	/* In --advance mode, advance the target ref */
+	if (result.clean == 1 && advance_name) {
+		printf("update %s %s %s\n",
+		       advance_name,
+		       oid_to_hex(&last_commit->object.oid),
+		       oid_to_hex(&onto->object.oid));
+	}
+
 	merge_finalize(&merge_opt, &result);
 	ret = result.clean;
 
 cleanup:
+	if (update_refs) {
+		strset_clear(update_refs);
+		free(update_refs);
+	}
 	release_revisions(&revs);
 
 	/* Return */

t/t3650-replay-basics.sh

@@ -80,4 +80,38 @@ test_expect_success 'using replay on bare repo to rebase with a conflict' '
 	test_expect_code 1 git -C bare replay --onto topic1 B..conflict
 '
 
+test_expect_success 'using replay to perform basic cherry-pick' '
+	# The differences between this test and previous ones are:
+	#   --advance vs --onto
+	# 2nd field of result is refs/heads/main vs. refs/heads/topic2
+	# 4th field of result is hash for main instead of hash for topic2
+
+	git replay --advance main topic1..topic2 >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines E D 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 'using replay on bare repo to perform basic cherry-pick' '
+	git -C bare replay --advance main topic1..topic2 >result-bare &&
+	test_cmp expect result-bare
+'
+
+test_expect_success 'replay on bare repo fails with both --advance and --onto' '
+	test_must_fail git -C bare replay --advance main --onto main topic1..topic2 >result-bare
+'
+
+test_expect_success 'replay fails when both --advance and --onto are omitted' '
+	test_must_fail git replay topic1..topic2 >result
+'
+
 test_done