Merge branch 'sa/replay-atomic-ref-updates' into seen

"git replay" (experimental) learned to perform ref updates itself
in a transaction by default, instead of emitting where each refs
should point at and leaving the actual update to another command.

* sa/replay-atomic-ref-updates:
  replay: make atomic ref updates the default behavior

Author: gitster

Documentation/git-replay.adoc

@@ -9,16 +9,16 @@ git-replay - EXPERIMENTAL: Replay commits on a new base, works with bare repos t
 SYNOPSIS
 --------
 [verse]
-(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch>) <revision-range>...
+(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch>) [--output-commands | --allow-partial] <revision-range>...
 
 DESCRIPTION
 -----------
 
 Takes ranges of commits and replays them onto a new location. Leaves
-the working tree and the index untouched, and updates no references.
-The output of this command is meant to be used as input to
-`git update-ref --stdin`, which would update the relevant branches
-(see the OUTPUT section below).
+the working tree and the index untouched, and by default updates the
+relevant references using atomic transactions. Use `--output-commands`
+to get the old default behavior where update commands that can be piped
+to `git update-ref --stdin` are emitted (see the OUTPUT section below).
 
 THIS COMMAND IS EXPERIMENTAL. THE BEHAVIOR MAY CHANGE.
 
@@ -42,6 +42,20 @@ 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).
 
+--output-commands::
+	Output update-ref commands instead of updating refs directly.
+	When this option is used, the output can be piped to `git update-ref --stdin`
+	for successive, relatively slow, ref updates. This is equivalent to the
+	old default behavior.
+
+--allow-partial::
+	Allow some ref updates to succeed even if others fail. By default,
+	ref updates are atomic (all succeed or all fail). With this option,
+	failed updates are reported as warnings rather than causing the entire
+	command to fail. The command exits with code 0 only if all updates
+	succeed; any failures result in exit code 1. Cannot be used with
+	`--output-commands`.
+
 <revision-range>::
 	Range of commits to replay. More than one <revision-range> can
 	be passed, but in `--advance <branch>` mode, they should have
@@ -54,15 +68,20 @@ include::rev-list-options.adoc[]
 OUTPUT
 ------
 
-When there are no conflicts, the output of this command is usable as
-input to `git update-ref --stdin`.  It is of the form:
+By default, when there are no conflicts, this command updates the relevant
+references using atomic transactions and produces no output. All ref updates
+succeed or all fail (atomic behavior). Use `--allow-partial` to allow some
+updates to succeed while others fail.
+
+When `--output-commands` is used, the output is usable as input to
+`git update-ref --stdin`. It is of the form:
 
 	update refs/heads/branch1 ${NEW_branch1_HASH} ${OLD_branch1_HASH}
 	update refs/heads/branch2 ${NEW_branch2_HASH} ${OLD_branch2_HASH}
 	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
+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).
 
@@ -77,41 +96,70 @@ is something other than 0 or 1.
 EXAMPLES
 --------
 
-To simply rebase `mybranch` onto `target`:
+To simply rebase `mybranch` onto `target` (default behavior):
 
 ------------
 $ 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.
+updates mybranch to point at the new commits and the second updates
+target to point at them.
+
+To get the old default behavior where update commands are emitted:
+
+------------
+$ git replay --output-commands --onto target origin/main..mybranch
+update refs/heads/mybranch ${NEW_mybranch_HASH} ${OLD_mybranch_HASH}
+------------
+
+To rebase multiple branches with partial failure tolerance:
+
+------------
+$ git replay --allow-partial --contained --onto origin/main origin/main..tipbranch
+------------
 
 What if you have a stack of branches, one depending upon another, and
 you'd really like to rebase the whole set?
 
 ------------
 $ git replay --contained --onto origin/main origin/main..tipbranch
+------------
+
+This automatically finds and rebases all branches contained within the
+`origin/main..tipbranch` range.
+
+Or if you want to see the old default behavior where update commands are emitted:
+
+------------
+$ git replay --output-commands --contained --onto origin/main origin/main..tipbranch
 update refs/heads/branch1 ${NEW_branch1_HASH} ${OLD_branch1_HASH}
 update refs/heads/branch2 ${NEW_branch2_HASH} ${OLD_branch2_HASH}
 update refs/heads/tipbranch ${NEW_tipbranch_HASH} ${OLD_tipbranch_HASH}
 ------------
 
 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:
+do. Here's an example where you explicitly specify which branches to rebase:
 
 ------------
 $ git replay --onto origin/main ^base branch1 branch2 branch3
+------------
+
+This gives you explicit control over exactly which branches are rebased,
+unlike the previous `--contained` example which automatically discovers them.
+
+To see the update commands that would be executed:
+
+------------
+$ git replay --output-commands --onto origin/main ^base branch1 branch2 branch3
 update refs/heads/branch1 ${NEW_branch1_HASH} ${OLD_branch1_HASH}
 update refs/heads/branch2 ${NEW_branch2_HASH} ${OLD_branch2_HASH}
 update refs/heads/branch3 ${NEW_branch3_HASH} ${OLD_branch3_HASH}

builtin/replay.c

@@ -284,6 +284,28 @@ static struct commit *pick_regular_commit(struct repository *repo,
 	return create_commit(repo, result->tree, pickme, replayed_base);
 }
 
+static int add_ref_to_transaction(struct ref_transaction *transaction,
+				  const char *refname,
+				  const struct object_id *new_oid,
+				  const struct object_id *old_oid,
+				  struct strbuf *err)
+{
+	return ref_transaction_update(transaction, refname, new_oid, old_oid,
+				      NULL, NULL, 0, "git replay", err);
+}
+
+static void print_rejected_update(const char *refname,
+				  const struct object_id *old_oid UNUSED,
+				  const struct object_id *new_oid UNUSED,
+				  const char *old_target UNUSED,
+				  const char *new_target UNUSED,
+				  enum ref_transaction_error err,
+				  void *cb_data UNUSED)
+{
+	const char *reason = ref_transaction_error_msg(err);
+	warning(_("failed to update %s: %s"), refname, reason);
+}
+
 int cmd_replay(int argc,
 	       const char **argv,
 	       const char *prefix,
@@ -294,6 +316,8 @@ int cmd_replay(int argc,
 	struct commit *onto = NULL;
 	const char *onto_name = NULL;
 	int contained = 0;
+	int output_commands = 0;
+	int allow_partial = 0;
 
 	struct rev_info revs;
 	struct commit *last_commit = NULL;
@@ -302,12 +326,15 @@ int cmd_replay(int argc,
 	struct merge_result result;
 	struct strset *update_refs = NULL;
 	kh_oid_map_t *replayed_commits;
+	struct ref_transaction *transaction = NULL;
+	struct strbuf transaction_err = STRBUF_INIT;
+	int commits_processed = 0;
 	int ret = 0;
 
-	const char * const replay_usage[] = {
+	const char *const replay_usage[] = {
 		N_("(EXPERIMENTAL!) git replay "
 		   "([--contained] --onto <newbase> | --advance <branch>) "
-		   "<revision-range>..."),
+		   "[--output-commands | --allow-partial] <revision-range>..."),
 		NULL
 	};
 	struct option replay_options[] = {
@@ -319,6 +346,10 @@ int cmd_replay(int argc,
 			   N_("replay onto given commit")),
 		OPT_BOOL(0, "contained", &contained,
 			 N_("advance all branches contained in revision-range")),
+		OPT_BOOL(0, "output-commands", &output_commands,
+			 N_("output update commands instead of updating refs")),
+		OPT_BOOL(0, "allow-partial", &allow_partial,
+			 N_("allow some ref updates to succeed even if others fail")),
 		OPT_END()
 	};
 
@@ -330,9 +361,12 @@ int cmd_replay(int argc,
 		usage_with_options(replay_usage, replay_options);
 	}
 
-	if (advance_name_opt && contained)
-		die(_("options '%s' and '%s' cannot be used together"),
-		    "--advance", "--contained");
+	die_for_incompatible_opt2(!!advance_name_opt, "--advance",
+				  contained, "--contained");
+
+	die_for_incompatible_opt2(allow_partial, "--allow-partial",
+				  output_commands, "--output-commands");
+
 	advance_name = xstrdup_or_null(advance_name_opt);
 
 	repo_init_revisions(repo, &revs, prefix);
@@ -389,6 +423,17 @@ int cmd_replay(int argc,
 	determine_replay_mode(repo, &revs.cmdline, onto_name, &advance_name,
 			      &onto, &update_refs);
 
+	if (!output_commands) {
+		unsigned int transaction_flags = allow_partial ? REF_TRANSACTION_ALLOW_FAILURE : 0;
+		transaction = ref_store_transaction_begin(get_main_ref_store(repo),
+							  transaction_flags,
+							  &transaction_err);
+		if (!transaction) {
+			ret = error(_("failed to begin ref transaction: %s"), transaction_err.buf);
+			goto cleanup;
+		}
+	}
+
 	if (!onto) /* FIXME: Should handle replaying down to root commit */
 		die("Replaying down to root commit is not supported yet!");
 
@@ -407,6 +452,8 @@ int cmd_replay(int argc,
 		khint_t pos;
 		int hr;
 
+		commits_processed = 1;
+
 		if (!commit->parents)
 			die(_("replaying down to root commit is not supported yet!"));
 		if (commit->parents->next)
@@ -434,21 +481,52 @@ int cmd_replay(int argc,
 			if (decoration->type == DECORATION_REF_LOCAL &&
 			    (contained || strset_contains(update_refs,
 							  decoration->name))) {
-				printf("update %s %s %s\n",
-				       decoration->name,
-				       oid_to_hex(&last_commit->object.oid),
-				       oid_to_hex(&commit->object.oid));
+				if (output_commands) {
+					printf("update %s %s %s\n",
+					       decoration->name,
+					       oid_to_hex(&last_commit->object.oid),
+					       oid_to_hex(&commit->object.oid));
+				} else if (add_ref_to_transaction(transaction, decoration->name,
+								  &last_commit->object.oid,
+								  &commit->object.oid,
+								  &transaction_err) < 0) {
+					ret = error(_("failed to add ref update to transaction: %s"), transaction_err.buf);
+					goto cleanup;
+				}
 			}
 			decoration = decoration->next;
 		}
 	}
 
 	/* 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));
+		if (output_commands) {
+			printf("update %s %s %s\n",
+			       advance_name,
+			       oid_to_hex(&last_commit->object.oid),
+			       oid_to_hex(&onto->object.oid));
+		} else if (add_ref_to_transaction(transaction, advance_name,
+						  &last_commit->object.oid,
+						  &onto->object.oid,
+						  &transaction_err) < 0) {
+			ret = error(_("failed to add ref update to transaction: %s"), transaction_err.buf);
+			goto cleanup;
+		}
+	}
+
+	/* Commit the ref transaction if we have one */
+	if (transaction && result.clean == 1) {
+		if (ref_transaction_commit(transaction, &transaction_err)) {
+			if (allow_partial) {
+				warning(_("some ref updates failed: %s"), transaction_err.buf);
+				ref_transaction_for_each_rejected_update(transaction,
+									 print_rejected_update, NULL);
+				ret = 0; /* Set failure even with allow_partial */
+			} else {
+				ret = error(_("failed to update refs: %s"), transaction_err.buf);
+				goto cleanup;
+			}
+		}
 	}
 
 	merge_finalize(&merge_opt, &result);
@@ -457,9 +535,17 @@ int cmd_replay(int argc,
 		strset_clear(update_refs);
 		free(update_refs);
 	}
-	ret = result.clean;
+
+	/* Handle empty ranges: if no commits were processed, treat as success */
+	if (!commits_processed)
+		ret = 1; /* Success - no commits to replay is not an error */
+	else
+		ret = result.clean;
 
 cleanup:
+	if (transaction)
+		ref_transaction_free(transaction);
+	strbuf_release(&transaction_err);
 	release_revisions(&revs);
 	free(advance_name);