builtin/refs: add '--skip-reflog' flag to bypass reflog migration

The 'git-refs(1)' migrate subcommand, which transfers repositories
between reference backends, currently migrates reflogs by default as of
246cebe (refs: add support for migrating reflogs, 2024-12-16).

While this behavior is desirable for most client-side repositories,
server-side repositories are not expected to contain reflogs. However,
due to historical reasons, some may still have them. This could be
caused, for example, by bugs, misconfiguration, or an administrator
enabling reflogs on the server for debugging purposes.

To address this, introduce the --skip-reflog flag, allowing users to
bypass reflog migration. This ensures that the repository ends up in the
expected state after migration.

Helped-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

builtin/refs.c

@@ -30,6 +30,9 @@ static int cmd_refs_migrate(int argc, const char **argv, const char *prefix,
 		OPT_BIT(0, "dry-run", &flags,
 			N_("perform a non-destructive dry-run"),
 			REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN),
+		OPT_BIT(0, "skip-reflog", &flags,
+			N_("skip migrating reflogs"),
+			REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG),
 		OPT_END(),
 	};
 	struct strbuf errbuf = STRBUF_INIT;

refs.c

@@ -3035,9 +3035,11 @@ int repo_migrate_ref_storage_format(struct repository *repo,
 	if (ret < 0)
 		goto done;
 
-	ret = refs_for_each_reflog(old_refs, migrate_one_reflog, &data);
-	if (ret < 0)
-		goto done;
+	if (!(flags & REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG)) {
+		ret = refs_for_each_reflog(old_refs, migrate_one_reflog, &data);
+		if (ret < 0)
+			goto done;
+	}
 
 	ret = ref_transaction_commit(transaction, errbuf);
 	if (ret < 0)

refs.h

@@ -1157,8 +1157,11 @@ int is_pseudo_ref(const char *refname);
  *   - REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN: perform a dry-run migration
  *     without touching the main repository. The result will be written into a
  *     temporary ref storage directory.
+ *
+ *   - REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG: skip migration of reflogs.
  */
-#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN (1 << 0)
+#define REPO_MIGRATE_REF_STORAGE_FORMAT_DRYRUN      (1 << 0)
+#define REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG (1 << 1)
 
 /*
  * Migrate the ref storage format used by the repository to the

t/t1460-refs-migrate.sh

@@ -9,14 +9,21 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 # Migrate the provided repository from one format to the other and
 # verify that the references and logs are migrated over correctly.
-# Usage: test_migration <repo> <format> <skip_reflog_verify>
+# Usage: test_migration <repo> <format> [<skip_reflog_verify> [<options...>]]
 #   <repo> is the relative path to the repo to be migrated.
 #   <format> is the ref format to be migrated to.
-#   <skip_reflog_verify> (true or false) whether to skip reflog verification.
+#   <skip_reflog_verify> (default: false) whether to skip reflog verification.
+#   <options...> are other options be passed directly to 'git refs migrate'.
 test_migration () {
 	repo=$1 &&
 	format=$2 &&
-	skip_reflog_verify=${3:-false} &&
+	shift 2 &&
+	skip_reflog_verify=false &&
+	if test $# -ge 1
+	then
+		skip_reflog_verify=$1
+		shift
+	fi &&
 	git -C "$repo" for-each-ref --include-root-refs \
 		--format='%(refname) %(objectname) %(symref)' >expect &&
 	if ! $skip_reflog_verify
@@ -25,7 +32,7 @@ test_migration () {
 	   git -C "$repo" reflog list >expect_log_list
 	fi &&
 
-	git -C "$repo" refs migrate --ref-format="$2" &&
+	git -C "$repo" refs migrate --ref-format="$format" "$@" &&
 
 	git -C "$repo" for-each-ref --include-root-refs \
 		--format='%(refname) %(objectname) %(symref)' >actual &&
@@ -241,6 +248,19 @@ do
 				test_cmp expect.reflog actual.reflog
 			)
 		'
+
+		test_expect_success "$from_format -> $to_format: skip reflog with --skip-reflog" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			test_commit -C repo initial &&
+			# we see that the repository contains reflogs.
+			git -C repo reflog --all >reflogs &&
+			test_line_count = 2 reflogs &&
+			test_migration repo "$to_format" true --skip-reflog &&
+			# there should be no reflogs post migration.
+			git -C repo reflog --all >reflogs &&
+			test_must_be_empty reflogs
+		'
 	done
 done