builtin/refs: add '--no-reflog' flag to drop reflogs

The "git refs migrate" subcommand converts the backend used for ref
storage. It always migrates reflog data as well as refs. Introduce an
option to exclude reflogs from migration, allowing them to be discarded
when they are unnecessary.

This is particularly useful in server-side repositories, where reflogs
are typically not expected. However, some repositories may still have
them due to historical reasons, such as bugs, misconfigurations, or
administrative decisions to enable reflogs for debugging. In such
repositories, it would be optimal to drop reflogs during the migration.

To address this, introduce the '--no-reflog' flag, which prevents reflog
migration. When this flag is used, reflogs from the original reference
backend are migrated. Since only the new reference backend remains in
the repository, all previous reflogs are permanently discarded.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-refs.txt

diff --git a/Documentation/git-refs.txt b/Documentation/git-refs.txt
index 9829984..733ada7 100644
--- a/Documentation/git-refs.txt
+++ b/Documentation/git-refs.txt
@@ -8,9 +8,9 @@
 
 SYNOPSIS
 --------
-[verse]
-'git refs migrate' --ref-format=<format> [--dry-run]
-'git refs verify' [--strict] [--verbose]
+[synopsis]
+git refs migrate --ref-format=<format> [--no-reflog] [--dry-run]
+git refs verify [--strict] [--verbose]
 
 DESCRIPTION
 -----------
@@ -43,6 +43,11 @@
 	can be used to double check that the migration works as expected before
 	performing the actual migration.
 
+--reflog::
+--no-reflog::
+	Choose between migrating the reflog data to the new backend,
+	and discarding them.  The default is "--reflog", to migrate.
+
 The following options are specific to 'git refs verify':
 
 --strict::

builtin/refs.c

diff --git a/builtin/refs.c b/builtin/refs.c
index a29f195..c459507 100644
--- a/builtin/refs.c
+++ b/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, "no-reflog", &flags,
+			N_("drop reflogs entirely during the migration"),
+			REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG),
 		OPT_END(),
 	};
 	struct strbuf errbuf = STRBUF_INIT;

refs.c

diff --git a/refs.c b/refs.c
index f4094a3..5e8f5c0 100644
--- a/refs.c
+++ b/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

diff --git a/refs.h b/refs.h
index a0cdd99..ccee8fc 100644
--- a/refs.h
+++ b/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

diff --git a/t/t1460-refs-migrate.sh b/t/t1460-refs-migrate.sh
index a6d9b35..2ab97e1 100755
--- a/t/t1460-refs-migrate.sh
+++ b/t/t1460-refs-migrate.sh
@@ -9,14 +9,21 @@
 
 # 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 ! $skip_reflog_verify
@@ -25,7 +32,7 @@
 	   git -C "$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 "$repo" for-each-ref --include-root-refs \
 		--format='%(refname) %(objectname) %(symref)' >actual &&
@@ -241,6 +248,19 @@
 				test_cmp expect.reflog actual.reflog
 			)
 		'
+
+		test_expect_success "$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 --no-reflog &&
+			# there should be no reflogs post migration.
+			git -C repo reflog --all >reflogs &&
+			test_must_be_empty reflogs
+		'
 	done
 done