[PATCH v4] builtin/refs: add '--no-reflog' flag to drop reflogs

[Karthik Nayak <karthik.188@gmail.com>]

The 'git-refs(1)' migrate subcommand, which transfers repositories
between reference backends, currently migrates reflogs by default as of
246cebe320 (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 handle this, introduce the '--no-reflog' flag, which skips reflog
migration. When this flag is used, reflogs from the original reference
backend are not transferred, and since only the new reference backend
remains in the repository, all previous reflogs are permanently removed.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
 builtin/refs.c          |  3 +++
 refs.c                  |  8 +++++---
 refs.h                  |  5 ++++-
 t/t1460-refs-migrate.sh | 28 ++++++++++++++++++++++++----
 4 files changed, 36 insertions(+), 8 deletions(-)

Changes in v4:
- Modify the flag to `--no-reflog` to better indicate that reflogs will
  be dropped during the migration. This is also reflected in the help text
  and the commit message.

Changes in v3:
- Make changes to the test:
  - Use "$@" instead of $@
  - Mark optional arguments correctly
  - Use <options...> instead of <...options> as the former is more widely
    used.
- Link to v2: https://lore.kernel.org/r/20250211-477-refs-migrate-add-a-flag-to-ignore-reflogs-during-migration-v2-1-991a2ec9a796@gmail.com

Changes in v2:
- Fix typo in commit mesasge and clarify the intent.
- Modify the test to use `test_line_count` and `test_must_be_empty`.
- Link to v1: https://lore.kernel.org/r/20250207-477-refs-migrate-add-a-flag-to-ignore-reflogs-during-migration-v1-1-7d40f3b4e30b@gmail.com

Range-diff:

1:  42c40d9617 ! 1:  3a02e8e526 builtin/refs: add '--skip-reflog' flag to bypass reflog migration
    @@ Metadata
     Author: Karthik Nayak <karthik.188@gmail.com>
     
      ## Commit message ##
    -    builtin/refs: add '--skip-reflog' flag to bypass reflog migration
    +    builtin/refs: add '--no-reflog' flag to drop reflogs
     
         The 'git-refs(1)' migrate subcommand, which transfers repositories
         between reference backends, currently migrates reflogs by default as of
    @@ Commit message
         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.
    +    To handle this, introduce the '--no-reflog' flag, which skips reflog
    +    migration. When this flag is used, reflogs from the original reference
    +    backend are not transferred, and since only the new reference backend
    +    remains in the repository, all previous reflogs are permanently removed.
     
         Helped-by: Patrick Steinhardt <ps@pks.im>
         Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
    @@ builtin/refs.c: static int cmd_refs_migrate(int argc, const char **argv, const c
      		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"),
    ++		OPT_BIT(0, "no-reflog", &flags,
    ++			N_("drop reflogs entirely during the migration"),
     +			REPO_MIGRATE_REF_STORAGE_FORMAT_SKIP_REFLOG),
      		OPT_END(),
      	};
    @@ t/t1460-refs-migrate.sh: do
     +			# we see that the repository contains reflogs.
     +			git -C repo reflog --all >reflogs &&
     +			test_line_count = 2 reflogs &&
    -+			test_migration repo "$to_format" true --skip-reflog &&
    ++			test_migration repo "$to_format" true --no-reflog &&
     +			# there should be no reflogs post migration.
     +			git -C repo reflog --all >reflogs &&
     +			test_must_be_empty reflogs

---

diff --git a/builtin/refs.c b/builtin/refs.c
index a29f195834..c459507d51 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;
diff --git a/refs.c b/refs.c
index f4094a326a..5e8f5c06fa 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)
diff --git a/refs.h b/refs.h
index a0cdd99250..ccee8fc670 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
diff --git a/t/t1460-refs-migrate.sh b/t/t1460-refs-migrate.sh
index a6d9b35a46..2ab97e1b7d 100755
--- a/t/t1460-refs-migrate.sh
+++ b/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 --no-reflog &&
+			# there should be no reflogs post migration.
+			git -C repo reflog --all >reflogs &&
+			test_must_be_empty reflogs
+		'
 	done
 done
 
-- 
2.47.2