refs/files: add count field to ref_lock

When refs are updated in the files-backend, a lock is obtained for the
corresponding file path. This is the case even for reflogs, i.e. a lock
is obtained on the reference path instead of the reflog path. This
works, since generally, reflogs are updated alongside the ref.

The upcoming patches will add support for reflog updates in ref
transaction. This means, in a particular transaction we want to have ref
updates and reflog updates. For a given ref in a given transaction there
can be at most one update. But we can theoretically have multiple reflog
updates for a given ref in a given transaction. A great example of this
would be when migrating reflogs from one backend to another. There we
would batch all the reflog updates for a given reference in a single
transaction.

The current flow does not support this, because currently refs & reflogs
are treated as a single entity and capture the lock together. To
separate this, add a count field to ref_lock. With this, multiple
updates can hold onto a single ref_lock and the lock will only be
released when all of them release the lock.

This patch only adds the `count` field to `ref_lock` and adds the logic
to increment and decrement the lock. In a follow up commit, we'll
separate the reflog update logic from ref updates and utilize this
functionality.

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

Author: KarthikNayak

refs/files-backend.c

@@ -71,6 +71,7 @@ struct ref_lock {
 	char *ref_name;
 	struct lock_file lk;
 	struct object_id old_oid;
+	unsigned int count; /* track users of the lock (ref update + reflog updates) */
 };
 
 struct files_ref_store {
@@ -638,9 +639,12 @@ int parse_loose_ref_contents(const struct git_hash_algo *algop,
 
 static void unlock_ref(struct ref_lock *lock)
 {
-	rollback_lock_file(&lock->lk);
-	free(lock->ref_name);
-	free(lock);
+	lock->count--;
+	if (!lock->count) {
+		rollback_lock_file(&lock->lk);
+		free(lock->ref_name);
+		free(lock);
+	}
 }
 
 /*
@@ -696,6 +700,7 @@ static int lock_raw_ref(struct files_ref_store *refs,
 	*lock_p = CALLOC_ARRAY(lock, 1);
 
 	lock->ref_name = xstrdup(refname);
+	lock->count = 1;
 	files_ref_path(refs, &ref_file, refname);
 
 retry:
@@ -1169,6 +1174,7 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
 		goto error_return;
 
 	lock->ref_name = xstrdup(refname);
+	lock->count = 1;
 
 	if (raceproof_create_file(ref_file.buf, create_reflock, &lock->lk)) {
 		unable_to_lock_message(ref_file.buf, errno, err);
@@ -2535,6 +2541,12 @@ static int check_old_oid(struct ref_update *update, struct object_id *oid,
 	return -1;
 }
 
+struct files_transaction_backend_data {
+	struct ref_transaction *packed_transaction;
+	int packed_refs_locked;
+	struct strmap ref_locks;
+};
+
 /*
  * Prepare for carrying out update:
  * - Lock the reference referred to by update.
@@ -2557,11 +2569,14 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 {
 	struct strbuf referent = STRBUF_INIT;
 	int mustexist = ref_update_expects_existing_old_ref(update);
+	struct files_transaction_backend_data *backend_data;
 	int ret = 0;
 	struct ref_lock *lock;
 
 	files_assert_main_repository(refs, "lock_ref_for_update");
 
+	backend_data = transaction->backend_data;
+
 	if ((update->flags & REF_HAVE_NEW) && ref_update_has_null_new_value(update))
 		update->flags |= REF_DELETING;
 
@@ -2572,18 +2587,25 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 			goto out;
 	}
 
-	ret = lock_raw_ref(refs, update->refname, mustexist,
-			   affected_refnames,
-			   &lock, &referent,
-			   &update->type, err);
-	if (ret) {
-		char *reason;
+	lock = strmap_get(&backend_data->ref_locks, update->refname);
+	if (lock) {
+		lock->count++;
+	} else {
+		ret = lock_raw_ref(refs, update->refname, mustexist,
+				   affected_refnames,
+				   &lock, &referent,
+				   &update->type, err);
+		if (ret) {
+			char *reason;
+
+			reason = strbuf_detach(err, NULL);
+			strbuf_addf(err, "cannot lock ref '%s': %s",
+				    ref_update_original_update_refname(update), reason);
+			free(reason);
+			goto out;
+		}
 
-		reason = strbuf_detach(err, NULL);
-		strbuf_addf(err, "cannot lock ref '%s': %s",
-			    ref_update_original_update_refname(update), reason);
-		free(reason);
-		goto out;
+		strmap_put(&backend_data->ref_locks, update->refname, lock);
 	}
 
 	update->backend_data = lock;
@@ -2730,11 +2752,6 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 	return ret;
 }
 
-struct files_transaction_backend_data {
-	struct ref_transaction *packed_transaction;
-	int packed_refs_locked;
-};
-
 /*
  * Unlock any references in `transaction` that are still locked, and
  * mark the transaction closed.
@@ -2767,6 +2784,8 @@ static void files_transaction_cleanup(struct files_ref_store *refs,
 		if (backend_data->packed_refs_locked)
 			packed_refs_unlock(refs->packed_ref_store);
 
+		strmap_clear(&backend_data->ref_locks, 0);
+
 		free(backend_data);
 	}
 
@@ -2796,6 +2815,7 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 		goto cleanup;
 
 	CALLOC_ARRAY(backend_data, 1);
+	strmap_init(&backend_data->ref_locks);
 	transaction->backend_data = backend_data;
 
 	/*