refs: allow multiple reflog entries for the same refname

The reference transaction only allows a update for a given reference to
avoid conflicts. This, however, isn't an issue for reflogs. There are no
conflicts to be resolved in reflogs and when migrating reflogs between
backends we'd have multiple reflog entries for the same refname.

So allow multiple reflog updates within a single transaction. Also the
reflog creation logic isn't exposed to the end user. While this might
change in the future, currently, this reduces the scope of issues to
think about.

This is required to add reflog migration support to `git refs migrate`
which currently doesn't support it.

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

Author: KarthikNayak

refs/files-backend.c

@@ -2612,6 +2612,9 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 
 	update->backend_data = lock;
 
+	if (update->flags & REF_LOG_ONLY)
+		goto out;
+
 	if (update->type & REF_ISSYMREF) {
 		if (update->flags & REF_NO_DEREF) {
 			/*
@@ -2830,13 +2833,16 @@ static int files_transaction_prepare(struct ref_store *ref_store,
 	 */
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
-		struct string_list_item *item =
-			string_list_append(&affected_refnames, update->refname);
+		struct string_list_item *item;
 
 		if ((update->flags & REF_IS_PRUNING) &&
 		    !(update->flags & REF_NO_DEREF))
 			BUG("REF_IS_PRUNING set without REF_NO_DEREF");
 
+		if (update->flags & REF_LOG_ONLY)
+			continue;
+
+		item = string_list_append(&affected_refnames, update->refname);
 		/*
 		 * We store a pointer to update in item->util, but at
 		 * the moment we never use the value of this field
@@ -3036,8 +3042,9 @@ static int files_transaction_finish_initial(struct files_ref_store *refs,
 
 	/* Fail if a refname appears more than once in the transaction: */
 	for (i = 0; i < transaction->nr; i++)
-		string_list_append(&affected_refnames,
-				   transaction->updates[i]->refname);
+		if (!(transaction->updates[i]->flags & REF_LOG_ONLY))
+			string_list_append(&affected_refnames,
+					   transaction->updates[i]->refname);
 	string_list_sort(&affected_refnames);
 	if (ref_update_reject_duplicates(&affected_refnames, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;

refs/reftable-backend.c

@@ -990,8 +990,9 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		if (ret)
 			goto done;
 
-		string_list_append(&affected_refnames,
-				   transaction->updates[i]->refname);
+		if (!(transaction->updates[i]->flags & REF_LOG_ONLY))
+			string_list_append(&affected_refnames,
+					   transaction->updates[i]->refname);
 	}
 
 	/*
@@ -1302,6 +1303,7 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 	struct ident_split committer_ident = {0};
 	size_t logs_nr = 0, logs_alloc = 0, i;
 	const char *committer_info;
+	struct strintmap logs_ts;
 	int ret = 0;
 
 	committer_info = git_committer_info(0);
@@ -1310,6 +1312,8 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 
 	QSORT(arg->updates, arg->updates_nr, transaction_update_cmp);
 
+	strintmap_init(&logs_ts, ts);
+
 	reftable_writer_set_limits(writer, ts, ts);
 
 	for (i = 0; i < arg->updates_nr; i++) {
@@ -1391,6 +1395,7 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 
 			if (create_reflog) {
 				struct ident_split c;
+				uint64_t update_index;
 
 				ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
 				log = &logs[logs_nr++];
@@ -1405,7 +1410,11 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 				}
 
 				fill_reftable_log_record(log, &c);
-				log->update_index = ts;
+
+				update_index = strintmap_get(&logs_ts, u->refname);
+				log->update_index = update_index;
+				strintmap_set(&logs_ts, u->refname, update_index+1);
+
 				log->refname = xstrdup(u->refname);
 				memcpy(log->value.update.new_hash,
 				       u->new_oid.hash, GIT_MAX_RAWSZ);
@@ -1476,6 +1485,7 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 
 done:
 	assert(ret != REFTABLE_API_ERROR);
+	strintmap_clear(&logs_ts);
 	for (i = 0; i < logs_nr; i++)
 		reftable_log_record_release(&logs[i]);
 	free(logs);