refs: allow multiple reflog entries for the same refname

The reference transaction only allows a single 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.

In the reftable backend, the writer sorts all updates based on the
update_index before writing to the block. When there are multiple
reflogs for a given refname, it is essential that the order of the
reflogs is maintained. So add the `index` value to the `update_index`.
The `index` field is only set when multiple reflog entries for a given
refname are added and as such in most scenarios the old behavior
remains.

This is required to add reflog migration support to `git refs migrate`.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -2611,6 +2611,9 @@ 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) {
 			/*
@@ -2829,13 +2832,16 @@ 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 =
+		struct string_list_item *item;
-			string_list_append(&affected_refnames, update->refname);
 
 		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
@@ -3035,6 +3041,7 @@ 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++)
+		if (!(transaction->updates[i]->flags & REF_LOG_ONLY))
 			string_list_append(&affected_refnames,
 					   transaction->updates[i]->refname);
 	string_list_sort(&affected_refnames);

refs/reftable-backend.c

@@ -990,6 +990,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		if (ret)
 			goto done;
 
+		if (!(transaction->updates[i]->flags & REF_LOG_ONLY))
 			string_list_append(&affected_refnames,
 					   transaction->updates[i]->refname);
 	}
@@ -1301,6 +1302,7 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 	struct reftable_log_record *logs = NULL;
 	struct ident_split committer_ident = {0};
 	size_t logs_nr = 0, logs_alloc = 0, i;
+	uint64_t max_update_index = ts;
 	const char *committer_info;
 	int ret = 0;
 
@@ -1405,7 +1407,19 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 				}
 
 				fill_reftable_log_record(log, &c);
-				log->update_index = ts;
+
+				/*
+				 * Updates are sorted by the writer. So updates for the same
+				 * refname need to contain different update indices.
+				 */
+				log->update_index = ts + u->index;
+
+				/*
+				 * Note the max update_index so the limit can be set later on.
+				 */
+				if (log->update_index > max_update_index)
+					max_update_index = log->update_index;
+
 				log->refname = xstrdup(u->refname);
 				memcpy(log->value.update.new_hash,
 				       u->new_oid.hash, GIT_MAX_RAWSZ);
@@ -1469,6 +1483,8 @@ static int write_transaction_table(struct reftable_writer *writer, void *cb_data
 	 * and log blocks.
 	 */
 	if (logs) {
+		reftable_writer_set_limits(writer, ts, max_update_index);
+
 		ret = reftable_writer_add_logs(writer, logs, logs_nr);
 		if (ret < 0)
 			goto done;