memtx: check for ephemeral spaces in a uniform way

In our SQL implementation temporary spaces are used. They come to
MVCC engine in two variants - NULL or ephemeral. In both cases
MVCC engine must not do anything, there are several checks for
that in different parts of code.

Normalize these checks and make them similar to each other.

Part of tarantool#8648
Part of tarantool#8654

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

src/box/memtx_tx.c

@@ -2025,7 +2025,7 @@ memtx_tx_history_add_stmt(struct txn_stmt *stmt, struct tuple *old_tuple,
 			  struct tuple **result)
 {
 	assert(stmt != NULL);
-	assert(stmt->space != NULL);
+	assert(stmt->space != NULL && !stmt->space->def->opts.is_ephemeral);
 	assert(new_tuple != NULL || old_tuple != NULL);
 	assert(new_tuple == NULL || !tuple_has_flag(new_tuple, TUPLE_IS_DIRTY));
 
@@ -2666,11 +2666,7 @@ static void
 memtx_tx_track_read_story(struct txn *txn, struct space *space,
 			  struct memtx_story *story, uint64_t index_mask)
 {
-	if (txn == NULL)
-		return;
-	if (space == NULL)
-		return;
-	if (space->def->opts.is_ephemeral)
+	if (txn == NULL || space == NULL || space->def->opts.is_ephemeral)
 		return;
 	(void)space;
 	assert(story != NULL);
@@ -2715,11 +2711,7 @@ memtx_tx_track_read(struct txn *txn, struct space *space, struct tuple *tuple)
 {
 	if (tuple == NULL)
 		return;
-	if (txn == NULL)
-		return;
-	if (space == NULL)
-		return;
-	if (space->def->opts.is_ephemeral)
+	if (txn == NULL || space == NULL || space->def->opts.is_ephemeral)
 		return;
 
 	if (tuple_has_flag(tuple, TUPLE_IS_DIRTY)) {

src/box/memtx_tx.h

@@ -282,10 +282,7 @@ memtx_tx_track_point(struct txn *txn, struct space *space,
 {
 	if (!memtx_tx_manager_use_mvcc_engine)
 		return;
-	if (txn == NULL)
-		return;
-	/* Skip ephemeral spaces. */
-	if (space == NULL || space->def->id == 0)
+	if (txn == NULL || space == NULL || space->def->opts.is_ephemeral)
 		return;
 	memtx_tx_track_point_slow(txn, index, key);
 }
@@ -315,10 +312,7 @@ memtx_tx_track_gap(struct txn *txn, struct space *space, struct index *index,
 {
 	if (!memtx_tx_manager_use_mvcc_engine)
 		return;
-	if (txn == NULL)
-		return;
-	/* Skip ephemeral spaces. */
-	if (space == NULL || space->def->id == 0)
+	if (txn == NULL || space == NULL || space->def->opts.is_ephemeral)
 		return;
 	memtx_tx_track_gap_slow(txn, space, index, successor,
 				type, key, part_count);
@@ -346,10 +340,7 @@ memtx_tx_track_full_scan(struct txn *txn, struct space *space,
 {
 	if (!memtx_tx_manager_use_mvcc_engine)
 		return;
-	if (txn == NULL)
-		return;
-	/* Skip ephemeral spaces. */
-	if (space == NULL || space->def->id == 0)
+	if (txn == NULL || space == NULL || space->def->opts.is_ephemeral)
 		return;
 	memtx_tx_track_full_scan_slow(txn, index);
 }