MDEV-26206 gap lock is not set if implicit lock exists

Post-push fix for 10.5+.

The fix influence MDEV-14479. Before the fix
lock_rec_convert_impl_to_expl() did not create explicit lock if caller's
transaction owns found implicit lock(see MDEV-14479 for details). After the fix
lock_rec_convert_impl_to_expl() can create explicit lock under the above
conditions if the requested lock mode is not LOCK_REC_NOT_GAP. And that
is why we need to check if the table is X-locked before
lock_rec_convert_impl_to_expl() call.

storage/innobase/lock/lock0lock.cc

@@ -5640,7 +5640,9 @@ lock_sec_rec_read_check_and_lock(
 	if the max trx id for the page >= min trx id for the trx list or a
 	database recovery is running. */
 
-	if (!page_rec_is_supremum(rec)
+	trx_t *trx = thr_get_trx(thr);
+	if (!lock_table_has(trx, index->table, LOCK_X)
+	    && !page_rec_is_supremum(rec)
 	    && page_get_max_trx_id(block->frame) >= trx_sys.get_min_trx_id()
 	    && lock_rec_convert_impl_to_expl(thr_get_trx(thr), block, rec,
 					     index, offsets)
@@ -5650,7 +5652,6 @@ lock_sec_rec_read_check_and_lock(
 	}
 
 #ifdef WITH_WSREP
-	trx_t *trx= thr_get_trx(thr);
 	/* If transaction scanning an unique secondary key is wsrep
 	high priority thread (brute force) this scanning may involve
 	GAP-locking in the index. As this locking happens also when
@@ -5724,8 +5725,10 @@ lock_clust_rec_read_check_and_lock(
 
 	heap_no = page_rec_get_heap_no(rec);
 
-	if (heap_no != PAGE_HEAP_NO_SUPREMUM
-	    && lock_rec_convert_impl_to_expl(thr_get_trx(thr), block, rec,
+	trx_t *trx = thr_get_trx(thr);
+	if (!lock_table_has(trx, index->table, LOCK_X)
+	    && heap_no != PAGE_HEAP_NO_SUPREMUM
+	    && lock_rec_convert_impl_to_expl(trx, block, rec,
 					     index, offsets)
 	    && gap_mode == LOCK_REC_NOT_GAP) {
 		/* We already hold an implicit exclusive lock. */