Applied the fix for bug #54235 taken from one of the mysql trees.

The fix aligns join_null_complements() with join_matching_records()
making both call generate_full_extensions().
There should not be any difference between how the WHERE clause
is applied to NULL-complemented records from a partial join and how
it is applied to other partially joined records:the latter happens in
join_matching_records(), precisely in generate_full_extensions().

mysql-test/r/join_cache.result

@@ -5330,4 +5330,44 @@ a
 9
 set join_cache_level = default;
 DROP TABLE t1;
+#
+# Bug#54235 Extra rows with join_cache_level=6,8 and two LEFT JOINs
+#
+CREATE TABLE t1 (a int);
+CREATE TABLE t2 (a int);
+CREATE TABLE t3 (a int);
+CREATE TABLE t4 (a int);
+INSERT INTO t1 VALUES (null), (2), (null), (1);
+set join_cache_level = 6;
+EXPLAIN
+SELECT t1.a 
+FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.a) ON 0 
+WHERE t1.a OR t3.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	0	Using where; Using join buffer
+1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	0	Using where; Using join buffer
+SELECT t1.a 
+FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.a) ON 0 
+WHERE t1.a OR t3.a;
+a
+2
+1
+EXPLAIN
+SELECT t1.a 
+FROM t1 LEFT JOIN (t2 LEFT JOIN (t3 LEFT JOIN t4 ON 1) ON t2.a) ON 0
+WHERE t1.a OR t4.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	0	Using where; Using join buffer
+1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	0	Using where; Using join buffer
+1	SIMPLE	t4	ALL	NULL	NULL	NULL	NULL	0	Using where; Using join buffer
+SELECT t1.a 
+FROM t1 LEFT JOIN (t2 LEFT JOIN (t3 LEFT JOIN t4 ON 1) ON t2.a) ON 0
+WHERE t1.a OR t4.a;
+a
+2
+1
+set join_cache_level = default;
+DROP TABLE t1,t2,t3,t4;
 set @@optimizer_switch=@save_optimizer_switch;

mysql-test/t/join_cache.test

@@ -2104,5 +2104,37 @@ SELECT t.a FROM t1 t, t1 s FORCE INDEX(idx)
 set join_cache_level = default;
 DROP TABLE t1;
 
+
+--echo #
+--echo # Bug#54235 Extra rows with join_cache_level=6,8 and two LEFT JOINs
+--echo #
+
+CREATE TABLE t1 (a int);
+CREATE TABLE t2 (a int);
+CREATE TABLE t3 (a int);
+CREATE TABLE t4 (a int);
+
+INSERT INTO t1 VALUES (null), (2), (null), (1);
+
+set join_cache_level = 6;
+EXPLAIN
+SELECT t1.a 
+  FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.a) ON 0 
+    WHERE t1.a OR t3.a;
+SELECT t1.a 
+  FROM t1 LEFT JOIN (t2 LEFT JOIN t3 ON t2.a) ON 0 
+    WHERE t1.a OR t3.a;
+
+EXPLAIN
+SELECT t1.a 
+  FROM t1 LEFT JOIN (t2 LEFT JOIN (t3 LEFT JOIN t4 ON 1) ON t2.a) ON 0
+    WHERE t1.a OR t4.a;
+SELECT t1.a 
+  FROM t1 LEFT JOIN (t2 LEFT JOIN (t3 LEFT JOIN t4 ON 1) ON t2.a) ON 0
+    WHERE t1.a OR t4.a;
+
+set join_cache_level = default;
+DROP TABLE t1,t2,t3,t4;
+
 # this must be the last command in the file
 set @@optimizer_switch=@save_optimizer_switch;

sql/sql_join_cache.cc

@@ -2295,35 +2295,11 @@ enum_nested_loop_state JOIN_CACHE::join_null_complements(bool skip_last)
       /* The outer row is complemented by nulls for each inner table */
       restore_record(join_tab->table, s->default_values);
       mark_as_null_row(join_tab->table);  
-      /* Check all pushdown conditions attached to the inner table */
-      join_tab->first_unmatched->found= 1;
-      if (join_tab->select && join_tab->select->skip_record(join->thd) <= 0)
-        continue;
-      if (is_last_inner)
-      { 
-        JOIN_TAB *first_upper= join_tab->first_unmatched->first_upper;
-        while (first_upper && first_upper->last_inner == join_tab)
-        {
-          set_match_flag_if_none(first_upper, get_curr_rec());
-          for (JOIN_TAB* tab= first_upper; tab <= join_tab; tab++)
-          {
-            if (tab->select && tab->select->skip_record(join->thd) <= 0)
-              goto next;
-          }
-          first_upper= first_upper->first_upper;
-        }
-      }
-      /* Find all matches for the remaining join tables */
-      rc= (*join_tab->next_select)(join, join_tab+1, 0);
+      rc= generate_full_extensions(get_curr_rec());
       if (rc != NESTED_LOOP_OK && rc != NESTED_LOOP_NO_MORE_ROWS)
-      {
-        reset(TRUE);
         goto finish;
     }
   }
-  next:
-    ;
-  }
 
 finish:
   return rc;