MDEV-26402: A SEGV in Item_field::used_tables/update_depend_map_for_order...

When doing condition pushdown from HAVING into WHERE,
Item_equal::create_pushable_equalities() calls
item->set_extraction_flag(IMMUTABLE_FL) for constant items.
Then, Item::cleanup_excluding_immutables_processor() checks for this flag
to see if it should call item->cleanup() or leave the item as-is.

The failure happens when a constant item has a non-constant one inside it,
like:

   (tbl.col=0 AND impossible_cond)

item->walk(cleanup_excluding_immutables_processor) works in a bottom-up
way so it
1. will call Item_func_eq(tbl.col=0)->cleanup()
2. will not call Item_cond_and->cleanup (as the AND is constant)

This creates an item tree where a fixed Item has an un-fixed Item inside
it which eventually causes an assertion failure.

Fixed by introducing this rule: instead of just calling

  item->set_extraction_flag(IMMUTABLE_FL);

we call Item::walk() to set the flag for all sub-items of the item.

mysql-test/main/having_cond_pushdown.result

@@ -4924,3 +4924,18 @@ SELECT a FROM t1 GROUP BY a HAVING a = ( SELECT MIN(c) FROM t2 );
 a
 2
 DROP TABLE t1,t2;
+#
+# MDEV-26402: A SEGV in Item_field::used_tables/update_depend_map_for_order or Assertion `fixed == 1'
+#
+CREATE TABLE t1 (i int NOT NULL);
+SELECT * FROM t1 GROUP BY i HAVING i IN ( i IS NULL);
+i
+SELECT * FROM t1 GROUP BY i HAVING i IN ( i IS NULL AND 'x' = 0);
+i
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'x'
+SELECT * FROM t1 GROUP BY i HAVING i='1' IN ( i IS NULL AND 'x' = 0);
+i
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'x'
+DROP TABLE t1;

mysql-test/main/having_cond_pushdown.test

@@ -1440,3 +1440,13 @@ eval EXPLAIN FORMAT=JSON $q;
 eval $q;
 
 DROP TABLE t1,t2;
+
+--echo #
+--echo # MDEV-26402: A SEGV in Item_field::used_tables/update_depend_map_for_order or Assertion `fixed == 1'
+--echo #
+
+CREATE TABLE t1 (i int NOT NULL);
+SELECT * FROM t1 GROUP BY i HAVING i IN ( i IS NULL);
+SELECT * FROM t1 GROUP BY i HAVING i IN ( i IS NULL AND 'x' = 0);
+SELECT * FROM t1 GROUP BY i HAVING i='1' IN ( i IS NULL AND 'x' = 0);
+DROP TABLE t1;

sql/item.h

@@ -1938,6 +1938,12 @@ class Item: public Value_source,
     return 0;
   }
 
+  virtual bool set_extraction_flag_processor(void *arg)
+  {
+    set_extraction_flag(*(int*)arg);
+    return 0;
+  }
+
   /**
     Check db/table_name if they defined in item and match arg values
 

sql/item_cmpfunc.cc

@@ -7533,7 +7533,17 @@ bool Item_equal::create_pushable_equalities(THD *thd,
     if (!eq ||  equalities->push_back(eq, thd->mem_root))
       return true;
     if (!clone_const)
-      right_item->set_extraction_flag(IMMUTABLE_FL);
+    {
+      /*
+        Also set IMMUTABLE_FL for any sub-items of the right_item.
+        This is needed to prevent Item::cleanup_excluding_immutables_processor
+        from peforming cleanup of the sub-items and so creating an item tree
+        where a fixed item has non-fixed items inside it.
+      */
+      int new_flag= IMMUTABLE_FL;
+      right_item->walk(&Item::set_extraction_flag_processor, false,
+                       (void*)&new_flag);
+    }
   }
 
   while ((item=it++))