MDEV-28268: Server crashes in Expression_cache_tracker::fetch_current_stats

Expression_cache_tmptable object uses an Expression_cache_tracker object
to report the statistics.

In the common scenario, Expression_cache_tmptable destructor sets
tracker->cache=NULL. The tracker object survives after the expression
cache is deleted and one may call cache_tracker->fetch_current_stats()
for it with no harm.

However a degenerate cache with no parameters does not set
tracker->cache=NULL in Expression_cache_tmptable destructor which
results in an attempt to use freed data in the
cache_tracker->fetch_current_stats() call.

Fixed by setting tracker->cache to NULL and wrapping the assignment into
a function.

mysql-test/r/subselect4.result

@@ -2841,4 +2841,79 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	DEPENDENT SUBQUERY	tn	eq_ref	PRIMARY	PRIMARY	32	test.tms.key1	1	Using where
 set optimizer_switch=@tmp_os;
 drop table t1, t10, t11;
+#
+# MDEV-28268: Server crashes in Expression_cache_tracker::fetch_current_stats
+#
+CREATE TABLE t1 (a INT, b INT);
+INSERT INTO t1 VALUES (1,2),(3,4);
+ANALYZE FORMAT=JSON
+SELECT DISTINCT
+(SELECT MIN(a) FROM t1 WHERE b <= ANY (SELECT a FROM t1)) AS f
+FROM t1;
+ANALYZE
+{
+  "query_block": {
+    "select_id": 1,
+    "r_loops": 1,
+    "r_total_time_ms": "REPLACED",
+    "duplicate_removal": {
+      "temporary_table": {
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "r_loops": 1,
+          "rows": 2,
+          "r_rows": 2,
+          "r_total_time_ms": "REPLACED",
+          "filtered": 100,
+          "r_filtered": 100
+        },
+        "subqueries": [
+          {
+            "expression_cache": {
+              "state": "disabled",
+              "r_loops": 0,
+              "query_block": {
+                "select_id": 2,
+                "r_loops": 1,
+                "r_total_time_ms": "REPLACED",
+                "table": {
+                  "table_name": "t1",
+                  "access_type": "ALL",
+                  "r_loops": 1,
+                  "rows": 2,
+                  "r_rows": 2,
+                  "r_total_time_ms": "REPLACED",
+                  "filtered": 100,
+                  "r_filtered": 50,
+                  "attached_condition": "<nop>(<in_optimizer>(t1.b,(subquery#3) >= 4))"
+                },
+                "subqueries": [
+                  {
+                    "query_block": {
+                      "select_id": 3,
+                      "r_loops": 1,
+                      "r_total_time_ms": "REPLACED",
+                      "table": {
+                        "table_name": "t1",
+                        "access_type": "ALL",
+                        "r_loops": 1,
+                        "rows": 2,
+                        "r_rows": 2,
+                        "r_total_time_ms": "REPLACED",
+                        "filtered": 100,
+                        "r_filtered": 100
+                      }
+                    }
+                  }
+                ]
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
+DROP TABLE t1;
 # End of 10.2 tests

mysql-test/t/subselect4.test

@@ -2351,4 +2351,19 @@ set optimizer_switch=@tmp_os;
 
 drop table t1, t10, t11;
 
+--echo #
+--echo # MDEV-28268: Server crashes in Expression_cache_tracker::fetch_current_stats
+--echo #
+CREATE TABLE t1 (a INT, b INT);
+INSERT INTO t1 VALUES (1,2),(3,4);
+
+--source include/analyze-format.inc
+ANALYZE FORMAT=JSON
+SELECT DISTINCT
+  (SELECT MIN(a) FROM t1 WHERE b <= ANY (SELECT a FROM t1)) AS f
+FROM t1;
+
+# Cleanup
+DROP TABLE t1;
+
 --echo # End of 10.2 tests

sql/sql_expression_cache.cc

@@ -63,7 +63,7 @@ void Expression_cache_tmptable::disable_cache()
   cache_table= NULL;
   update_tracker();
   if (tracker)
-    tracker->cache= NULL;
+    tracker->detach_from_cache();
 }
 
 
@@ -188,6 +188,8 @@ Expression_cache_tmptable::~Expression_cache_tmptable()
   else
   {
     update_tracker();
+    if (tracker)
+      tracker->detach_from_cache();
     tracker= NULL;
   }
 }

sql/sql_expression_cache.h

@@ -83,7 +83,11 @@ class Expression_cache_tracker :public Sql_alloc
     cache(c), hit(0), miss(0), state(UNINITED)
   {}
 
+private:
+  // This can be NULL if the cache is already deleted
   Expression_cache *cache;
+
+public:
   ulong hit, miss;
   enum expr_cache_state state;
 
@@ -91,6 +95,7 @@ class Expression_cache_tracker :public Sql_alloc
   void set(ulong h, ulong m, enum expr_cache_state s)
   {hit= h; miss= m; state= s;}
 
+  void detach_from_cache() { cache= NULL; }
   void fetch_current_stats()
   {
     if (cache)