MDEV-4520: Assertion `0' fails in Query_cache::end_of_result on concurrent drop event and event execution

If there is no net.vio then query cache cant't get data via net_real_write() so it is better just do not try to cache such query.

mysql-test/r/init_connection_query_cache.result

@@ -0,0 +1,19 @@
+#
+# MDEV-4520: Assertion `0' fails in Query_cache::end_of_result on
+# concurrent drop event and event execution
+#
+set GLOBAL query_cache_size=1355776;
+create user mysqltest1@localhost;
+grant SELECT on test.* to mysqltest1@localhost;
+create table t1 (a int);
+# This explain put here to be sure that init connection query
+# has 'Impossible WHERE'.
+explain extended select * from test.t1 where 0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where 0
+revoke all privileges, grant option from mysqltest1@localhost;
+drop user mysqltest1@localhost;
+drop table t1;
+set GLOBAL query_cache_size=default;

mysql-test/t/init_connection_query_cache-master.opt

@@ -0,0 +1 @@
+--init_connect="select * from test.t1 where 0"

mysql-test/t/init_connection_query_cache.test

@@ -0,0 +1,26 @@
+-- source include/have_query_cache.inc
+
+--echo #
+--echo # MDEV-4520: Assertion `0' fails in Query_cache::end_of_result on
+--echo # concurrent drop event and event execution
+--echo #
+set GLOBAL query_cache_size=1355776;
+
+create user mysqltest1@localhost;
+grant SELECT on test.* to mysqltest1@localhost;
+
+create table t1 (a int);
+
+connect (con1,localhost,mysqltest1,,);
+connection con1;
+--echo # This explain put here to be sure that init connection query
+--echo # has 'Impossible WHERE'.
+explain extended select * from test.t1 where 0;
+connection default;
+disconnect con1;
+revoke all privileges, grant option from mysqltest1@localhost;
+drop user mysqltest1@localhost;
+
+drop table t1;
+
+set GLOBAL query_cache_size=default;

sql/sql_cache.cc

@@ -4000,7 +4000,8 @@ Query_cache::is_cacheable(THD *thd, LEX *lex,
   if (thd->lex->safe_to_cache_query &&
       (thd->variables.query_cache_type == 1 ||
        (thd->variables.query_cache_type == 2 && (lex->select_lex.options &
-						 OPTION_TO_QUERY_CACHE))))
+						 OPTION_TO_QUERY_CACHE))) &&
+      thd->net.vio)
   {
     DBUG_PRINT("qcache", ("options: %lx  %lx  type: %u",
                           (long) OPTION_TO_QUERY_CACHE,
@@ -4022,11 +4023,12 @@ Query_cache::is_cacheable(THD *thd, LEX *lex,
   }
 
   DBUG_PRINT("qcache",
-	     ("not interesting query: %d or not cacheable, options %lx %lx  type: %u",
+	     ("not interesting query: %d or not cacheable, options %lx %lx  type: %u  net->vio present: %u",
 	      (int) lex->sql_command,
 	      (long) OPTION_TO_QUERY_CACHE,
 	      (long) lex->select_lex.options,
-	      (int) thd->variables.query_cache_type));
+	      (int) thd->variables.query_cache_type,
+              (uint) test(thd->net.vio)));
   DBUG_RETURN(0);
 }
 

sql/sql_class.cc

@@ -4579,7 +4579,7 @@ void xid_cache_delete(XID_STATE *xid_state)
 int THD::decide_logging_format(TABLE_LIST *tables)
 {
   DBUG_ENTER("THD::decide_logging_format");
-  DBUG_PRINT("info", ("query: %s", query()));
+  DBUG_PRINT("info", ("Query: %s", query()));
   DBUG_PRINT("info", ("variables.binlog_format: %lu",
                       variables.binlog_format));
   DBUG_PRINT("info", ("lex->get_stmt_unsafe_flags(): 0x%x",