MDEV-14056 DROP TEMPORARY TABLE IF EXISTS causes error 1290 with read_only option

if it's a DROP TABLE, we cannot detect whether a table is
temporary by looking in thd->temporary_tables - because the
table might simply not exist at all.

mysql-test/r/read_only.result

@@ -47,6 +47,9 @@ delete t1 from t1,t3 where t1.a=t3.a;
 drop table t1;
 insert into t1 values(1);
 ERROR HY000: The MariaDB server is running with the --read-only option so it cannot execute this statement
+drop temporary table if exists t1;
+Warnings:
+Note	1051	Unknown table 't1'
 connection default;
 set global read_only=0;
 lock table t1 write;

mysql-test/t/read_only.test

@@ -114,6 +114,11 @@ drop table t1;
 --error ER_OPTION_PREVENTS_STATEMENT
 insert into t1 values(1);
 
+#
+# MDEV-14056 DROP TEMPORARY TABLE IF EXISTS causes error 1290 with read_only option
+#
+drop temporary table if exists t1;
+
 #
 # Bug#11733 COMMITs should not happen if read-only is set
 #

sql/sql_parse.cc

@@ -811,8 +811,7 @@ bool do_command(THD *thd)
     @retval FALSE The statement isn't updating any relevant tables.
 */
 
-static my_bool deny_updates_if_read_only_option(THD *thd,
-                                                TABLE_LIST *all_tables)
+static bool deny_updates_if_read_only_option(THD *thd, TABLE_LIST *all_tables)
 {
   DBUG_ENTER("deny_updates_if_read_only_option");
 
@@ -821,11 +820,7 @@ static my_bool deny_updates_if_read_only_option(THD *thd,
 
   LEX *lex= thd->lex;
 
-  const my_bool user_is_super=
-    ((ulong)(thd->security_ctx->master_access & SUPER_ACL) ==
-     (ulong)SUPER_ACL);
-
-  if (user_is_super)
+  if (thd->security_ctx->master_access & SUPER_ACL)
     DBUG_RETURN(FALSE);
 
   if (!(sql_command_flags[lex->sql_command] & CF_CHANGES_DATA))
@@ -835,30 +830,26 @@ static my_bool deny_updates_if_read_only_option(THD *thd,
   if (lex->sql_command == SQLCOM_UPDATE_MULTI)
     DBUG_RETURN(FALSE);
 
+  if (lex->sql_command == SQLCOM_CREATE_DB ||
+      lex->sql_command == SQLCOM_DROP_DB)
+    DBUG_RETURN(TRUE);
+
   /*
     a table-to-be-created is not in the temp table list yet,
     so CREATE TABLE needs a special treatment
   */
-  const bool update_real_tables=
-    lex->sql_command == SQLCOM_CREATE_TABLE
-    ? !(lex->create_info.options & HA_LEX_CREATE_TMP_TABLE)
-    : some_non_temp_table_to_be_updated(thd, all_tables);
-
-  const bool create_or_drop_databases=
-    (lex->sql_command == SQLCOM_CREATE_DB) ||
-    (lex->sql_command == SQLCOM_DROP_DB);
-
-  if (update_real_tables || create_or_drop_databases)
-  {
-      /*
-        An attempt was made to modify one or more non-temporary tables.
-      */
-      DBUG_RETURN(TRUE);
-  }
+  if (lex->sql_command == SQLCOM_CREATE_TABLE)
+    DBUG_RETURN(!(lex->create_info.options & HA_LEX_CREATE_TMP_TABLE));
 
+  /*
+    a table-to-be-dropped might not exist (DROP TEMPORARY TABLE IF EXISTS),
+    cannot use the temp table list either.
+  */
+  if (lex->sql_command == SQLCOM_DROP_TABLE && lex->drop_temporary)
+    DBUG_RETURN(FALSE);
 
-  /* Assuming that only temporary tables are modified. */
-  DBUG_RETURN(FALSE);
+  /* Now, check thd->temporary_tables list */
+  DBUG_RETURN(some_non_temp_table_to_be_updated(thd, all_tables));
 }
 
 /**