Bug#39843 DELETE requires write access to table in subquery in where clause

An unnecessarily restrictive lock were taken on sub-SELECTs during DELETE.

During parsing, a global structure is reused for sub-SELECTs and the attribute
keeping track of lock options were not reset properly.
This patch introduces a new attribute to keep track on the syntactical lock
option elements found in a sub-SELECT and then sets the lock options accordingly.

Now the sub-SELECTs will try to acquire a READ lock if possible
instead of a WRITE lock as inherited from the outer DELETE statement.

mysql-test/r/lock.result

@@ -166,4 +166,31 @@ ERROR HY000: View's SELECT refers to a temporary table 't2'
 Cleanup.
 
 drop table t2, t3;
+#
+# Bug#39843 DELETE requires write access to table in subquery in where clause
+#
+DROP TABLE IF EXISTS t1,t2;
+CREATE TABLE t1 (
+table1_rowid SMALLINT NOT NULL
+);
+CREATE TABLE t2 (
+table2_rowid SMALLINT NOT NULL
+);
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (1);
+LOCK TABLES t1 WRITE, t2 READ;
+# Sub-select should not try to aquire a write lock.
+DELETE FROM t1 
+WHERE EXISTS 
+( 
+SELECT 'x' 
+FROM t2
+WHERE t1.table1_rowid = t2.table2_rowid
+) ;
+# While implementing the patch we didn't break old behavior;
+# The following sub-select should still requires a write lock:
+SELECT * FROM t1 WHERE 1 IN (SELECT * FROM t2 FOR UPDATE);
+ERROR HY000: Table 't2' was locked with a READ lock and can't be updated
+UNLOCK TABLES;
+DROP TABLE t1,t2;
 End of 5.1 tests.

mysql-test/t/lock.test

@@ -214,4 +214,34 @@ create view v_bug5719 as select * from t2;
 --echo
 drop table t2, t3;
 
+--echo #
+--echo # Bug#39843 DELETE requires write access to table in subquery in where clause
+--echo #
+--disable_warnings
+DROP TABLE IF EXISTS t1,t2;
+--enable_warnings
+CREATE TABLE t1 (
+table1_rowid SMALLINT NOT NULL
+);
+CREATE TABLE t2 (
+table2_rowid SMALLINT NOT NULL
+);
+INSERT INTO t1 VALUES (1);
+INSERT INTO t2 VALUES (1);
+LOCK TABLES t1 WRITE, t2 READ;
+--echo # Sub-select should not try to aquire a write lock.
+DELETE FROM t1 
+WHERE EXISTS 
+( 
+SELECT 'x' 
+FROM t2
+WHERE t1.table1_rowid = t2.table2_rowid
+) ;
+--echo # While implementing the patch we didn't break old behavior;
+--echo # The following sub-select should still requires a write lock:
+--error ER_TABLE_NOT_LOCKED_FOR_WRITE
+SELECT * FROM t1 WHERE 1 IN (SELECT * FROM t2 FOR UPDATE);
+UNLOCK TABLES;
+DROP TABLE t1,t2;
+
 --echo End of 5.1 tests.

sql/sql_lex.cc

@@ -1559,6 +1559,7 @@ void st_select_lex::init_query()
   exclude_from_table_unique_test= no_wrap_view_item= FALSE;
   nest_level= 0;
   link_next= 0;
+  lock_option= TL_READ_DEFAULT;
 }
 
 void st_select_lex::init_select()

sql/sql_lex.h

@@ -688,6 +688,15 @@ public:
   int cur_pos_in_select_list;
 
   List<udf_func>     udf_list;                  /* udf function calls stack */
+
+  /**
+    Per sub-query locking strategy.
+    Note: This variable might interfer with the corresponding statement-level
+    variable Lex::lock_option because on how different parser rules depend
+    on eachother.
+  */
+  thr_lock_type lock_option;
+
   /* 
     This is a copy of the original JOIN USING list that comes from
     the parser. The parser :

sql/sql_parse.cc

@@ -5580,6 +5580,14 @@ void mysql_reset_thd_for_next_command(THD *thd)
 }
 
 
+/**
+  Resets the lex->current_select object.
+  @note It is assumed that lex->current_select != NULL
+
+  This function is a wrapper around select_lex->init_select() with an added
+  check for the special situation when using INTO OUTFILE and LOAD DATA.
+*/
+
 void
 mysql_init_select(LEX *lex)
 {
@@ -5594,6 +5602,18 @@ mysql_init_select(LEX *lex)
 }
 
 
+/**
+  Used to allocate a new SELECT_LEX object on the current thd mem_root and
+  link it into the relevant lists.
+
+  This function is always followed by mysql_init_select.
+
+  @see mysql_init_select
+
+  @retval TRUE An error occurred
+  @retval FALSE The new SELECT_LEX was successfully allocated.
+*/
+
 bool
 mysql_new_select(LEX *lex, bool move_down)
 {
@@ -6411,7 +6431,6 @@ void st_select_lex::set_lock_for_tables(thr_lock_type lock_type)
   DBUG_ENTER("set_lock_for_tables");
   DBUG_PRINT("enter", ("lock_type: %d  for_update: %d", lock_type,
 		       for_update));
-
   for (TABLE_LIST *tables= (TABLE_LIST*) table_list.first;
        tables;
        tables= tables->next_local)

sql/sql_yacc.yy

@@ -6489,7 +6489,8 @@ select_option:
           {
             if (check_simple_select())
               MYSQL_YYABORT;
-            Lex->lock_option= TL_READ_HIGH_PRIORITY;
+            Lex->lock_option=  TL_READ_HIGH_PRIORITY;
+            Lex->current_select->lock_option= TL_READ_HIGH_PRIORITY;
           }
         | DISTINCT         { Select->options|= SELECT_DISTINCT; }
         | SQL_SMALL_RESULT { Select->options|= SELECT_SMALL_RESULT; }
@@ -6535,6 +6536,7 @@ select_lock_type:
           {
             LEX *lex=Lex;
             lex->current_select->set_lock_for_tables(TL_WRITE);
+            lex->current_select->lock_option= TL_WRITE;
             lex->safe_to_cache_query=0;
           }
         | LOCK_SYM IN_SYM SHARE_SYM MODE_SYM
@@ -6542,6 +6544,7 @@ select_lock_type:
             LEX *lex=Lex;
             lex->current_select->
               set_lock_for_tables(TL_READ_WITH_SHARED_LOCKS);
+            lex->current_select->lock_option= TL_READ_WITH_SHARED_LOCKS;
             lex->safe_to_cache_query=0;
           }
         ;
@@ -12909,6 +12912,18 @@ subselect_start:
 subselect_end:
           {
             LEX *lex=Lex;
+            /*
+              Set the required lock level for the tables associated with the
+              current sub-select. This will overwrite previous lock options set
+              using st_select_lex::add_table_to_list in any of the following
+              rules: single_multi, table_wild_one, load_data, table_alias_ref,
+              table_factor.
+              The default lock level is TL_READ_DEFAULT but it can be modified
+              with query options specific for a certain (sub-)SELECT.
+            */
+            lex->current_select->
+              set_lock_for_tables(lex->current_select->lock_option);
+
             lex->pop_context();
             SELECT_LEX *child= lex->current_select;
             lex->current_select = lex->current_select->return_after_parsing();