Fix for bug #55273 "FLUSH TABLE tm WITH READ LOCK for Merge

table causes assert failure".

Attempting to use FLUSH TABLE table_list WITH READ LOCK
statement for a MERGE table led to an assertion failure if
one of its children was not present in the list of tables
to be flushed. The problem was not visible in non-debug builds.

The assertion failure was caused by the fact that in such
situations FLUSH TABLES table_list WITH READ LOCK implementation
tried to use (e.g. lock) such child tables without acquiring
metadata lock on them. This happened because when opening tables
we assumed metadata locks on all tables were already acquired
earlier during statement execution and a such assumption was
false for MERGE children.

This patch fixes the problem by ensuring at open_tables() time
that we try to acquire metadata locks on all tables to be opened. 
For normal tables such requests are satisfied instantly since
locks are already acquired for them. For MERGE children metadata
locks are acquired in normal fashion.

Note that FLUSH TABLES merge_table WITH READ LOCK will lock for
read both the MERGE table and its children but will flush only 
the MERGE table. To flush children one has to mention them in table
list explicitly. This is expected behavior and it is consistent with
usage patterns for this statement (e.g. in mysqlhotcopy script).

mysql-test/r/flush.result:
  Added test case for bug #55273 "FLUSH TABLE tm WITH READ LOCK
  for Merge table causes assert failure".
mysql-test/t/flush.test:
  Added test case for bug #55273 "FLUSH TABLE tm WITH READ LOCK
  for Merge table causes assert failure".
sql/sql_base.cc:
  Changed lock_table_names() to support newly introduced
  MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK flag.
sql/sql_base.h:
  Introduced MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK flag for
  open_tables() and lock_table_names() which allows to skip
  acquiring of global and schema-scope locks when SNW, SNRW or
  X metadata locks are acquired.
sql/sql_reload.cc:
  Changed "FLUSH TABLES table_list WITH READ LOCK" code not to
  cause assert about missing metadata locks when MERGE table is
  flushed without one of its underlying tables.
  To achieve this we no longer call open_and_lock_tables() with
  MYSQL_OPEN_HAS_MDL_LOCK flag so this function automatically
  acquires metadata locks on MERGE children if such lock has
  not been already acquired at earlier stage. Instead we call
  this function with MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK flag to
  suppress acquiring of global IX lock in order to keep FLUSH
  TABLES  table_list WITH READ LOCK compatible with FLUSH TABLE
  WITH READ LOCK.
  Also changed implementation to use lock_table_names() function
  for pre-acquiring of metadata locks instead of custom code.
  To implement this change moved setting of open_type member for
  table list elements to parser.
sql/sql_yacc.yy:
  Now we set acceptable type of table for FLUSH TABLES table_list
  WITH READ LOCK at parsing time instead of execution time.

mysql-test/r/flush.result

@@ -373,3 +373,53 @@ commit;
 # --> connection con2
 # --> connection default
 drop table t1;
+#
+# Test for bug #55273 "FLUSH TABLE tm WITH READ LOCK for Merge table
+#                      causes assert failure".
+#
+drop table if exists t1, t2, tm;
+create table t1 (i int);
+create table t2 (i int);
+create table tm (i int) engine=merge union=(t1, t2);
+insert into t1 values (1), (2);
+insert into t2 values (3), (4);
+# The below statement should succeed and lock merge
+# table for read. Only merge table gets flushed and
+# not underlying tables.
+flush tables tm with read lock;
+select * from tm;
+i
+1
+2
+3
+4
+# Check that underlying tables are locked.
+select * from t1;
+i
+1
+2
+select * from t2;
+i
+3
+4
+unlock tables;
+# This statement should succeed as well and flush
+# all tables in the list.
+flush tables tm, t1, t2 with read lock;
+select * from tm;
+i
+1
+2
+3
+4
+# Naturally, underlying tables should be locked in this case too.
+select * from t1;
+i
+1
+2
+select * from t2;
+i
+3
+4
+unlock tables;
+drop tables tm, t1, t2;

mysql-test/t/flush.test

@@ -546,3 +546,34 @@ disconnect con2;
 connection default;
 drop table t1;
 
+
+--echo #
+--echo # Test for bug #55273 "FLUSH TABLE tm WITH READ LOCK for Merge table
+--echo #                      causes assert failure".
+--echo #
+--disable_warnings
+drop table if exists t1, t2, tm;
+--enable_warnings
+create table t1 (i int);
+create table t2 (i int);
+create table tm (i int) engine=merge union=(t1, t2);
+insert into t1 values (1), (2);
+insert into t2 values (3), (4);
+--echo # The below statement should succeed and lock merge
+--echo # table for read. Only merge table gets flushed and
+--echo # not underlying tables.
+flush tables tm with read lock;
+select * from tm;
+--echo # Check that underlying tables are locked.
+select * from t1;
+select * from t2;
+unlock tables;
+--echo # This statement should succeed as well and flush
+--echo # all tables in the list.
+flush tables tm, t1, t2 with read lock;
+select * from tm;
+--echo # Naturally, underlying tables should be locked in this case too.
+select * from t1;
+select * from t2;
+unlock tables;
+drop tables tm, t1, t2;

sql/sql_base.cc

@@ -4522,13 +4522,15 @@ lock_table_names(THD *thd,
            ! (flags & MYSQL_OPEN_SKIP_TEMPORARY) &&
            find_temporary_table(thd, table))))
     {
-      if (schema_set.insert(table))
+      if (! (flags & MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK) &&
+          schema_set.insert(table))
         return TRUE;
       mdl_requests.push_front(&table->mdl_request);
     }
   }
 
-  if (! mdl_requests.is_empty())
+  if (! (flags & MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK) &&
+      ! mdl_requests.is_empty())
   {
     /*
       Scoped locks: Take intention exclusive locks on all involved

sql/sql_base.h

@@ -122,6 +122,11 @@ TABLE *open_ltable(THD *thd, TABLE_LIST *table_list, thr_lock_type update,
   (LONG_TIMEOUT = 1 year) rather than the user-supplied timeout value.
 */
 #define MYSQL_LOCK_IGNORE_TIMEOUT               0x0800
+/**
+  When acquiring "strong" (SNW, SNRW, X) metadata locks on tables to
+  be open do not acquire global and schema-scope IX locks.
+*/
+#define MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK         0x1000
 
 /** Please refer to the internals manual. */
 #define MYSQL_OPEN_REOPEN  (MYSQL_OPEN_IGNORE_FLUSH |\

sql/sql_reload.cc

@@ -328,7 +328,6 @@ bool reload_acl_and_cache(THD *thd, unsigned long options,
   -------------------------------------
   - you can't flush WITH READ LOCK a non-existent table
   - you can't flush WITH READ LOCK under LOCK TABLES
-  - currently incompatible with the GRL (@todo: fix)
 
   Effect on views and temporary tables.
   ------------------------------------
@@ -338,6 +337,13 @@ bool reload_acl_and_cache(THD *thd, unsigned long options,
   if there is a base table, it's used, otherwise ER_NO_SUCH_TABLE
   is returned.
 
+  Handling of MERGE tables
+  ------------------------
+  For MERGE table this statement will open and lock child tables
+  for read (it is impossible to lock parent table without it).
+  Child tables won't be flushed unless they are explicitly present
+  in the statement's table list.
+
   Implicit commit
   ---------------
   This statement causes an implicit commit before and
@@ -354,7 +360,6 @@ bool flush_tables_with_read_lock(THD *thd, TABLE_LIST *all_tables)
 {
   Lock_tables_prelocking_strategy lock_tables_prelocking_strategy;
   TABLE_LIST *table_list;
-  MDL_request_list mdl_requests;
 
   /*
     This is called from SQLCOM_FLUSH, the transaction has
@@ -368,17 +373,13 @@ bool flush_tables_with_read_lock(THD *thd, TABLE_LIST *all_tables)
   }
 
   /*
-    Acquire SNW locks on tables to be flushed. We can't use
-    lock_table_names() here as this call will also acquire global IX
-    and database-scope IX locks on the tables, and this will make
+    Acquire SNW locks on tables to be flushed. Don't acquire global
+    IX and database-scope IX locks on the tables as this will make
     this statement incompatible with FLUSH TABLES WITH READ LOCK.
   */
-  for (table_list= all_tables; table_list;
-       table_list= table_list->next_global)
-    mdl_requests.push_front(&table_list->mdl_request);
-
-  if (thd->mdl_context.acquire_locks(&mdl_requests,
-                                     thd->variables.lock_wait_timeout))
+  if (lock_table_names(thd, all_tables, NULL,
+                       thd->variables.lock_wait_timeout,
+                       MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK))
     goto error;
 
   DEBUG_SYNC(thd,"flush_tables_with_read_lock_after_acquire_locks");
@@ -390,21 +391,24 @@ bool flush_tables_with_read_lock(THD *thd, TABLE_LIST *all_tables)
     tdc_remove_table(thd, TDC_RT_REMOVE_UNUSED,
                      table_list->db,
                      table_list->table_name, FALSE);
-
-    /* Skip views and temporary tables. */
-    table_list->required_type= FRMTYPE_TABLE; /* Don't try to flush views. */
-    table_list->open_type= OT_BASE_ONLY;      /* Ignore temporary tables. */
+    /* Reset ticket to satisfy asserts in open_tables(). */
+    table_list->mdl_request.ticket= NULL;
   }
 
   /*
     Before opening and locking tables the below call also waits
     for old shares to go away, so the fact that we don't pass
     MYSQL_LOCK_IGNORE_FLUSH flag to it is important.
+    Also we don't pass MYSQL_OPEN_HAS_MDL_LOCK flag as we want
+    to open underlying tables if merge table is flushed.
+    For underlying tables of the merge the below call has to
+    acquire SNW locks to ensure that they can be locked for
+    read without further waiting.
   */
-  if  (open_and_lock_tables(thd, all_tables, FALSE,
-                            MYSQL_OPEN_HAS_MDL_LOCK,
-                            &lock_tables_prelocking_strategy) ||
-       thd->locked_tables_list.init_locked_tables(thd))
+  if (open_and_lock_tables(thd, all_tables, FALSE,
+                           MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK,
+                           &lock_tables_prelocking_strategy) ||
+      thd->locked_tables_list.init_locked_tables(thd))
   {
     goto error;
   }

sql/sql_yacc.yy

@@ -11278,7 +11278,11 @@ opt_with_read_lock:
             TABLE_LIST *tables= Lex->query_tables;
             Lex->type|= REFRESH_READ_LOCK;
             for (; tables; tables= tables->next_global)
+            {
               tables->mdl_request.set_type(MDL_SHARED_NO_WRITE);
+              tables->required_type= FRMTYPE_TABLE; /* Don't try to flush views. */
+              tables->open_type= OT_BASE_ONLY;      /* Ignore temporary tables. */
+            }
           }
         ;