Backport of revno: 2617.68.13

Introduce a counter for protection against global read lock on thread level.

The functions for protection against global read lock sometimes need a local
variable to signal when the protection is set, and hence need to be released.
It would be better to control this behaviour via a counter on the THD struct,
telling how many times the protection has been claimed by the current thread.
A side-effect of the fix is that if protection is claimed twice for a thread,
only a simple increment is required for the second claim, instead of a
mutex-protected increment of the global variable protect_against_global_read_lock.

sql/lock.cc

@@ -1098,6 +1098,19 @@ static volatile uint waiting_for_read_lock=0;
 #define GOT_GLOBAL_READ_LOCK               1
 #define MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT 2
 
+/**
+  Take global read lock, wait if there is protection against lock.
+
+  If the global read lock is already taken by this thread, then nothing is done.
+
+  See also "Handling of global read locks" above.
+
+  @param thd     Reference to thread.
+
+  @retval False  Success, global read lock set, commits are NOT blocked.
+  @retval True   Failure, thread was killed.
+*/
+
 bool lock_global_read_lock(THD *thd)
 {
   DBUG_ENTER("lock_global_read_lock");
@@ -1164,6 +1177,16 @@ bool lock_global_read_lock(THD *thd)
 }
 
 
+/**
+  Unlock global read lock.
+
+  Commits may or may not be blocked when this function is called.
+
+  See also "Handling of global read locks" above.
+
+  @param thd    Reference to thread.
+*/
+
 void unlock_global_read_lock(THD *thd)
 {
   uint tmp;
@@ -1190,6 +1213,25 @@ void unlock_global_read_lock(THD *thd)
   DBUG_VOID_RETURN;
 }
 
+/**
+  Wait if the global read lock is set, and optionally seek protection against
+  global read lock.
+
+  See also "Handling of global read locks" above.
+
+  @param thd              Reference to thread.
+  @param abort_on_refresh If True, abort waiting if a refresh occurs,
+                          do NOT seek protection against GRL.
+                          If False, wait until the GRL is released and seek
+                          protection against GRL.
+  @param is_not_commit    If False, called from a commit operation,
+                          wait only if commit blocking is also enabled.
+
+  @retval False  Success, protection against global read lock is set
+                          (if !abort_on_refresh)
+  @retval True   Failure, wait was aborted or thread was killed.
+*/
+
 #define must_wait (global_read_lock &&                             \
                    (is_not_commit ||                               \
                     global_read_lock_blocks_commit))
@@ -1201,6 +1243,16 @@ bool wait_if_global_read_lock(THD *thd, bool abort_on_refresh,
   bool result= 0, need_exit_cond;
   DBUG_ENTER("wait_if_global_read_lock");
 
+  /*
+    If we already have protection against global read lock,
+    just increment the counter.
+  */
+  if (unlikely(thd->global_read_lock_protection > 0))
+  {
+    if (!abort_on_refresh)
+      thd->global_read_lock_protection++;
+    DBUG_RETURN(FALSE);
+  }
   /*
     Assert that we do not own LOCK_open. If we would own it, other
     threads could not close their tables. This would make a pretty
@@ -1237,7 +1289,12 @@ bool wait_if_global_read_lock(THD *thd, bool abort_on_refresh,
       result=1;
   }
   if (!abort_on_refresh && !result)
+  {
+    thd->global_read_lock_protection++;
     protect_against_global_read_lock++;
+    DBUG_PRINT("sql_lock", ("protect_against_global_read_lock incr: %u",
+                            protect_against_global_read_lock));
+  }
   /*
     The following is only true in case of a global read locks (which is rare)
     and if old_message is set
@@ -1250,10 +1307,31 @@ bool wait_if_global_read_lock(THD *thd, bool abort_on_refresh,
 }
 
 
+/**
+  Release protection against global read lock and restart
+  global read lock waiters.
+
+  Should only be called if we have protection against global read lock.
+
+  See also "Handling of global read locks" above.
+
+  @param thd     Reference to thread.
+*/
+
 void start_waiting_global_read_lock(THD *thd)
 {
   bool tmp;
   DBUG_ENTER("start_waiting_global_read_lock");
+  /*
+    Ignore request if we do not have protection against global read lock.
+    (Note that this is a violation of the interface contract, hence the assert).
+  */
+  DBUG_ASSERT(thd->global_read_lock_protection > 0);
+  if (unlikely(thd->global_read_lock_protection == 0))
+    DBUG_VOID_RETURN;
+  /* Decrement local read lock protection counter, return if we still have it */
+  if (unlikely(--thd->global_read_lock_protection > 0))
+    DBUG_VOID_RETURN;
   if (unlikely(thd->global_read_lock))
     DBUG_VOID_RETURN;
   (void) pthread_mutex_lock(&LOCK_global_read_lock);
@@ -1267,6 +1345,21 @@ void start_waiting_global_read_lock(THD *thd)
 }
 
 
+/**
+  Make global read lock also block commits.
+
+  The scenario is:
+   - This thread has the global read lock.
+   - Global read lock blocking of commits is not set.
+
+  See also "Handling of global read locks" above.
+
+  @param thd     Reference to thread.
+
+  @retval False  Success, global read lock set, commits are blocked.
+  @retval True   Failure, thread was killed.
+*/
+
 bool make_global_read_lock_block_commit(THD *thd)
 {
   bool error;

sql/sql_class.cc

@@ -452,6 +452,7 @@ THD::THD()
    examined_row_count(0),
    warning_info(&main_warning_info),
    stmt_da(&main_da),
+   global_read_lock_protection(0),
    global_read_lock(0),
    is_fatal_error(0),
    transaction_rollback_request(0),

sql/sql_class.h

@@ -1874,6 +1874,7 @@ class THD :public Statement,
   ulong	     rand_saved_seed1, rand_saved_seed2;
   pthread_t  real_id;                           /* For debugging */
   my_thread_id  thread_id;
+  uint       global_read_lock_protection;// GRL protection count
   uint	     tmp_table, global_read_lock;
   uint	     server_status,open_options;
   enum enum_thread_type system_thread;

sql/sql_parse.cc

@@ -1762,7 +1762,6 @@ int
 mysql_execute_command(THD *thd)
 {
   int res= FALSE;
-  bool need_start_waiting= FALSE; // have protection against global read lock
   int  up_result= 0;
   LEX  *lex= thd->lex;
   /* first SELECT_LEX (have special meaning for many of non-SELECTcommands) */
@@ -2039,7 +2038,7 @@ mysql_execute_command(THD *thd)
       break;
 
     if (!thd->locked_tables_mode && lex->protect_against_global_read_lock &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+        wait_if_global_read_lock(thd, 0, 1))
       break;
 
     res= execute_sqlcom_select(thd, all_tables);
@@ -2309,10 +2308,9 @@ case SQLCOM_PREPARE:
       read lock when it succeeds. This needs to be released by
       start_waiting_global_read_lock(). We protect the normal CREATE
       TABLE in the same way. That way we avoid that a new table is
-      created during a gobal read lock.
+      created during a global read lock.
     */
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     {
       res= 1;
       goto end_with_restore_list;
@@ -2617,8 +2615,7 @@ case SQLCOM_PREPARE:
                             "INDEX DIRECTORY");
       create_info.data_file_name= create_info.index_file_name= NULL;
 
-      if (!thd->locked_tables_mode &&
-          !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+      if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
       {
         res= 1;
         break;
@@ -2852,8 +2849,7 @@ case SQLCOM_PREPARE:
     DBUG_ASSERT(first_table == all_tables && first_table != 0);
     if (update_precheck(thd, all_tables))
       break;
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
       goto error;
     DBUG_ASSERT(select_lex->offset_limit == 0);
     unit->set_limit(select_lex);
@@ -2891,7 +2887,7 @@ case SQLCOM_PREPARE:
     */
     if (!thd->locked_tables_mode &&
         lex->sql_command == SQLCOM_UPDATE_MULTI &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+        wait_if_global_read_lock(thd, 0, 1))
       goto error;
 
     res= mysql_multi_update_prepare(thd);
@@ -2993,8 +2989,7 @@ case SQLCOM_PREPARE:
     if ((res= insert_precheck(thd, all_tables)))
       break;
 
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     {
       res= 1;
       break;
@@ -3033,8 +3028,7 @@ case SQLCOM_PREPARE:
 
     unit->set_limit(select_lex);
 
-    if (! thd->locked_tables_mode &&
-        ! (need_start_waiting= ! wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     {
       res= 1;
       break;
@@ -3104,7 +3098,7 @@ case SQLCOM_PREPARE:
                  ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
       goto error;
     }
-    if (!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (wait_if_global_read_lock(thd, 0, 1))
       goto error;
     res= mysql_truncate(thd, first_table, 0);
     break;
@@ -3116,8 +3110,7 @@ case SQLCOM_PREPARE:
     DBUG_ASSERT(select_lex->offset_limit == 0);
     unit->set_limit(select_lex);
 
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     {
       res= 1;
       break;
@@ -3137,8 +3130,7 @@ case SQLCOM_PREPARE:
       (TABLE_LIST *)thd->lex->auxiliary_table_list.first;
     multi_delete *del_result;
 
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     {
       res= 1;
       break;
@@ -3282,8 +3274,7 @@ case SQLCOM_PREPARE:
     if (check_one_table_access(thd, privilege, all_tables))
       goto error;
 
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
       goto error;
 
     res= mysql_load(thd, lex->exchange, first_table, lex->field_list,
@@ -3357,7 +3348,7 @@ case SQLCOM_PREPARE:
                            FALSE, UINT_MAX, FALSE))
       goto error;
     if (lex->protect_against_global_read_lock &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+        wait_if_global_read_lock(thd, 0, 1))
       goto error;
 
     init_mdl_requests(all_tables);
@@ -4575,7 +4566,7 @@ case SQLCOM_PREPARE:
   res= TRUE;
 
 finish:
-  if (need_start_waiting)
+  if (thd->global_read_lock_protection > 0)
   {
     /*
       Release the protection against the global read lock and wake

sql/sql_table.cc

@@ -1780,16 +1780,16 @@ void write_bin_log(THD *thd, bool clear_error,
 bool mysql_rm_table(THD *thd,TABLE_LIST *tables, my_bool if_exists,
                     my_bool drop_temporary)
 {
-  bool error= FALSE, need_start_waiting= FALSE;
+  bool error;
   Drop_table_error_handler err_handler(thd->get_internal_handler());
+
   DBUG_ENTER("mysql_rm_table");
 
   /* mark for close and remove all cached entries */
 
   if (!drop_temporary)
   {
-    if (!thd->locked_tables_mode &&
-        !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+    if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
       DBUG_RETURN(TRUE);
   }
 
@@ -1797,8 +1797,7 @@ bool mysql_rm_table(THD *thd,TABLE_LIST *tables, my_bool if_exists,
   error= mysql_rm_table_part2(thd, tables, if_exists, drop_temporary, 0, 0);
   thd->pop_internal_handler();
 
-
-  if (need_start_waiting)
+  if (thd->global_read_lock_protection > 0)
     start_waiting_global_read_lock(thd);
 
   if (error)

sql/sql_trigger.cc

@@ -328,7 +328,6 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   TABLE *table;
   bool result= TRUE;
   String stmt_query;
-  bool need_start_waiting= FALSE;
   bool lock_upgrade_done= FALSE;
   MDL_ticket *mdl_ticket= NULL;
 
@@ -386,8 +385,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
     LOCK_open is not enough because global read lock is held without holding
     LOCK_open).
   */
-  if (!thd->locked_tables_mode &&
-      !(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
+  if (!thd->locked_tables_mode && wait_if_global_read_lock(thd, 0, 1))
     DBUG_RETURN(TRUE);
 
   if (!create)
@@ -521,7 +519,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   if (thd->locked_tables_mode && tables && lock_upgrade_done)
     mdl_ticket->downgrade_exclusive_lock();
 
-  if (need_start_waiting)
+  if (thd->global_read_lock_protection > 0)
     start_waiting_global_read_lock(thd);
 
   if (!result)