Add locks for sequence's to ensure that there is only one writer or many readers

This is needed for MyISAM and other storage engines which normally
relies on THR_LOCK's to ensure that one is not writing the same block
one could be reading from.

sql/sql_sequence.cc

@@ -342,14 +342,46 @@ bool sequence_insert(THD *thd, LEX *lex, TABLE_LIST *table_list)
 
 SEQUENCE::SEQUENCE() :all_values_used(0), initialized(SEQ_UNINTIALIZED), table(0)
 {
-  mysql_mutex_init(key_LOCK_SEQUENCE, &mutex, MY_MUTEX_INIT_SLOW);
+  mysql_rwlock_init(key_LOCK_SEQUENCE, &mutex);
 }
 
 SEQUENCE::~SEQUENCE()
 {
-  mysql_mutex_destroy(&mutex);
+  mysql_rwlock_destroy(&mutex);
 }
 
+/*
+  The following functions is to ensure that we when reserve new values
+  trough sequence object sequence we have only one writer at at time.
+  A sequence table can have many readers (trough normal SELECT's).
+
+  We mark that we have a write lock in the table object so that
+  ha_sequence::ha_write() can check if we have a lock. If already locked, then
+  ha_write() knows that we are running a sequence operation. If not, then
+  ha_write() knows that it's an INSERT.
+*/
+
+void SEQUENCE::write_lock(TABLE *table)
+{
+  DBUG_ASSERT(((ha_sequence*) table->file)->is_locked() == 0);
+  mysql_rwlock_wrlock(&mutex);
+  ((ha_sequence*) table->file)->write_lock();
+}
+void SEQUENCE::write_unlock(TABLE *table)
+{
+  ((ha_sequence*) table->file)->unlock();
+  mysql_rwlock_unlock(&mutex);
+}
+void SEQUENCE::read_lock(TABLE *table)
+{
+  if (!((ha_sequence*) table->file)->is_locked())
+    mysql_rwlock_rdlock(&mutex);
+}
+void SEQUENCE::read_unlock(TABLE *table)
+{
+  if (!((ha_sequence*) table->file)->is_locked())
+    mysql_rwlock_unlock(&mutex);
+}
 
 /**
    Read values from the sequence tables to table_share->sequence.
@@ -366,7 +398,7 @@ int SEQUENCE::read_initial_values(TABLE *table_arg)
   if (likely(initialized != SEQ_UNINTIALIZED))
     DBUG_RETURN(0);
   table= table_arg;
-  mysql_mutex_lock(&mutex);
+  write_lock(table);
   if (likely(initialized == SEQ_UNINTIALIZED))
   {
     MYSQL_LOCK *lock;
@@ -422,7 +454,7 @@ int SEQUENCE::read_initial_values(TABLE *table_arg)
     if (!has_active_transaction && !thd->transaction.stmt.is_empty())
       trans_commit_stmt(thd);
   }
-  mysql_mutex_unlock(&mutex);
+  write_unlock(table);
   DBUG_RETURN(error);
 }
 
@@ -436,7 +468,6 @@ int SEQUENCE::read_stored_values()
   int error;
   my_bitmap_map *save_read_set;
   DBUG_ENTER("SEQUENCE::read_stored_values");
-  mysql_mutex_assert_owner(&mutex);
 
   save_read_set= tmp_use_all_columns(table, table->read_set);
   error= table->file->ha_read_first_row(table->record[0], MAX_KEY);
@@ -546,6 +577,7 @@ int sequence_definition::write(TABLE *table, bool all_fields)
 {
   int error;
   MY_BITMAP *save_rpl_write_set, *save_write_set;
+  DBUG_ASSERT(((ha_sequence*) table->file)->is_locked());
 
   save_rpl_write_set= table->rpl_write_set;
   if (likely(!all_fields))
@@ -563,11 +595,8 @@ int sequence_definition::write(TABLE *table, bool all_fields)
   save_write_set= table->write_set;
   table->write_set= &table->s->all_set;
   store_fields(table);
-  /* Tell ha_sequence::write_row that we already hold the mutex */
-  ((ha_sequence*) table->file)->sequence_locked= 1;
   if ((error= table->file->ha_write_row(table->record[0])))
     table->file->print_error(error, MYF(0));
-  ((ha_sequence*) table->file)->sequence_locked= 0;
   table->rpl_write_set= save_rpl_write_set;
   table->write_set= save_write_set;
   return error;
@@ -610,7 +639,7 @@ longlong SEQUENCE::next_value(TABLE *table, bool second_round, int *error)
 
   *error= 0;
   if (!second_round)
-    lock();
+    write_lock(table);
 
   res_value= next_free_value;
   next_free_value= increment_value(next_free_value);
@@ -618,7 +647,7 @@ longlong SEQUENCE::next_value(TABLE *table, bool second_round, int *error)
   if ((real_increment > 0 && res_value < reserved_until) ||
       (real_increment < 0 && res_value > reserved_until))
   {
-    unlock();
+    write_unlock(table);
     DBUG_RETURN(res_value);
   }
 
@@ -677,11 +706,11 @@ longlong SEQUENCE::next_value(TABLE *table, bool second_round, int *error)
     next_free_value= res_value;
   }
 
-  unlock();
+  write_unlock(table);
   DBUG_RETURN(res_value);
 
 err:
-  unlock();
+  write_unlock(table);
   my_error(ER_SEQUENCE_RUN_OUT, MYF(0), table->s->db.str,
            table->s->table_name.str);
   *error= ER_SEQUENCE_RUN_OUT;
@@ -740,7 +769,7 @@ bool SEQUENCE::set_value(TABLE *table, longlong next_val, ulonglong next_round,
   ulonglong org_round= round;
   DBUG_ENTER("SEQUENCE::set_value");
 
-  lock();
+  write_lock(table);
   if (is_used)
     next_val= increment_value(next_val);
 
@@ -782,7 +811,7 @@ bool SEQUENCE::set_value(TABLE *table, longlong next_val, ulonglong next_round,
   error= 0;
 
 end:
-  unlock();
+  write_unlock(table);
   DBUG_RETURN(error);
 }
 
@@ -871,6 +900,7 @@ bool Sql_cmd_alter_sequence::execute(THD *thd)
     goto end;
   }
 
+  table->s->sequence->write_lock(table);
   if (!(error= new_seq->write(table, 1)))
   {
     /* Store the sequence values in table share */
@@ -878,6 +908,7 @@ bool Sql_cmd_alter_sequence::execute(THD *thd)
   }
   else
     table->file->print_error(error, MYF(0));
+  table->s->sequence->write_unlock(table);
   if (trans_commit_stmt(thd))
     error= 1;
   if (trans_commit_implicit(thd))