Fix pool_of_threads test case

mysql-test/r/pool_of_threads.result

@@ -2084,10 +2084,10 @@ fld6	char(4)	latin1_swedish_ci	NO				#
 show full columns from t2 from test like 's%';
 Field	Type	Collation	Null	Key	Default	Extra	Privileges	Comment
 show keys from t2;
-Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
-t2	0	PRIMARY	1	auto	A	1199	NULL	NULL		BTREE	
-t2	0	fld1	1	fld1	A	1199	NULL	NULL		BTREE	
-t2	1	fld3	1	fld3	A	NULL	NULL	NULL		BTREE	
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment
+t2	0	PRIMARY	1	auto	A	1199	NULL	NULL		BTREE		
+t2	0	fld1	1	fld1	A	1199	NULL	NULL		BTREE		
+t2	1	fld3	1	fld3	A	NULL	NULL	NULL		BTREE		
 drop table t4, t3, t2, t1;
 CREATE TABLE t1 (
 cont_nr int(11) NOT NULL auto_increment,
@@ -2153,7 +2153,7 @@ Warning	1052	Column 'kundentyp' in group statement is ambiguous
 drop table t1;
 SELECT sleep(5);
 SELECT sleep(5);
-# -- Success: more than --thread-pool-size normal connections not possible
+# -- Success: more than --thread_pool_max_threads normal connections not possible
 sleep(5)
 0
 sleep(5)

mysql-test/t/pool_of_threads.cnf

@@ -2,7 +2,8 @@
 
 [mysqld.1]
 loose-thread-handling=   pool-of-threads
-loose-thread_pool_size=  2
+loose-thread_pool_size=  1
+loose-thread_pool_max_threads=  2
 extra-port=        @ENV.MASTER_EXTRA_PORT
 extra-max-connections=1
 

mysql-test/t/pool_of_threads.test

@@ -13,7 +13,7 @@
 # connections on the extra port.
 
 # First set two connections running, and check that extra connection
-# on normal port fails due to--thread-pool-size=2
+# on normal port fails due to--thread-pool-max_threads=2
 connection default;
 send SELECT sleep(5);
 
@@ -32,11 +32,11 @@ connect(con3,localhost,root,,);
 let $error = $mysql_errno;
 if (!$error)
 {
-  --echo # -- Error: managed to establish more than --thread-pool-size connections
+  --echo # -- Error: managed to establish more than --thread_pool_max_threads connections
 }
 if ($error)
 {
-  --echo # -- Success: more than --thread-pool-size normal connections not possible
+  --echo # -- Success: more than --thread_pool_max_threads normal connections not possible
 }
 
 connection default;

sql/threadpool_unix.cc

@@ -624,14 +624,22 @@ static pool_event_t * listener(worker_thread_t *current_thread,
 
 
 
-/* Creates a new worker thread. thread_mutex must be held when calling this function */
+/* 
+  Creates a new worker thread. 
+  thread_mutex must be held when calling this function 
+
+  NOTE: in rare cases, the number of threads can exceed
+  threadpool_max_threads, because we need at least 2 threads
+  per group to prevent deadlocks (one listener + one worker)
+*/
 
 static int create_worker(thread_group_t *thread_group)
 {
   pthread_t thread_id;
   int err;
   DBUG_ENTER("create_worker");
-  if (tp_stats.num_worker_threads >= (int)threadpool_max_threads)
+  if (tp_stats.num_worker_threads >= (int)threadpool_max_threads
+     && thread_group->thread_count >= 2)
   {
     DBUG_PRINT("info", 
      ("Cannot create new thread (maximum allowed threads reached)"));
@@ -667,6 +675,9 @@ static int wake_or_create_thread(thread_group_t *thread_group)
   if (thread_group->pending_thread_start_count > 0)
     DBUG_RETURN(-1);
 
+  if (thread_group->thread_count > thread_group->connection_count)
+    DBUG_RETURN(-1);
+
   if (thread_group->thread_count < 4)
   {
     DBUG_RETURN(create_worker(thread_group));