MDEV-33070 Thread pool starvation at oversubscribe

Allow one more worker at the oversubscribe threshold so a group
only becomes oversubscribed after it exceeds the configured
limit.

Add a regression test that reproduces the starvation case in the
generic thread pool and verifies queued work still drains.

Signed-off-by: Varun Deep Saini <varun.23bcs10048@ms.sst.scaler.com>

mysql-test/main/mdev-33070.cnf

@@ -0,0 +1,7 @@
+!include include/default_my.cnf
+
+[mysqld.1]
+extra-port=        @ENV.MASTER_EXTRA_PORT
+
+[ENV]
+MASTER_EXTRA_PORT= @OPT.port

mysql-test/main/mdev-33070.opt

@@ -0,0 +1 @@
+--thread-handling=pool-of-threads --loose-thread-pool-mode=generic --thread-pool-size=1 --thread-pool-max-threads=3 --thread-pool-oversubscribe=1 --thread-pool-stall-limit=10000 --thread-pool-dedicated-listener

mysql-test/main/mdev-33070.result

@@ -0,0 +1,35 @@
+# MDEV-33070 Thread pool starvation at oversubscribe threshold
+CREATE TABLE t1 (a INT);
+connect  c1, localhost, root,,;
+connect  c2, localhost, root,,;
+connect  c3, localhost, root,,;
+connect  extra_con, 127.0.0.1, root,,test,$MASTER_EXTRA_PORT,;
+connection c1;
+SELECT SLEEP(1000);
+connection extra_con;
+connection c2;
+SELECT SLEEP(1000);
+connection extra_con;
+connection c3;
+INSERT INTO t1 VALUES (1);
+connection extra_con;
+KILL QUERY c1_id;
+SELECT COUNT(*) = 1 FROM t1;
+COUNT(*) = 1
+1
+connection c1;
+disconnect c1;
+connection extra_con;
+KILL QUERY c2_id;
+connection c2;
+disconnect c2;
+connection c3;
+disconnect c3;
+connection extra_con;
+SELECT COUNT(*) = 1 FROM t1;
+COUNT(*) = 1
+1
+disconnect extra_con;
+connection default;
+DROP TABLE t1;
+# End of MDEV-33070 tests

mysql-test/main/mdev-33070.test

@@ -0,0 +1,83 @@
+--source include/not_embedded.inc
+--source include/have_pool_of_threads.inc
+
+--echo # MDEV-33070 Thread pool starvation at oversubscribe threshold
+
+# The server is started with a single thread group, a dedicated listener,
+# one worker thread, and thread_pool_oversubscribe=1.
+#
+# c1 and c2 run long queries and occupy the one worker plus the one allowed
+# oversubscribe slot. c3 submits a trivial INSERT which the listener can only
+# enqueue, because it is sent only after both c1 and c2 are confirmed running.
+#
+# Before the fix, after c1 was killed the worker stayed at the oversubscribe
+# threshold and went to sleep instead of dequeuing c3. Because
+# thread_pool_dedicated_listener=ON, only the worker can drain the queue, so if
+# c3 is still pending while c1 and c2 run and completes only after c1 is
+# killed, then the worker resumed dequeuing.
+
+CREATE TABLE t1 (a INT);
+
+--connect (c1, localhost, root,,)
+--connect (c2, localhost, root,,)
+--connect (c3, localhost, root,,)
+--connect (extra_con, 127.0.0.1, root,,test,$MASTER_EXTRA_PORT,)
+
+--connection c1
+--let $c1_id= `SELECT CONNECTION_ID()`
+--send SELECT SLEEP(1000)
+
+--connection extra_con
+let $wait_condition=
+  SELECT COUNT(*) > 0 FROM INFORMATION_SCHEMA.PROCESSLIST
+  WHERE STATE='User sleep' AND ID=$c1_id;
+--source include/wait_condition.inc
+
+--connection c2
+--let $c2_id= `SELECT CONNECTION_ID()`
+--send SELECT SLEEP(1000)
+
+--connection extra_con
+let $wait_condition=
+  SELECT COUNT(*) > 0 FROM INFORMATION_SCHEMA.PROCESSLIST
+  WHERE STATE='User sleep' AND ID=$c2_id;
+--source include/wait_condition.inc
+
+--connection c3
+--send INSERT INTO t1 VALUES (1)
+
+--connection extra_con
+--replace_result $c1_id c1_id
+eval KILL QUERY $c1_id;
+
+let $wait_timeout= 10;
+let $wait_condition=
+  SELECT COUNT(*) = 1 FROM t1;
+--source include/wait_condition.inc
+SELECT COUNT(*) = 1 FROM t1;
+
+--connection c1
+--error 0,ER_QUERY_INTERRUPTED,ER_STATEMENT_TIMEOUT
+--reap
+--disconnect c1
+
+--connection extra_con
+--replace_result $c2_id c2_id
+eval KILL QUERY $c2_id;
+
+--connection c2
+--error 0,ER_QUERY_INTERRUPTED
+--reap
+--disconnect c2
+
+--connection c3
+--reap
+--disconnect c3
+
+--connection extra_con
+SELECT COUNT(*) = 1 FROM t1;
+--disconnect extra_con
+
+--connection default
+DROP TABLE t1;
+--echo # End of MDEV-33070 tests

sql/threadpool_generic.cc

@@ -1139,7 +1139,7 @@ static void queue_put(thread_group_t *thread_group, TP_connection_generic *conne
 
 static bool too_many_threads(thread_group_t *thread_group)
 {
-  return (thread_group->active_thread_count >= 1+(int)threadpool_oversubscribe
+  return (thread_group->active_thread_count > 1+(int)threadpool_oversubscribe
    && !thread_group->stalled);
 }