MDEV-4566 : Failing DBUG_ASSERT() in SELECT SLEEP(), with threadpool.

This bug only happens with long sleep()s ( > 5 sec), and in debug version.

Analysis: 
The assertion is caused by nested thd_wait_begin() calls, which is not an expected condition.
-  "outer" thd_wait_begin()) , in Item_func_sleep::val_int()
-  "inner" thd_wait_begin() in Interruptible_wait::wait(). This function periodically checks whether connection is still valid, via THD::is_connection(), which ends up  calling vio_io_wait()  with timeout parameter set to  0.


Fix is not to call thd wait callback in vio_io_wait(), if timeout parameter is 0. There is no "waiting" in this case.

2 files changed, 6 insertions, 3 deletions

diff --git a/mysql-test/r/pool_of_threads.result b/mysql-test/r/pool_of_threads.result
index 7acb45121d6..c151805e151 100644
--- a/mysql-test/r/pool_of_threads.result
+++ b/mysql-test/r/pool_of_threads.result
@@ -2157,10 +2157,10 @@ Warnings:
 Warning	1052	Column 'kundentyp' in group statement is ambiguous
 drop table t1;
 SET optimizer_switch=@save_optimizer_switch;
-SELECT sleep(5);
+SELECT sleep(5.5);
 SELECT sleep(5);
 # -- Success: more than --thread_pool_max_threads normal connections not possible
-sleep(5)
+sleep(5.5)
 0
 sleep(5)
 0
diff --git a/mysql-test/t/pool_of_threads.test b/mysql-test/t/pool_of_threads.test
index 4600128ff43..24e0218db62 100644
--- a/mysql-test/t/pool_of_threads.test
+++ b/mysql-test/t/pool_of_threads.test
@@ -17,7 +17,10 @@ SET optimizer_switch=@save_optimizer_switch;
 # First set two connections running, and check that extra connection
 # on normal port fails due to--thread-pool-max_threads=2
 connection default;
-send SELECT sleep(5);
+
+# Sleep for slightly longer than 5 sec to trigger MDEV-4566 
+# (abort in interruptible wait connection check)
+send SELECT sleep(5.5); 
 --sleep  1 
 
 connect(con2,localhost,root,,);