MDEV-21101 unexpected wait_timeout with pool-of-threads

Due to restricted size of the threadpool, execution of client queries can
be delayed (queued) for a while. This delay was interpreted as client
inactivity, and connection is closed, if client idle time + queue time
exceeds wait_timeout.

But users did not expect queue time to be included into wait_timeout.

This patch changes the behavior. We don't close connection anymore,
if there is some unread data present on connection,
even if wait_timeout is exceeded. Unread data means that client
was not idle, it sent a query, which we did not have time to process yet.

1 files changed, 53 insertions, 0 deletions

diff --git a/mysql-test/main/mdev-21101.test b/mysql-test/main/mdev-21101.test
new file mode 100644
index 00000000000..9b43c60ec88
--- /dev/null
+++ b/mysql-test/main/mdev-21101.test
@@ -0,0 +1,53 @@
+# Test that wait_timeout does not cause connection to be closed, when connection is delayed due to
+# threadpool internal problems, e.g misconfiguration - too few threads and queueing.
+# So if client did not cause wait_timeout, do not report it either.
+# See MDEV-21101 for details.
+
+# Intentionally misconfigure threadpool to have at most 1 or 2 threads (
+# depends on the implementation). Use minimal wait_timeout, do some slow queries from
+# different connections simultaneously, to force queueing occurs.
+# Verify connections are intact, even if queueing time exceeds wait_timeout
+
+SELECT
+  @@global.wait_timeout, @@global.thread_pool_max_threads, @@global.thread_pool_size,
+  @@global.thread_pool_oversubscribe,  @@global.thread_pool_stall_limit
+INTO
+  @_wait_timeout,@_thread_pool_max_threads,@_thread_pool_size,
+  @_thread_pool_oversubscribe,@_thread_pool_stall_limit;
+
+SET @@global.wait_timeout=1,
+    @@global.thread_pool_max_threads=2,
+    @@global.thread_pool_size=1,
+    @@global.thread_pool_oversubscribe=1,
+    @@global.thread_pool_stall_limit=10;
+ 
+--connect (c1, localhost, root,,)
+--connect (c2, localhost, root,,)
+--connect (c3, localhost, root,,)
+--connection c1
+--send select sleep(1.1)
+--connection c2
+--send select sleep(1.1)
+--connection c3
+--send select sleep(1.1)
+--connection default
+--send select sleep(1.1)
+--connection c1
+--reap
+--connection c2
+--reap
+--connection c3
+--reap
+--connection default
+--reap
+--disconnect c1
+--disconnect c2
+--disconnect c3
+--connection default
+
+SET @@global.wait_timeout=@_wait_timeout,
+    @@global.thread_pool_max_threads=@_thread_pool_max_threads,
+    @@global.thread_pool_size=@_thread_pool_size,
+    @@global.thread_pool_oversubscribe=@_thread_pool_oversubscribe,
+    @@global.thread_pool_stall_limit=@_thread_pool_stall_limit;
+