MDEV-37890 - sys_vars.thread_cache_size_func fails sporadically

Removed redundant FLUSH STATUS, doesn't serve any purpose in this test.

Shrinking thread_cache_size is done without appropriate mutex protection
and it doesn't flush thread cache. It means we may have more threads in
the cache than thread_cache_size allows.

First FLUSH THREADS (line 36) was added just in case
thread_cache_size_func-master.opt file is removed and raising
thread_cache_size from 0 to 3 becomes shrinking from the default value
to 3.

Second FLUSH THREADS (line 81) was added to fix timeout in wait
condition (line 100). It serves as a barrier that guarantees fourth
preceding disconnect won't pick one of subsequent connects and in turn
leave 2 threads in the cache.

Threads_cached cannot be queried for exact value because there can
(or cannot) be disconnecting threads from previous tests or mtr utility
connection. There is also a gap between server_threads.erase() (aka
removal from I_S.PROCESSLIST) and thread_cache.park() (aka
Threads_cached++). All in all it makes Threads_cached value
unpredictable after thread_cache_size is set to 3.

Instead of querying for exact value, just make sure Threads_cached
doesn't exceed thread_cache_size.

Author: svoj

mysql-test/suite/sys_vars/r/thread_cache_size_func.result

@@ -1,25 +1,24 @@
 SET @global_thread_cache_size = @@GLOBAL.thread_cache_size;
-FLUSH STATUS;
 '# Test1#'
 SET @@GLOBAL.thread_cache_size=3;
-SHOW STATUS LIKE 'Threads_cached';
-Variable_name	Value
-Threads_cached	0
-0 Expected
+FLUSH THREADS;
+select variable_value<=@@global.thread_cache_size from information_schema.global_status where variable_name="Threads_cached";
+variable_value<=@@global.thread_cache_size
+1
 CONNECT  conn1,localhost,root,,;
 CONNECT  conn2,localhost,root,,;
 CONNECT  conn3,localhost,root,,;
 CONNECT  conn4,localhost,root,,;
 connection default;
-SHOW STATUS LIKE 'Threads_cached';
-Variable_name	Value
-Threads_cached	0
-0 Expected
+select variable_value<=@@global.thread_cache_size from information_schema.global_status where variable_name="Threads_cached";
+variable_value<=@@global.thread_cache_size
+1
 disconnect conn1;
 disconnect conn2;
 disconnect conn3;
 disconnect conn4;
 SET @@GLOBAL.thread_cache_size= 1;
+FLUSH THREADS;
 CONNECT  conn1,localhost,root,,;
 CONNECT  conn2,localhost,root,,;
 connection default;

mysql-test/suite/sys_vars/t/thread_cache_size_func.test

@@ -31,12 +31,10 @@
 --source include/one_thread_per_connection.inc
 SET @global_thread_cache_size = @@GLOBAL.thread_cache_size;
 
-FLUSH STATUS;
-
 -- ECHO '# Test1#'
 SET @@GLOBAL.thread_cache_size=3;
-SHOW STATUS LIKE 'Threads_cached';
---echo 0 Expected
+FLUSH THREADS;
+select variable_value<=@@global.thread_cache_size from information_schema.global_status where variable_name="Threads_cached";
 
 ##################################
 # Make 4 connections             #
@@ -53,8 +51,7 @@ CONNECTION default;
 let $wait_condition= SELECT COUNT(*)= 5 FROM INFORMATION_SCHEMA.PROCESSLIST;
 --source include/wait_condition.inc
 
-SHOW STATUS LIKE 'Threads_cached';
---echo 0 Expected
+select variable_value<=@@global.thread_cache_size from information_schema.global_status where variable_name="Threads_cached";
 
 ####################################
 #Disconnecting all the connections #
@@ -81,6 +78,7 @@ let $wait_condition= SELECT variable_value = 3 FROM INFORMATION_SCHEMA.global_st
 # Decreasing cache size to 1
 #
 SET @@GLOBAL.thread_cache_size= 1;
+FLUSH THREADS;
 
 CONNECT (conn1,localhost,root,,);
 CONNECT (conn2,localhost,root,,);