Distinguish "manager stopped" from "manager not started"

This way, if manager thread somehow starts and stops again quickly before
main thread wakes up to check if it started correctly, we will not hang.

Patch suggested by Monty as follow-up to
7f498fbaSigned-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

sql/sql_manager.cc

@@ -26,6 +26,10 @@
 #include "sql_manager.h"
 #include "sql_base.h"                           // flush_tables
 
+/*
+  Values for manager_thread_in_use: 0 means "not started". 1 means "started
+  and active". 2 means "stopped".
+*/
 static bool volatile manager_thread_in_use = 0;
 static bool abort_manager = false;
 
@@ -44,7 +48,7 @@ static struct handler_cb *cb_list; // protected by LOCK_manager
 bool mysql_manager_submit(void (*action)(void *), void *data)
 {
   bool result= FALSE;
-  DBUG_ASSERT(manager_thread_in_use);
+  DBUG_ASSERT(manager_thread_in_use == 1);
   struct handler_cb **cb;
   mysql_mutex_lock(&LOCK_manager);
   cb= &cb_list;
@@ -119,7 +123,7 @@ pthread_handler_t handle_manager(void *arg __attribute__((unused)))
     mysql_mutex_lock(&LOCK_manager);
   }
   DBUG_ASSERT(cb_list == NULL);
-  manager_thread_in_use = 0;
+  manager_thread_in_use = 2;
   mysql_mutex_unlock(&LOCK_manager);
   mysql_mutex_destroy(&LOCK_manager);
   mysql_cond_destroy(&COND_manager);
@@ -148,6 +152,15 @@ void start_handle_manager()
     }
 
     mysql_mutex_lock(&LOCK_manager);
+    /*
+      Wait for manager thread to have started, otherwise in extreme cases the
+      server may start up and have initiated shutdown at the time the manager
+      thread even starts to run.
+
+      Allow both values 1 and 2 for manager_thread_in_use, so that we will not
+      get stuck here if the manager thread somehow manages to start up and
+      abort again before we have time to test it here.
+    */
     while (!manager_thread_in_use)
       mysql_cond_wait(&COND_manager, &LOCK_manager);
     mysql_mutex_unlock(&LOCK_manager);