MDEV-9156 : Fix tp_add_connection()'s error handling

Avoid possible my_thread_end() in the main polling thread.

sql/threadpool.h

@@ -27,6 +27,7 @@ extern uint threadpool_oversubscribe;  /* Maximum active threads in group */
 
 
 /* Common thread pool routines, suitable for different implementations */
+extern void threadpool_cleanup_connection(THD *thd);
 extern void threadpool_remove_connection(THD *thd);
 extern int  threadpool_process_request(THD *thd);
 extern int  threadpool_add_connection(THD *thd);

sql/threadpool_common.cc

@@ -168,22 +168,28 @@ int threadpool_add_connection(THD *thd)
   return retval;
 }
 
+/*
+  threadpool_cleanup_connection() does the bulk of connection shutdown work.
+  Usually called from threadpool_remove_connection(), but rarely it might
+  be called also in the main polling thread if connection initialization fails.
+*/
+void threadpool_cleanup_connection(THD *thd)
+{
+  thd->net.reading_or_writing = 0;
+  end_connection(thd);
+  close_connection(thd, 0);
+  unlink_thd(thd);
+  mysql_cond_broadcast(&COND_thread_count);
+}
+
 
 void threadpool_remove_connection(THD *thd)
 {
-
   Worker_thread_context worker_context;
   worker_context.save();
-
   thread_attach(thd);
-  thd->net.reading_or_writing= 0;
-
-  end_connection(thd);
-  close_connection(thd, 0);
-
-  unlink_thd(thd);
-  mysql_cond_broadcast(&COND_thread_count);
 
+  threadpool_cleanup_connection(thd);
   /*
     Free resources associated with this connection: 
     mysys thread_var and PSI thread.

sql/threadpool_unix.cc

@@ -1255,7 +1255,7 @@ void tp_add_connection(THD *thd)
   else
   {
     /* Allocation failed */
-    threadpool_remove_connection(thd);
+    threadpool_cleanup_connection(thd);
   } 
   DBUG_VOID_RETURN;
 }

sql/threadpool_win.cc

@@ -667,7 +667,7 @@ void tp_add_connection(THD *thd)
   if(!con)
   {
     tp_log_warning("Allocation failed", "tp_add_connection");
-    threadpool_remove_connection(thd);
+    threadpool_cleanup_connection(thd);
     return;
   }
 
@@ -685,7 +685,7 @@ void tp_add_connection(THD *thd)
   else
   {
     /* Likely memory pressure */
-    login_callback(NULL, con, NULL); /* deletes connection if something goes wrong */
+    threadpool_cleanup_connection(thd);
   }
 }