Error 500

[sci2tech]

Imagine this:
1. User1 add subdomain -> engine-mgr is running -> ispcp-sub-mng is running and altering /etc/apache2/sites-available/ispcp.conf
2. reseller / admin modify another domain -> ispcp-dmn/sub/als/subals-mngr is running and try to alter same file.
Now if user1 lock engine, second user will not be able to run engine an must wait until a third user trigger engine.
How is supposed to work:
1. User trigger engine
2. Engine create lock.
3. Second user need to trigger engine. Gui check if engine lock exists, If yes, transmit header to prevent browser to drop connection, sleep sleep and retry until engine is unlocked.
Error 500 occur when waiting time >20/30 seconds (depends on configure).
Racing condition occur when 2`nd user trigger engine before 1`st instance create lock.
How can we avoid raceing?
1. Gui test daemon is running (otherwise we can lock the engine permanently). If not trigger error message and stop.
2. Gui test for lock file. If exists, wait, then recheck.
3. Create lock file, then trigger engine.
4. Engine perform request, then remove lock.
Permission will not be a problem as long as engine runs with root privilege.