Raise KeyboardInterrupt on SIGTERM - Workaround

Catch SIGTERM signal and call the signal handler method.
The signal handler then raises KeyboardInterrupt. The
KeyboardInterrupt won't be caught by any 'except Exception'
clauses.

Without this the service does not stop periodic workers,
tear down pxe filters etc as it is supposed to on shutdown.

NOTE: Calling shutdown() directly from the signal handler
causes the below error. This is why the signal handler
raises KeyboardInterrupt.
 AssertionError: Cannot switch to MAINLOOP from MAINLOOP

Related-Bug: #1765700

Story: 2001890
Task: 14374
Change-Id: If0e24eae767b7806243fa4ae34fedb30ae9af25a

ironic_inspector/wsgi_service.py

@@ -43,6 +43,7 @@ class WSGIService(object):
         self._periodics_worker = None
         self._shutting_down = semaphore.Semaphore()
         signal.signal(signal.SIGHUP, self._handle_sighup)
+        signal.signal(signal.SIGTERM, self._handle_sigterm)
 
     def _init_middleware(self):
         """Initialize WSGI middleware.
@@ -198,6 +199,12 @@ class WSGIService(object):
     def _handle_sighup(self, *args):
         eventlet.spawn(self._handle_sighup_bg, *args)
 
+    def _handle_sigterm(self, *args):
+        # This is a workaround to ensure that shutdown() is done when recieving
+        # SIGTERM. Raising KeyboardIntrerrupt which won't be caught by any
+        # 'except Exception' clauses.
+        raise KeyboardInterrupt
+
 
 def periodic_clean_up():  # pragma: no cover
     try: