Propagate SIGTERM & SIGINT to supervised processes

Introduces icky global state, but I can't think of another
way to do this with signals, since those are inherently global.

We exit ourselves after SIGTERM & SIGINT signals have been
propagated. Maybe we should wait for our child processes to
actually finish?

simpervisor/atexitasync.py

@@ -0,0 +1,27 @@
+"""
+asyncio aware version of atexit.
+
+Handles SIGINT and SIGTERM, unlike atexit
+"""
+import signal
+import asyncio
+import sys
+
+_handlers = []
+
+signal_handler_set = False
+
+def add_handler(handler):
+    if not signal_handler_set:
+        loop = asyncio.get_event_loop()
+        loop.add_signal_handler(signal.SIGINT, _handle_signal, signal.SIGINT)
+        loop.add_signal_handler(signal.SIGTERM, _handle_signal, signal.SIGTERM)
+    _handlers.append(handler)
+
+def remove_handler(handler):
+    _handlers.remove(handler)
+
+def _handle_signal(signum):
+    for handler in _handlers:
+        handler(signum)
+    sys.exit(0)

simpervisor/process.py

@@ -1,8 +1,10 @@
 """
 Simple asynchronous process supervisor
 """
+import signal
 import asyncio
 import logging
+from simpervisor import atexitasync
 
 
 class SupervisedProcess:
@@ -35,6 +37,10 @@ def _debug_log(self, action, message, extras=None):
             base_extras.update(extras)
         self.log.debug(message, extra=base_extras)
 
+    def _handle_signal(self, signal):
+        self.proc.send_signal(signal)
+        self._debug_log('signal', f'Propagated signal {signal} to {self.name}')
+
     async def start(self):
         """
         Start the process
@@ -54,9 +60,11 @@ async def start(self):
 
         # Spin off a coroutine to watch, reap & restart process if needed
         asyncio.ensure_future(self._restart_process_if_needed())
+        atexitasync.add_handler(self._handle_signal)
 
     async def _restart_process_if_needed(self):
         retcode = await self.proc.wait()
+        atexitasync.remove_handler(self._handle_signal)
         self._debug_log(
             'exited', f'{self.name} exited with code {retcode}',
             {'code': retcode}