Added _is_single_user_process_alive to allow subclasses to reimplement

this without reimplementing the whole poll method.

jupyterhub/spawner.py

@@ -1468,15 +1468,22 @@ class LocalProcessSpawner(Spawner):
             self.clear_state()
             return 0
 
-        # send signal 0 to check if PID exists
-        # this doesn't work on Windows, but that's okay because we don't support Windows.
-        alive = await self._signal(0)
+        alive = await self._is_single_user_process_alive()
         if not alive:
             self.clear_state()
             return 0
         else:
             return None
 
+    async def _is_single_user_process_alive(self):
+        """Returns True if the process still exists, False otherwise.
+        
+        Allows subclasses to implement this in a platform specific manner.
+        """
+        # send signal 0 to check if PID exists
+        # this doesn't work on Windows, but that's okay because we don't support Windows.
+        return await self._signal(0)
+
     async def _signal(self, sig):
         """Send given signal to a single-user server's process.