Add async option to add_spawn_function

Some things may need to be started synchronously (if they're functions
that themselves start an asynchronous loop for instance).

malcolm/core/servercomms.py

@@ -12,7 +12,8 @@ def __init__(self, name, process):
         self.q = self.process.create_queue()
         self.add_spawn_function(self.send_loop,
                                 self.make_default_stop_func(self.q))
-        self.add_spawn_function(self.start_recv_loop, self.stop_recv_loop)
+        self.add_spawn_function(self.start_recv_loop, self.stop_recv_loop,
+                                async=False)
 
     def send_loop(self):
         """Service self.q, sending responses to client"""

malcolm/core/spawnable.py

@@ -22,14 +22,17 @@ def start(self, process=None):
         if process is None:
             process = self.process
         self._initialize()
-        for (func, stop_func) in self._spawn_functions:
-            self._spawned.append(process.spawn(func))
+        for (func, stop_func, async) in self._spawn_functions:
+            spawned = process.spawn(func)
+            self._spawned.append(spawned)
+            if not async:
+                spawned.wait()
 
     def stop(self):
         """Call registered stop functions"""
 
         self._initialize()
-        for (func, stop_func) in reversed(self._spawn_functions):
+        for (func, stop_func, async) in reversed(self._spawn_functions):
             if stop_func is not None:
                 stop_func()
 
@@ -39,15 +42,15 @@ def wait(self, timeout=None):
             spawned.wait(timeout=timeout)
         self._spawned = []
 
-    def add_spawn_function(self, func, stop_func=None):
+    def add_spawn_function(self, func, stop_func=None, async=True):
         """Register functions to be triggered by self.start and self.stop
 
         Args:
             func: function to be spawned
             stop_func: function to halt the spawned function (default None)
         """
         self._initialize()
-        self._spawn_functions.append((func, stop_func))
+        self._spawn_functions.append((func, stop_func, async))
 
     def make_default_stop_func(self, q):
         """Convenience function for creating a default stop function that puts