ntfy-waiter: never drop notifications, but rather sleep until client is ready to receive them

hosts/by-name/servo/services/ntfy/ntfy-waiter

@@ -24,15 +24,15 @@ class Client:
     def __cmp__(self, other: 'Client'):
         return cmp(self.addr_info, other.addr_info)
 
-    def maybe_notify(self, message: bytes) -> bool:
+    def try_notify(self, message: bytes) -> bool:
         """
-        returns false if we wanted to notify the client but failed (e.g. socket died).
+        returns true if we send a packet to notify client.
-        true otherwise.
+        fals otherwise (e.g. the socket is dead).
         """
-        if time.time() < self.live_after:
+        ttl = self.live_after - time.time()
-            # TODO: we want to still notify, just with a delay!
+        if ttl > 0:
-            logger.debug(f"not notifying client because it's not ready: {self.addr_info}")
+            logger.debug(f"sleeping {ttl:.2f}s until client {self.addr_info} is ready to receive notification")
-            return True
+            time.sleep(ttl)
 
         try:
             self.sock.sendall(message)
@@ -77,12 +77,13 @@ class Adapter:
             self.add_client(Client(conn, addr_info, live_after = time.time() + self.silence))
 
     def notify_clients(self, message: bytes = WAKE_MESSAGE):
-        # notify every client, and drop any which have disconnected
+        # notify every client, and drop any which have disconnected.
-        # XXX: paranoia about me not knowing Python's guarantees for mutation during set iteration
+        # note that we notify based on age (oldest -> youngest)
-        clients = set(self.clients)
+        # because notifying young clients might entail sleeping until they're ready.
+        clients = sorted(self.clients, key=lambda c: (c.live_after, c.addr_info))
 
         dead_clients = [
-            c for c in clients if not c.maybe_notify(message)
+            c for c in clients if not c.try_notify(message)
         ]
         for c in dead_clients:
             self.clients.remove(c)