wake the mqtt worker when a publish is queued (#24580)

publish() only queued the message, and the worker was blocked waiting on the broker socket for up to a second, so on a quiet connection messages went out up to 1s late. A socketpair registered in the worker's selector now interrupts the wait when a publish is queued or stop() is called.

frigate/comms/mqtt.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import logging
 import queue
 import selectors
+import socket
 import threading
 import time
 from collections.abc import Callable
@@ -57,6 +58,11 @@ class MqttClient(Communicator):
         self._next_connect_time = 0.0
         self._last_on_connect_dispatch = 0.0
 
+        # lets other threads interrupt the worker's socket wait
+        self._wake_recv, self._wake_send = socket.socketpair()
+        self._wake_recv.setblocking(False)
+        self._wake_send.setblocking(False)
+
     def subscribe(self, receiver: Callable) -> None:
         """Wrapper for allowing dispatcher to subscribe."""
         self._dispatcher = receiver
@@ -86,6 +92,7 @@ class MqttClient(Communicator):
             return
 
         self._publish_queue.put(QueuedPublish(full_topic, payload, retain))
+        self._wake_worker()
 
     def stop(self) -> None:
         if self._worker is None:
@@ -101,9 +108,11 @@ class MqttClient(Communicator):
                     publish_done,
                 )
             )
+            self._wake_worker()
             publish_done.wait(MQTT_SHUTDOWN_FLUSH_TIMEOUT)
 
         self._stop_event.set()
+        self._wake_worker()
 
         if self.client is not None:
             try:
@@ -369,11 +378,17 @@ class MqttClient(Communicator):
                 exc_info=True,
             )
 
+    def _wake_worker(self) -> None:
+        try:
+            self._wake_send.send(b"\0")
+        except BlockingIOError:
+            # the buffer is full, so a wake is already pending
+            pass
+
     def _loop_client(self, timeout: float) -> int:
         """Drive Paho without select()'s limit on socket file descriptors."""
+        assert self.client is not None
         client = self.client
-        if client is None:
-            return mqtt.MQTT_ERR_NO_CONN
         sock = client.socket()
         if sock is None:
             return mqtt.MQTT_ERR_NO_CONN
@@ -385,11 +400,19 @@ class MqttClient(Communicator):
         pending = hasattr(sock, "pending") and sock.pending() > 0
         with selectors.DefaultSelector() as selector:
             selector.register(sock, events)
-            ready = selector.select(0.0 if pending else timeout)
+            selector.register(self._wake_recv, selectors.EVENT_READ)
+            ready = {
+                key.fileobj: mask
+                for key, mask in selector.select(0.0 if pending else timeout)
+            }
 
-        ready_events = 0
-        for _, mask in ready:
-            ready_events |= mask
+        if self._wake_recv in ready:
+            try:
+                self._wake_recv.recv(4096)
+            except BlockingIOError:
+                pass
+
+        ready_events = ready.get(sock, 0)
         if pending or ready_events & selectors.EVENT_READ:
             result = client.loop_read()
             if result != mqtt.MQTT_ERR_SUCCESS or client.socket() is None: