Fix SSE fanout backlog causing delayed bursty updates

tests/test_sse.py

@@ -6,6 +6,8 @@ import queue
 import time
 import uuid
 
+import pytest
+
 from utils.sse import subscribe_fanout_queue
 
 
@@ -13,8 +15,8 @@ def _channel_key(prefix: str) -> str:
     return f"{prefix}-{uuid.uuid4()}"
 
 
-def test_fanout_does_not_drain_source_queue_without_subscribers() -> None:
+def test_fanout_drains_source_queue_without_subscribers() -> None:
-    """Queued messages should remain buffered while no SSE clients are connected."""
+    """Queued messages should be dropped while no SSE clients are connected."""
     source = queue.Queue()
     channel_key = _channel_key("sse-idle")
 
@@ -25,19 +27,18 @@ def test_fanout_does_not_drain_source_queue_without_subscribers() -> None:
     source.put({"type": "aprs", "callsign": "N0CALL"})
     time.sleep(0.05)
 
-    assert source.qsize() == 1
+    assert source.qsize() == 0
 
 
-def test_fanout_delivers_buffered_message_after_re_subscribe() -> None:
+def test_fanout_does_not_replay_stale_message_after_re_subscribe() -> None:
-    """A message queued while disconnected should be delivered on reconnect."""
+    """A message queued while disconnected should not be replayed on reconnect."""
     source = queue.Queue()
     channel_key = _channel_key("sse-resub")
 
     _, unsubscribe = subscribe_fanout_queue(source, channel_key=channel_key, source_timeout=0.01)
     unsubscribe()
 
-    expected = {"type": "aprs", "callsign": "K1ABC"}
+    source.put({"type": "aprs", "callsign": "K1ABC"})
-    source.put(expected)
 
     subscriber, unsubscribe2 = subscribe_fanout_queue(
         source,
@@ -45,8 +46,12 @@ def test_fanout_delivers_buffered_message_after_re_subscribe() -> None:
         source_timeout=0.01,
     )
     try:
+        with pytest.raises(queue.Empty):
+            subscriber.get(timeout=0.1)
+        live = {"type": "aprs", "callsign": "LIVE01"}
+        source.put(live)
         got = subscriber.get(timeout=0.25)
     finally:
         unsubscribe2()
 
-    assert got == expected
+    assert got == live

utils/sse.py

@@ -26,13 +26,26 @@ _fanout_channels_lock = threading.Lock()
 
 def _run_fanout(channel: _QueueFanoutChannel) -> None:
     """Drain source queue and fan out each message to all subscribers."""
+    idle_drain_batch = 512
+
     while True:
         with channel.lock:
-            has_subscribers = bool(channel.subscribers)
+            subscribers = tuple(channel.subscribers)
 
-        if not has_subscribers:
+        if not subscribers:
-            # Do not drain source_queue when no clients are connected.
+            # Keep ingest pipelines responsive even if UI clients disconnect:
-            time.sleep(channel.source_timeout)
+            # drain and drop stale backlog while idle so producer threads do
+            # not block on full source queues.
+            drained = 0
+            for _ in range(idle_drain_batch):
+                try:
+                    channel.source_queue.get_nowait()
+                    drained += 1
+                except queue.Empty:
+                    break
+
+            if drained == 0:
+                time.sleep(channel.source_timeout)
             continue
 
         try:
@@ -40,17 +53,6 @@ def _run_fanout(channel: _QueueFanoutChannel) -> None:
         except queue.Empty:
             continue
 
-        with channel.lock:
-            subscribers = tuple(channel.subscribers)
-
-        if not subscribers:
-            # Subscriber set changed after we dequeued; requeue best-effort.
-            try:
-                channel.source_queue.put_nowait(msg)
-            except queue.Full:
-                pass
-            continue
-
         for subscriber in subscribers:
             try:
                 subscriber.put_nowait(msg)