Fix SSE fanout packet loss on reconnect

tests/test_sse.py

@@ -0,0 +1,52 @@
+"""Tests for SSE fanout queue behavior."""
+
+from __future__ import annotations
+
+import queue
+import time
+import uuid
+
+from utils.sse import subscribe_fanout_queue
+
+
+def _channel_key(prefix: str) -> str:
+    return f"{prefix}-{uuid.uuid4()}"
+
+
+def test_fanout_does_not_drain_source_queue_without_subscribers() -> None:
+    """Queued messages should remain buffered while no SSE clients are connected."""
+    source = queue.Queue()
+    channel_key = _channel_key("sse-idle")
+
+    # Start fanout distributor, then remove the only subscriber.
+    _, unsubscribe = subscribe_fanout_queue(source, channel_key=channel_key, source_timeout=0.01)
+    unsubscribe()
+
+    source.put({"type": "aprs", "callsign": "N0CALL"})
+    time.sleep(0.05)
+
+    assert source.qsize() == 1
+
+
+def test_fanout_delivers_buffered_message_after_re_subscribe() -> None:
+    """A message queued while disconnected should be delivered 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(expected)
+
+    subscriber, unsubscribe2 = subscribe_fanout_queue(
+        source,
+        channel_key=channel_key,
+        source_timeout=0.01,
+    )
+    try:
+        got = subscriber.get(timeout=0.25)
+    finally:
+        unsubscribe2()
+
+    assert got == expected