fix(dashboard): stop server event loop on correct thread

When idle shutdown triggered _stop_server(), it was creating a new event
loop and calling server.stop() on it, but the daemon thread was still
running loop.run_forever() on the original event loop. This left sockets
bound, causing "address already in use" on restart.

Fix by storing references to the server's event loop and thread, then
using call_soon_threadsafe(loop.stop) to signal the correct loop to exit.
The thread join ensures sockets are released before the next server starts.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/py_dvt_ate/app/dashboard/app.py

@@ -35,6 +35,8 @@ IDLE_SHUTDOWN_SECONDS = int(os.environ.get("IDLE_SHUTDOWN_SECONDS", "300"))
 _last_activity_time: float = time.time()
 _idle_checker_started = False
 _server_ref: SimulationServer | None = None  # Reference for idle checker thread
+_server_loop: asyncio.AbstractEventLoop | None = None  # Event loop running the server
+_server_thread: threading.Thread | None = None  # Thread running the server event loop
 
 
 def _idle_checker() -> None:
@@ -54,17 +56,20 @@ def _idle_checker() -> None:
 
 def _stop_server() -> None:
     """Stop the server and clear caches for fresh restart."""
-    global _server_ref, _idle_checker_started
-    if _server_ref is not None:
-        # Stop the server
-        loop = asyncio.new_event_loop()
-        try:
-            loop.run_until_complete(_server_ref.stop())
-        except Exception:
-            pass
-        finally:
-            loop.close()
-        _server_ref = None
+    global _server_ref, _idle_checker_started, _server_loop, _server_thread
+
+    if _server_loop is not None and _server_thread is not None:
+        # Schedule stop on the correct event loop (the one actually running the server)
+        # This causes loop.run_forever() to exit in the daemon thread
+        _server_loop.call_soon_threadsafe(_server_loop.stop)
+
+        # Wait for thread to exit (with timeout to avoid hanging)
+        _server_thread.join(timeout=5.0)
+
+        _server_loop = None
+        _server_thread = None
+
+    _server_ref = None
 
     # Clear Streamlit's cached server so next visitor gets fresh instance
     get_or_create_server.clear()
@@ -136,6 +141,8 @@ def get_or_create_server() -> SimulationServer:
     Returns:
         The simulation server instance.
     """
+    global _server_loop, _server_thread
+
     server = SimulationServer(
         ServerConfig(
             host="127.0.0.1",
@@ -151,7 +158,9 @@ def get_or_create_server() -> SimulationServer:
 
     def run_server() -> None:
         """Run the async server in a new event loop."""
+        global _server_loop
         loop = asyncio.new_event_loop()
+        _server_loop = loop  # Store reference for _stop_server to use
         asyncio.set_event_loop(loop)
         try:
             loop.run_until_complete(server.start())
@@ -171,6 +180,7 @@ def get_or_create_server() -> SimulationServer:
 
     thread = threading.Thread(target=run_server, daemon=True)
     thread.start()
+    _server_thread = thread  # Store reference for _stop_server to use
 
     # Wait for server to be fully started (up to 5 seconds)
     if not server_ready.wait(timeout=5.0):