Start recv_events only after attributes are initialized.

Else, a race condition could lead to accessing self.pong_waiters
before it is defined.

src/websockets/asyncio/connection.py

@@ -101,14 +101,6 @@ def __init__(
         # Protect sending fragmented messages.
         self.fragmented_send_waiter: asyncio.Future[None] | None = None
 
-        # Exception raised while reading from the connection, to be chained to
-        # ConnectionClosed in order to show why the TCP connection dropped.
-        self.recv_exc: BaseException | None = None
-
-        # Completed when the TCP connection is closed and the WebSocket
-        # connection state becomes CLOSED.
-        self.connection_lost_waiter: asyncio.Future[None] = self.loop.create_future()
-
         # Mapping of ping IDs to pong waiters, in chronological order.
         self.pong_waiters: dict[bytes, tuple[asyncio.Future[float], float]] = {}
 
@@ -128,6 +120,14 @@ def __init__(
         # Task that sends keepalive pings. None when ping_interval is None.
         self.keepalive_task: asyncio.Task[None] | None = None
 
+        # Exception raised while reading from the connection, to be chained to
+        # ConnectionClosed in order to show why the TCP connection dropped.
+        self.recv_exc: BaseException | None = None
+
+        # Completed when the TCP connection is closed and the WebSocket
+        # connection state becomes CLOSED.
+        self.connection_lost_waiter: asyncio.Future[None] = self.loop.create_future()
+
         # Adapted from asyncio.FlowControlMixin
         self.paused: bool = False
         self.drain_waiters: collections.deque[asyncio.Future[None]] = (

src/websockets/sync/connection.py

@@ -101,19 +101,6 @@ def __init__(
         # Whether we are busy sending a fragmented message.
         self.send_in_progress = False
 
-        # Exception raised in recv_events, to be chained to ConnectionClosed
-        # in the user thread in order to show why the TCP connection dropped.
-        self.recv_exc: BaseException | None = None
-
-        # Receiving events from the socket. This thread is marked as daemon to
-        # allow creating a connection in a non-daemon thread and using it in a
-        # daemon thread. This mustn't prevent the interpreter from exiting.
-        self.recv_events_thread = threading.Thread(
-            target=self.recv_events,
-            daemon=True,
-        )
-        self.recv_events_thread.start()
-
         # Mapping of ping IDs to pong waiters, in chronological order.
         self.pong_waiters: dict[bytes, tuple[threading.Event, float, bool]] = {}
 
@@ -133,6 +120,21 @@ def __init__(
         # Thread that sends keepalive pings. None when ping_interval is None.
         self.keepalive_thread: threading.Thread | None = None
 
+        # Exception raised in recv_events, to be chained to ConnectionClosed
+        # in the user thread in order to show why the TCP connection dropped.
+        self.recv_exc: BaseException | None = None
+
+        # Receiving events from the socket. This thread is marked as daemon to
+        # allow creating a connection in a non-daemon thread and using it in a
+        # daemon thread. This mustn't prevent the interpreter from exiting.
+        self.recv_events_thread = threading.Thread(
+            target=self.recv_events,
+            daemon=True,
+        )
+
+        # Start recv_events only after all attributes are initialized.
+        self.recv_events_thread.start()
+
     # Public attributes
 
     @property