javaclojureundertow

Undertow seems to send corrupt messages when enabled with permessage-deflate

I am using undertow to run a websocket server that sends cursor positions. I am running undertow 2.3.5.Final, on openjdk 11.0.22 2024-01-16.

Problem: If I open two tabs, eventually I get a corrupt message.

Here's what a successful message looks like:

{"op":"refresh-presence","room-id":"123","data":{"5c494daf-3c68-4ef6-b649-69177cc10105":{"peer-id":"5c494daf-3c68-4ef6-b649-69177cc10105","user":null,"data":{}},"b4eb6d8a-562a-4b8c-8e44-ecd43cba3621":{"peer-id":"b4eb6d8a-562a-4b8c-8e44-ecd43cba3621","user":null,"data":{}},"88f03150-696d-4e09-afbd-1381a2fa2f7e":{"peer-id":"88f03150-696d-4e09-afbd-1381a2fa2f7e","user":null,"data":{"cursors-space-default--main-123":{"x":723,"y":345,"xPercent":56.484375,"yPercent":52.43161094224924}}}}}

And here is a failure:

ors-space-default--main-123":{"x":562,"y":12,"xPercent":43.9ab521,"r56nt":1b3273a68bf-fefe7d735"}

^ This looks like the begging part is "cut off". xPercent also looks weird: 43.9ab521, it's like some of the uuids have contaminated the values.

And another example:

This is a successful message:

{"op":"set-presence-ok","room-id":"123","client-event-id":"9093a7cc-96ea-42e5-943e-e7c7962bbfcc"}

And this is a failure

{"op":"set-presence-ok","room-id":"123","client-event-id":"6186021","u44-45ser"265-211b8cu44315"}

It looks like the uuid here got cut up

Setup

Here's how I set up the websocket connection:

(defn ws-request [^HttpServerExchange exchange ^IPersistentMap headers ^WebSocketConnectionCallback callback]
  (let [handler (->  (WebSocketProtocolHandshakeHandler. callback)
                     (.addExtension (PerMessageDeflateHandshake.
                                     true ; client? 
                                     6    ; deflaterLevel: is a number from 0 (no compression) to 9 (maximum compression)
                                     )))]
    (when headers
      (set-headers (.getResponseHeaders exchange) headers))
    (.handleRequest handler exchange)))

And here's how I send messages:

(defn send-json!
  "Serializes `obj` to json, and sends over a websocket."
  [obj ws-conn]
  (let [obj-json (->json obj)
        p (promise)
        _ (WebSockets/sendText
           ^String obj-json
           ^WebSocketChannel ws-conn
           (proxy [WebSocketCallback] []
             (complete [ws-conn context]
               (deliver p nil))
             (onError [ws-conn context throwable]
               (deliver p throwable))))
        ret @p]
    (when (instance? Throwable ret)
      (throw ret))))

Potential culprit: PerMessageDeflateHandshake

If I remove the PerMessageDeflateHandshake extension, I can no longer repro the corrupt messages.

I am not sure what the best way to approach this problem is.

Solution

  • The problem was that Websockets.sendText was used in a multi-threaded environment.

    According to this post, WebsocketChannel should be thread-safe. My guess is that when combined with PermessageDeflate, it is no longer thread-safe.

    Adding a lock around sendText fixed this for me.