Safari websocket compression is broken

[mohamedmansour]

Using go v1.17
Using gorilla websocket v1.4.2

iOS Safari 15 enabled by default for all users "NSURLSession WebSocket", by enabling this, it broke my application at (https://watchtheburn.com) if you have a Mac, you can inspect and see it not working (on an iOS device).

Going into Experimental WebKit Features and disabling "NSURLSession WebSocket" it works again.

@ejain opened this up and closed it for #696 Any ideas how I can resolve it?

[ghost]

Is the problem that iOS 15 does not support compressed fragmented messages?

If that is the problem, there are ways for the application to work around the bug in iOS.

[mohamedmansour]

@mamjsn is there a way to test it?

[ghost]

There are a couple of options.

Disable compression

Delete this line.

Prevent fragmentation

EDIT: The code below does not prevent fragmentation when compression is used.

Replace this section of code with:

 w, err := c.conn.WriteMessage(websocket.TextMessage, message)
 if err != nil {
     return
 }

WriteMessage does not fragment message. This change removes the optimization to send multiple application messages in a single websocket message. You should check to see if that optimization actually kicks in.

Here's how to retain the optimization at the cost of allocating memory:

  m := message[0:len(message):len(message)] // force new allocation on append
  n := len(c.send)
  for i := 0; i < n; i++ {
      m = append(m, '\n')
      m = append(m, <-c.send)
 }
 w, err := c.conn.WriteMessage(websocket.TextMessage, m)
 if err != nil {
       return
 }

[mohamedmansour]

@mamjsn how did you figure this out! thank you for taking your time and looking at the source code of my app!

I verified that removing compression works on iOS 15.
mohamedmansour/ethereum-burn-stats@d282823

I tried re-enabling compression, but it doesn't work, so disabling it instead. Unless I am doing it wrong.
mohamedmansour/ethereum-burn-stats@ceb5e2e

Thank you!

[nicks]

we're hitting this too in MacOS Monterey 😭 tilt-dev/tilt#4746

thanks @mamjsn for the workarounds above.

how hard would it be to have gorilla/websocket detect this scenario and workaround it? Like maybe we could check for Sec-WebSocket-Extensions: permessage-deflate and User-Agent: Safari? would be happy to try to put together a PR if no one is opposed to browser-specific workarounds.

[ghost]

@mohamedmansour: Based on Safari's past use of x-webkit-deflate-frame extension, I am guessing that Safari compresses frames instead of messages. The permessage-deflate extension compresses entire messages.

Thank you for trying the code to prevent message fragmentation. It turns out that my suggestion does not prevent fragmentation when compression is used. I apologize for the wasted effort there.

There's not easy path to disabling fragmentation: the code changes are not trivial, and the package must allocate memory to hold the entire compressed message in memory.

Here's another possible workaround for applications: Compressed messages do not fragment when the compressed message size plus websocket overhead is smaller than the write buffer size. Set the write buffer size to a value larger than any expected message. Use buffer pools to mitigate memory allocation costs.

[garyburd]

Some versions of Safari do not support compressed fragmented messages. Work around the Safari bug by disabling compression.

Other discussions on the topic:

• iOS 15 WKWebView websocket behaviour,
• Safari (macOS 12, iOS 15) and compression is broken uNetworking/uWebSockets#1347

[garyburd]

Applications should disable compression when working with buggy Safari clients.

There are reports that the problem is fixed in recent versions of Safari.