Detecting whether a client vanished

[Profpatsch]

Is there a way to detect whether a client has vanished during a request?

setOnClose will only fire at the very end as far as I can see.

The following code tries to use responseStream, and by flushing before every delay I hoped it would realize the client is gone:

bar :: IO ()
bar = Warp.runSettings
  ( Warp.defaultSettings
      & Warp.setPort 1234
      & Warp.setOnClose (\sockAddr -> print "connection vanished")
  )
  $ \req respond -> do
    respond $ Wai.responseStream ok200 [] $ \write flush -> do
      print "sleeping"
      traverse_
        ( \_ -> do
            flush
            print "loop"
            threadDelay 1_000_000
        )
        [1 .. 10]
      write "done"
      print "done"

When I run this and start a curl http://localhost:1234 which I Ctrl+c after ~1-2 seconds, it will print:

"sleeping"
"loop"
"loop"
"loop"
"loop"
"loop"
"loop"
"loop"
"loop"
"loop"
"loop"
"done"
"connection vanished"

regardless of my client going away.

There is an old-ish SO question that creates a workaround by trying to read from the client socket in regular intervals.

[Vlix]

This unfortunately is a long-standing issue (cf. #196 & #914), but there's plans to add functionality to GHC to get access to that information (cf. #937)

[Profpatsch]

I came to the conclusion that for our use-case, it would be best to add functionality like this to the application protocol, because layer 2/3 connection information really depends on the setup; e.g. when adding a proxy it might break the connection detection again.

[Vlix]

Could you elaborate on that?