Exception in onWSClose causes uncaught exception

[wch]

Run this in one R session:

library(httpuv)
app <- startServer("0.0.0.0", 8000, list(
  onWSOpen = function(ws) {
    ws$onClose(function() {
      message("onClose start")
      Sys.sleep(10)
      message("onClose end")
    })
  }
))

And this in another:

library(websocket)
ws <- websocket::WebSocket$new("ws://localhost:8000/")
ws$onMessage(function(msg) cat(msg, "\n"))
ws$close()

In the first R session, press Ctrl-C within 10 seconds. It will print:

Error in execCallbacks(timeoutSecs) : c++ exception (unknown reason)
Calls: <Anonymous> ... serviceApp -> service -> run_now -> execCallbacks -> .Call

This is a minimal reproducible case of rstudio/shiny#1967 (comment)