Infinite recursion when 3rd party libs are instrumenting XHR's

[brian-mann]

If a 3rd party lib such as new relic's js instrumentation library wraps XHR's the same way Cypress does, it's possible for there to be an infinite loop while we both call the backed up XHR methods.

I've paired down the root cause to this snippet of code which can cause it.

var handlers = ["onload", "onerror", "onreadystatechange"]

var wrapXhr = function(){
  var xhr = this

  handlers.forEach(function(handler){
    var bak = xhr[handler]

    xhr[handler] = function(){
      if (typeof bak === "function"){
        bak.apply(xhr, arguments)
      }
    }
  })
}

var xhr = new XMLHttpRequest
xhr.addEventListener("readystatechange", wrapXhr, false)
xhr.open("GET", "/comments.json")
xhr.onload = function(){
  // this should continue to hit
  debugger
}
xhr.send()

[brian-mann]

Fixed in 0.16.3.

[flotwig]

Having a similar issue in 3.3.1, when a user has the New Relic insturmentation code directly embedded in their page, New Relic will replace win.XMLHttpRequest with their own copy, which destroys the functions that we add on to the XMLHttpRequest object.

Doing something like this to rebind the XMLHttpRequest makes the tests pass roughly 50% of the time:

function stubXHR($win) {
  Cypress
  .state('server')
  .bindTo($win)
}

it('loads the page', () => {
  cy.server();
  cy.visit('https://' + server, {
    onBeforeLoad: stubXHR
  });
});