Error thrown in xhr.onreadystatechange when javascript execution is suspended/resumed on iOS

[ilplotkin]

On iOS, javascript execution is suspended when user switches to another tab in browser, closes browser, performs back/forward using swipe gestions.
In our application, if http request is occurred when javascript execution is suspended/resumed, sometimes (apprx one time per dozen tries) the following error is thrown:

TypeError: 'null' is not an object (evaluating 'xhr.readyState')

  src/ng/httpBackend.js
  xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {  // Error is thrown in this line

Debugging showed that xhr.onreadystatechange was invoked several times with readyState=4. On the first invocation xhr was nullified in completeRequest function, and then, on subsequent invocations, error was thrown.

Unfortunately, I could not reproduce the problem in small example. But I found that jQuery and Zepto have appropriate guards. Please see, for example, madrobby/zepto#633

The following change is proposed:
ilplotkin@a79004c

[IgorMinar]

very odd corner case

I'm triaging the issue for the next release. thanks for all the info and suggested fix.

[kole]

I can confirm this bug using 1.1.5. Attaching screenshot from sentry for reference.

[tbosch]

I could reproduce this bug with:

• iOS Simulator
• Backend that takes a long time to resolve an xhr

Procedure:

1. Create a test page that does an xhr
2. Send the browser into the background by going to the iOS start screen
3. Wait until the request is resolved
4. Open up the browser again.

Note: The iOS remote debugger somehow didn't show the error. Had to add manual try/catch inside the onreadystatechange function to see it...

[tbosch]

Thanks for submitting this issue!

[davidjnelson]

Reported as still occuring: #6530