panic: interface conversion: interface is nil, not goja.Value

[sniku]

xk6-browser panics with the attached script.
version: latest main eb13672

The panic is reproducible on my machine. It happened 4 times in the row.

➜  Downloads xk6-browser3 run reproducible-websocket-EOF-errors.js

          /\      |‾‾| /‾‾/   /‾‾/   
     /\  /  \     |  |/  /   /  /    
    /  \/    \    |     (   /   ‾‾\  
   /          \   |  |\  \ |  (‾)  | 
  / __________ \  |__| \__\ \_____/ .io

  execution: local
     script: reproducible-websocket-EOF-errors.js
     output: -

  scenarios: (100.00%) 1 scenario, 10 max VUs, 10m30s max duration (incl. graceful stop):
           * default: 10 looping VUs for 10m0s (gracefulStop: 30s)

[...]
ERRO[0030] err:websocket: close 1006 (abnormal closure): unexpected EOF  category="Connection:handleIOError" elapsed="0 ms" goroutine=209
ERRO[0030] cmd.Wait: signal: killed                      category="BrowserType:execute" elapsed="2 ms" goroutine=23
ERRO[0030] stdout.Close: close |0: file already closed   category="BrowserType:execute" elapsed="0 ms" goroutine=23
panic: interface conversion: interface is nil, not goja.Value
running (00m30.0s), 10/10 VUs, 22 complete and 0 interrupted iterations
goroutine 4320 [running]:------------------------] 10 VUs  00m30.0s/10m0s
github.com/grafana/xk6-browser/common.(*ElementHandle).waitForElementState(0xc000deb060?, {0x1687188, 0xc00372d500}, {0xc003b31530, 0x3, 0x3}, 0x6fc23ac00)
	github.com/grafana/[email protected]/common/element_handle.go:660 +0x145
github.com/grafana/xk6-browser/common.(*ElementHandle).newPointerAction.func1({0x1687188, 0xc00372d500}, 0xc000deb060)
	github.com/grafana/[email protected]/common/element_handle.go:1371 +0x105
github.com/grafana/xk6-browser/common.retryPointerAction({0x1687188, 0xc00372d500}, 0xc00022e460, 0xc00372d4a0)
	github.com/grafana/[email protected]/common/element_handle.go:1460 +0x165
github.com/grafana/xk6-browser/common.(*ElementHandle).newPointerAction.func2({0x1687188?, 0xc00372d500?}, 0x0?, 0xc001271fb0?)
	github.com/grafana/[email protected]/common/element_handle.go:1437 +0x4b
created by github.com/grafana/xk6-browser/common.callApiWithTimeout
	github.com/grafana/[email protected]/common/helpers.go:121 +0x17b
➜  Downloads 

import { check } from 'k6';
import launcher from 'k6/x/browser';

export let options = {
  vus: 10,
  duration: '1m'

} 

export default function () {
  const browser = launcher.launch('chromium', {
    headless: false,
  });
  const context = browser.newContext();
  const page = context.newPage();

  // Goto front page, find login link and click it
  page.goto('https://test.k6.io/', { waitUntil: 'networkidle' });
  const elem = page.$('a[href="/my_messages.php"]');
  elem.click();
  page.waitForLoadState();

  // Enter login credentials and login
  page.$('input[name="login"]').type('admin');
  page.$('input[name="password"]').type('123');
  page.$('input[type="submit"]').click();

  // We expect the above form submission to trigger a navigation, so wait for it
  // and the page to be loaded.
  page.waitForNavigation();

  check(page, {
    'header': page.$('h2').textContent() == 'Welcome, admin!',
  });

  page.close();
  browser.close();
}

panic-2022-06-27_16.32.27.mp4

[inancgumus]

A temporary fix seems easy, but the error points out a race condition issue.

xk6-browser/common/element_handle.go

Line 660 in eb13672

value := result.(goja.Value)

-  value := result.(goja.Value) 
+  value, ok := result.(goja.Value)
+  if !ok {
+    ...
+  }

[imiric]

Yeah, definitely caused by a race condition, but we shouldn't crash because of it, so always checking type assertions is the safe thing to do.

@inancgumus If you're planning to work on this, could you also do that change in all other places we assume a goja.Value? A helper function is probably a good idea.

[inancgumus]

I noticed in #425 that we mostly are going to crash when the type assertion fails 😒 At least, we'll say: hey, there is a type-mismatch. #142's time is approaching.

[inancgumus]

@sniku, before the fix, we were seeing the following errors:

...
panic: interface conversion: interface is nil, not goja.Value
...

After the fix, I rerun the script you shared, and we no longer get any of these errors. However, we still get the usual race condition errors as follows:

ERRO[0047] cannot query element for selector ("input[name=\"login\"]"): element handle cannot evaluate: cannot call function on expression ("\n\t\t(node, injected, selector) => {\n\t\t\treturn injected.querySelector(selector, node || document, false);\n\t\t}\n\t\n//# sourceURL=__xk6_browser_evaluation_script__\n") in execution context (3) in frame (8741591778908D9C86DDDB08002A6325) with session (1182CE87F40D061C621471FF52104A2D): Cannot find context with specified id (-32000)
        at reflect.methodValueCall (native)
        at file:///Users/inanc/grafana/xk6-browser/examples/pawel.js:24:2(49)
        at native  executor=constant-vus scenario=default source=stacktrace

The error above is not about this issue; we'll address race condition issues in #428 and #427.