page.screenshot hangs if an alert() on page ready

I’ve run into an issue with page.screenshot() that occurs whenever an alert() dialog on the page happens after page ready. The page.screenshot() hangs forever and if you end-task the node process, you get

 UnhandledPromiseRejectionWarning: Error: Protocol error (Page.captureScreenshot): Target closed.
    at Promise (/var/www/html/puppeteer-bug/node_modules/puppeteer/lib/Connection.js:200:56)
    at new Promise (<anonymous>)
    at CDPSession.send (/var/www/html/puppeteer-bug/node_modules/puppeteer/lib/Connection.js:199:12)
    at Page._screenshotTask (/var/www/html/puppeteer-bug/node_modules/puppeteer/lib/Page.js:738:39)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

This is running puppeteer 1.3.0 under Ubuntu with Node 5.6.0 and the Chromium version that ships with Puppeteer. Also replicated using full Chrome 65.

Basic Node app:

'use strict';
const puppeteer = require('puppeteer');

async function go(){
    const browser = await puppeteer.launch();
    console.log('opened browser');
    const page = await browser.newPage();
    console.log('created page');
    await page.goto('http://testing/puppeteer/', { timeout: 30000 });
    console.log('loaded page');
    await page.screenshot({
        path: './test.jpg'
    });
    console.log('took screenshot');
    await browser.close();
    console.log('closed browser');
}

go();

The html it’s loading is:

<div style="position: absolute; background-color: #99dd99; top: 10px; left: 40px; width: 300px; height: 250px; padding: 5px 15px; border: 1px solid #444;">Stuff</div>
<script src="https://code.jquery.com/jquery-3.3.1.min.js" integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8=" crossorigin="anonymous"></script>
<script>
	$(document).ready(function(){
		alert('test');
	});
</script>

If you comment out the alert, everything works. With the alert in place, you get a console log for ‘loaded page’ and then everything hangs indefinitely.

A workaround is to use Promise.race:

    await Promise.race([
        page.screenshot({
            path: '/test.jpg'
        }),
        new Promise((resolve, reject) => setTimeout(function(){ reject('Screenshot timeout');}, 3000))
    ]);

The screenshot still doesn’t complete, but at least it doesn’t hang the app.

[hoperyy]

my solution:

headlessPage.on('dialog', async (dialog) => {
     // "screenshot / evaluate" will go on by closing dialog
     await dialog.dismiss()
 })

[accamper]

This bug just caused me to waste an entire day. For me, it was page.click and page.evaluate that was hanging for the same underlying reason. Puppeteer should at least reject immediately in all functions if there is an active dialog