Puppeteer - Protocol error (Page.navigate): Target

As you can see with the sample code below, I'm using Puppeteer with a cluster of workers in Node to run multiple requests of websites screenshots by a given URL:

const cluster = require('cluster');
const express = require('express');
const bodyParser = require('body-parser');
const puppeteer = require('puppeteer');

async function getScreenshot(domain) {
    let screenshot;
    const browser = await puppeteer.launch({ args: ['--no-sandbox', '--disable-setuid-sandbox', '--disable-dev-shm-usage'] });
    const page = await browser.newPage();

    try {
        await page.goto('http://' + domain + '/', { timeout: 60000, waitUntil: 'networkidle2' });
    } catch (error) {
        try {
            await page.goto('http://' + domain + '/', { timeout: 120000, waitUntil: 'networkidle2' });
            screenshot = await page.screenshot({ type: 'png', encoding: 'base64' });
        } catch (error) {
            console.error('Connecting to: ' + domain + ' failed due to: ' + error);
        }

    await page.close();
    await browser.close();

    return screenshot;
}

if (cluster.isMaster) {
    const numOfWorkers = require('os').cpus().length;
    for (let worker = 0; worker < numOfWorkers; worker++) {
        cluster.fork();
    }

    cluster.on('exit', function (worker, code, signal) {
        console.debug('Worker ' + worker.process.pid + ' died with code: ' + code + ', and signal: ' + signal);
        Cluster.fork();
    });

    cluster.on('message', function (handler, msg) {
        console.debug('Worker: ' + handler.process.pid + ' has finished working on ' + msg.domain + '. Exiting...');
        if (Cluster.workers[handler.id]) {
            Cluster.workers[handler.id].kill('SIGTERM');
        }
    });
} else {
    const app = express();
    app.use(bodyParser.json());
    app.listen(80, function() {
        console.debug('Worker ' + process.pid + ' is listening to incoming messages');
    });

    app.post('/screenshot', (req, res) => {
        const domain = req.body.domain;

        getScreenshot(domain)
            .then((screenshot) =>
                try {
                    process.send({ domain: domain });
                } catch (error) {
                    console.error('Error while exiting worker ' + process.pid + ' due to: ' + error);
                }

                res.status(200).json({ screenshot: screenshot });
            })
            .catch((error) => {
                try {
                    process.send({ domain: domain });
                } catch (error) {
                    console.error('Error while exiting worker ' + process.pid + ' due to: ' + error);
                }

                res.status(500).json({ error: error });
            });
    });
}

Some explanation:

Each time a request arrives a worker will process it and kill itself at the end
Each worker creates a new browser instance with a single page, and if a page took more than 60sec to load, it will retry reloading it (in the same page because maybe some resources has already been loaded) with timeout of 120sec
Once finished both the page and the browser will be closed

My problem is that some legitimate domains get errors that I can't explain:

Error: Protocol error (Page.navigate): Target closed.

Error: Protocol error (Runtime.callFunctionOn): Session closed. Most likely the page has been closed.

I read at some git issue (that I can't find now) that it can happen when the page redirects and adds 'www' at the start, but I'm hoping it's false... Is there something I'm missing?

What "Target closed" means

When you launch a browser via puppeteer.launch it will start a browser and connect to it. From there on any function you execute on your opened browser (like page.goto) will be send via the Chrome DevTools Protocol to the browser. A target means a tab in this context.

The Target closed exception is thrown when you are trying to run a function, but the target (tab) was already closed.

Similar error messages

The error message was recently changed to give more meaningful information. It now gives the following message:

Error: Protocol error (Target.activateTarget): Session closed. Most likely the page has been closed.

Why does it happen

There are multiple reasons why this could happen.

You used a resource that was already closed

Most likely, you are seeing this message because you closed the tab/browser and are still trying to use the resource. To give an simple example:
```
const browser = await puppeteer.launch();
const page = await browser.newPage();

await browser.close();
await page.goto('http://www.google.com');
```
In this case the browser was closed and after that, a page.goto was called resulting in the error message. Most of the time, it will not be that obvious. Maybe an error handler already closed the page during a cleanup task, while your script is still crawling.
The browser crashed or was unable to initialize

I also experience this every few hundred requests. There is an issue about this on the puppeteer repository as well. It seems to be the case, when you are using a lot of memory or CPU power. Maybe you are spawning a lot of browser? In these cases the browser might crash or disconnect.

I found no "silver bullet" solution to this problem. But you might want to check out the library puppeteer-cluster (disclaimer: I'm the author) which handles these kind of error cases and let's you retry the URL when the error happens. It can also manage a pool of browser instances and would also simplify your code.