More details from Chrome 'Aw snap'

I’ve been experiencing this bug while using Chrome devtools and it’s gotten to the point where I wanted to dig deeper to find out what is causing it.

This is not about debugging Chrome itself, I don’t have the skills for that, but just trying to get some more details to figure out if it’s something I’ve done wrong in my code. Or if not, gather enough detail to report a useful bug.

The general idea of the bug is:

  1. launch a webpage with a service worker
  2. have the service worker trigger a POST
  3. (try to) inspect the body of the POST in the devtools network tab
  4. the tab will crash

It’s not all POST requests, I tried to make a minimal reproduction, but there’s something about the ones I make that causes this.

So the goal here is to see what it sent in that POST, just to confirm it’s what I thought, as I can’t use devtools to see that. For this we’ll use mitmproxy to intercept our network traffic.

  1. install mitmproxy
  2. run mitmproxy listening on port 8081 
    mitmproxy \
  --listen-port 8081 \
  --listen-host 0.0.0.0 \
  --ssl-insecure \
  --set block_global=false

Now we can start Chrome so it uses our proxy. I don’t want to mess with my main profile, plus it’s good to verify bugs on a clean profile, so let’s start Chrome with a throw-away profile and ask for extra logging output:

google-chrome-stable \
  --proxy-server=http://localhost:8081 \
  --user-data-dir=/tmp/chrome1 \
  --enable-logging \
  --v=1

Now we need Chrome to trust the HTTPS certificate that mitmproxy presents. We do that by:

  1. opening some HTTPS webpage (not google, that just seems to hang) and confirm that it fails with an invalid cert
  2. now open mitm.it, the proxy will intercept this and show you a page where you can download certs
  3. download the pem cert (by clicking other)
  4. open settings, search for “cert”, open the Manage Certificate item
  5. go to the Authorities tab
  6. click the import button
  7. select the pem cert you downloaded
  8. check “Trust this certificate for identifying websites”
  9. go back and refresh your failed page, it should work now because Chrome trusts the mitmproxy cert

Now, if you look in the running mitmproxy you will (should) see flows appearing as you browse with Chrome.

You’ll also see debug logs spewing from Chrome in your terminal where you launched it.

We now have everything in place to trigger the bug and then I can check in mitmproxy that my app is indeed sending what I expect. It was. It was a pretty boring JSON object too. I’ve reported the bug and I can’t get a minimal reproduction to work, so I’ll just have to wait and hope for a fix.

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