Endless H15 errors on Heroku with default install

[jeffblake]

I've used Scout in the past and loved the quick actionable insights to make improvements (n+1's, etc). I installed the heroku addon and deployed, and received thousands of H15 Idle connection errors, caused by actioncable connections. As soon as I uninstalled the gem and redeployed... no more errors.

Heroku considers these errors, so if you have the built-in "response error notification" enabled in Heroku, you will get emails forever about these H15 errors, and you will see in this graph the errors and no more errors with deploy eb7010db when I removed the scout gem.

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[dlanderson]

Thanks for the report @jeffblake - I'm surprised that we would interfere with ActionCable connections, since we don't have any instrumentation there. Can you provide some info on which rails version you're running and which Scout version was used? Was there any indication that it was happening on just one endpoint, or multiple? Happy to take a look at your Gemfile.lock if you can send it over to support@scoutapm.com as well as any logs from Heroku that you can send over showing the error and related lines.

[jeffblake]

Rails 7.0.1
Scout 5.1.1

It's happening on one endpoint, the actioncable mount path, all of the logs look like this:

at=error code=H15 desc="Idle connection" method=GET path="/cable" host=app.*.com request_id=c1977422-d02f-4d5e-8b1c-06f9eecd7b84 fwd="x.x.x.x" dyno=web.1 connect=0ms service=55003ms status=503 bytes= protocol=https

I've tried restarting the dynos multiple times to no avail, the only fix is to remove the Scout gem.