wrapping.md

👋 First off thanks for your product! Recently introduced it at my company and have had nothing but positive feedback so far.

I noticed an odd race condition from percy start --detached between spawning the agent and running the health check. This causes the command to error but still start the process in the background. It only occurs with the --detached flag.

I'm trying to start the agent from node script using execa.command. Unfortunately this error causes execa to bail which kills the agent.

I investigated a naive fix which seems to have worked around the problem though definitely not in a clean way.

current behavior

Note: The PERCY_TOKEN in the following examples was created to be purposefully "leaked" here

Here is the current behavior:

Notice how the health check fails and I can see that nothing is running on the port immediately after it finishes but a second or two later it shows up in lsof's output. If I use a fake PERCY_TOKEN (e.g. PERCY_TOKEN=foo yarn percy start --detached) I get the same health check error but the process never starts (since there is no valid project to create a build for).

To reproduce this behavior simply run the following in a new directory:

> yarn init
> yarn add @percy/script
> PERCY_TOKEN=<fill-in-your-token-here> yarn percy start --detached

naive fix

I tested a naive fix by inserting a simple 2 second sleep between the call to run Percy detached and the failing health check. Basically I inserted the following on L60 of that same file:

function sleep(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

await sleep(2000)

This avoids the race condition:

Note that when spawning via execa 2000ms isn't enough. Bumping up to 5000ms seemed to consistently succeed

I also tried awaiting the spawn call that I think is the culprit here but that didn't seem to have any effect.