feat: supply system log to trusted plugins via FD

[Skn0tt]

System logs are super useful for our internal observability. Giving system logs to (trusted) build plugins isn't straight-forward since plugins are run inside a child process. This PR is an attempt at bridging this, by sharing the system log file descriptor with the child process. This means build plugins will be able to write to system logs like this:

import { appendFileSync } from  "fs"

export const onBuild = function () {
  // ...
  appendFileSync("/dev/fd/4", "next.js customer using ISC, SSR and XYZ")
  // ...
}

/dev/fd/4 is a bit of a magic name - but it works.

There's still some tests failing, but I wanted to share the approach sooner than later, and get your feedback.

[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[Skn0tt]

Looks like tests are failing on Windows. There's a fair chance that this FD magic doesn't work on Windows, i'll have to investigate that.

[eduardoboucas]

I think this capability is nice, but I'm not a big fan of exposing the raw file descriptor to plugins. This feels leaky, and if we ever want to change the mechanism we use for system logs (like the structured logs idea that @JGAntunes shared) we'll have a hard time with existing plugins.

Could we instead add a systemLog function to the properties we expose to build plugin hooks, which currently just wraps the logic for writing to the file description if available (and perhaps act as a no-op if not available), and further down the line we can swap the internals of that while maintaining the user-facing interface?

[Skn0tt]

AFAICT, all properties we send to plugins are serialized because of the IPC bridge. If we send over a function, that would break during serialisation. If we had some sort of "bootstrap" code within the plugin, then we could set up said function in there. Thinking about it, we probably have something like that! I'll check that.

[eduardoboucas]

We have things like failBuild() inside plugins, so we should be able to wire things up in a similar way?

[Skn0tt]

Totally, yeah! Implemented in bbcad09

[eduardoboucas]

Nice! 🚀

[eduardoboucas]

We should add this to the types.

[Skn0tt]

done in 6fa00d8

[Skn0tt]

Tests failed on windows, because /dev/fd/4 isn't really a thing there. Makes sense. I won't put effort into making that work because we don't run CI in Windows - updated the tests instead.