Fix unmount of nested mounts in PHP runtime hotswap

[brandonpayton]

Motivation for the change, related issues

Today, if we nest a NODEFS mount inside another NODEFS mount, hot-swapping the PHP runtime fails when we attempt to unmount a nested mount from a parent that has already been unmounted. This PR fixes that issue.

cc @adamziel who mentioned an issue that looked like this one.

Implementation details

This PR changes PHP runtime hot-swapping to unmount mounts in reverse order, and the attempted unmount no longer fails.

Testing Instructions (or ideally a Blueprint)

• CI

[brandonpayton]

I'd like to add a test for this before we merge. It would be good to catch other issues with hot-swapping runtimes.

I thought we already had such tests but haven't been able to find any.

[brandonpayton]

ah, we do have tests:
https://github.com/WordPress/wordpress-playground/blob/03ae8ec97a54131d82a1acaf7b91fddb499c89ca/packages/php-wasm/node/src/test/rotate-php-runtime.spec.ts

[brandonpayton]

I'll add a test for nested NODEFS mounts and confirm it detects this issue prior to the fix.

[brandonpayton]

OK, I've added a runtime rotation test with nested mounts. When I locally revert this fix, the test begins to fail due to the bug. This should be good to go. Let's confirm the tests pass.

[adamziel]

Thank you! I also wonder if there'a a meta problem in that PHP even crashes in the first place on a failed require call. I wonder what's happening in native php-fpm for example. But, regardless, we this fix in place we can live with either behavior just fine. Thank you!

[brandonpayton]

I also wonder if there'a a meta problem in that PHP even crashes in the first place on a failed require call. I wonder what's happening in native php-fpm for example.

That's a great point! I'll make a note to poke around a bit, even just for better understanding on our part.