vfs: add vfs.mounted()

[pipobscure]

There was no way to find out which virtual file systems are mounted. That matters most for mounts a program did not make itself: those created by --vfs-mount and --vfs-load at startup, and the assets of a single executable application.

Add vfs.mounted(), which returns the VirtualFileSystem instances currently mounted in the calling thread, in the order they were mounted. A new array is returned on each call, so mutating it does not affect the mounts.

[bakkot]

(Moving the conversation from the other thread)

I really don't like that these aren't named. There's plenty of ways to handle duplicate names, the most obvious being to simply abort, or to define some precedence order (last-wins, except that NODE_OPTIONS always takes precedent over CLI, say). Naming programmatically created volumes is also easy; you just programmatically create names.

But iterating over every mount poking at the contents to see if they're what you want is not very appealing. Nor is having to maintain a mapping between the order in which the --vfs-mount arguments were passed and the actual place you need to look, especially given that adding a new one in NODE_OPTIONS bumps all the CLI ones.

[pipobscure]

What would you think about this:

In the VFS Provider creation you have the optional specifying of a name that's attached to the provider.
Then the CLI syntax would benode --vfs-mount [name=]<target> as in the name is again optional.
require('node:vfs').mounted() would still return VirtualFilesystem objects, but since that gives you access to the provider they could also have a vfs.name property that gets the name out.

This would be fairly easy to retrofit and still maintain all the characteristics I care about as well.

If that's something that you'd be on board with, I'd be happy to add that as a new PR. (This one wouldn't really be affected)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (dd5dfb5) to head (588a7fd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66116   +/-   ##
=======================================
  Coverage   90.27%   90.27%           
=======================================
  Files         790      790           
  Lines      271651   271667   +16     
  Branches    51842    51849    +7     
=======================================
+ Hits       245228   245243   +15     
+ Misses      16928    16925    -3     
- Partials     9495     9499    +4     

Files with missing lines	Coverage Δ
lib/internal/vfs/setup.js	87.13% <100.00%> (+0.16%)	⬆️
lib/vfs.js	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.