vfs: drop --vfs-mount, keep --vfs-load alone

[pipobscure]

vfs: drop --vfs-mount, pin the load mount point
--vfs-mount mounted a source without running it, and shared one ordered
list of sources with --vfs-load, so neither option could say which entry
it had contributed: the entry point was recovered from the position of
--vfs-load among the mounts, in a list NODE_OPTIONS could prepend to.
Nothing needs more than one mount from the command line: a program that
wants more can mount them itself through node:vfs, where it also gets
the instance.

Remove --vfs-mount, leaving --vfs-load with the single source it mounts
and runs, and reserve layer 0 for that source, numbering the file
systems a program mounts itself from 1. The source is then at the same
mount point in every thread, whatever else that thread mounts -
including a thread where a --require preload mounted a file system of
its own first - so a path into it stays valid in a worker.

A worker still does not run that entry point: it inherits the source but
not the decision to load from it. A worker created with its own execArgv
inherits neither, so the documentation now says that such a worker must
be given --experimental-vfs and --vfs-load again to run a script from
the mount, and that --experimental-vfs is also what makes node:vfs
available to the worker's own code.

ERR_VFS_INVALID_TARGET now names --vfs-load as the source's origin, and
the startup test moves to test-vfs-load.js, with the cases that covered
mounting without loading removed and cases for the reserved mount point
added.

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/startup

[pipobscure]

This is the alternate to #66119 so we can either have names for mounts, in which case --vfs-mount makes sense, or we go with this PR, which drops the idea of --vfs-mount entirely and just allows for --vfs-load.

@mcollina @jasnell @bakkot since you have been the most actively engaged on the topic and much more in tune with the philosophy of what should go into node-core, I'm asking for your guidance. I'm fine with either.

[bakkot]

Great, thanks, I think this ends up being a lot simpler/clearer.

[pipobscure]

I'm refactoring the reservation of layerId=0 into this branch and out of #66140. This way the two PRs are entirely independent of each other.

[pipobscure]

I'm refactoring the reservation of layerId=0 into this branch and out of #66140. This way the two PRs are entirely independent of each other. Plus I rebased on main and squashed.

[pipobscure]

Note: this refactor makes #66162 and #66140 are now entirel independent of each other

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77717/

[codecov]

Codecov Report

❌ Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (36dd044) to head (e86cbc1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/process/pre_execution.js	93.02%	3 Missing ⚠️
src/node.cc	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66162      +/-   ##
==========================================
- Coverage   90.29%   90.27%   -0.02%     
==========================================
  Files         790      790              
  Lines      272529   272495      -34     
  Branches    52031    52018      -13     
==========================================
- Hits       246083   246006      -77     
- Misses      16909    16954      +45     
+ Partials     9537     9535       -2     

Files with missing lines	Coverage Δ
lib/internal/errors.js	98.81% <100.00%> (ø)
lib/internal/main/worker_thread.js	95.02% <100.00%> (ø)
lib/internal/vfs/file_system.js	99.62% <100.00%> (+<0.01%)	⬆️
lib/internal/vfs/provider_registry.js	93.68% <100.00%> (ø)
src/node_options.cc	80.68% <ø> (-0.27%)	⬇️
src/node_options.h	95.63% <ø> (ø)
src/node_worker.cc	81.77% <ø> (ø)
src/node.cc	77.36% <0.00%> (+0.22%)	⬆️
lib/internal/process/pre_execution.js	96.67% <93.02%> (+0.91%)	⬆️

... and 30 files with indirect coverage changes

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[pipobscure]

I’m seeing the same three jobs fail on #66127, #66140 and #66162 and given I ran test on Linux, Mac and Windows, I’m assuming it’s not related to these PRs. Could someone have a look.

[pipobscure]

I have rebased on latest main in order to include:

• Ref: 20b3a69
• Ref: 3f76564
• Ref: 194eb69

Which were merged last night and hopefully deflake tests.

If someone can verify them and retrigger CI, I'd appreciate it.

[pipobscure]

Rebased on latest main as there was a conflict that wanted resolving.