vfs: add RC endpoint to insert virtual entries into the dir cache #9310

The associated forum post URL https://forum.rclone.org/t/add-an-entry-to-vfs-via-rc-api/51242

What is your current rclone version (output from rclone version)?

Latest stable (v1.69.1)

What problem are you trying to solve?

vfs/refresh replaces the cached directory listing with whatever the backend returns. On S3-compatible backends (Garage in our case) this causes two issues:

1. Race condition - if called shortly after an upload, the backend may not have committed the object yet, so the new entry is missing from the refreshed listing.

2. Temporary disappearance of existing entries - on large directories, an incomplete or eventually-consistent listing response replaces the cache, causing entries to vanish until the next refresh.
   Applications watching the mount interpret this as deletions.

There is currently no way to add a single entry to the dir cache without re-listing the entire directory.

How do you think rclone should be changed to solve that?

rclone already has Dir.AddVirtual(leaf, size, isDir) internally (used by the VFS write-back cache) which inserts entries without backend re-listing, survives subsequent vfs/refresh calls, and auto-promotes
to real entries once the backend confirms them.

I propose exposing this via two new RC endpoints:

rclone rc vfs/adddir dir=data/projects/new-project
rclone rc vfs/addfile file=data/projects/report.pdf size=4521984

Use case: multiple machines mounting the same S3 backend via rclone. Machine A uploads content and notifies machine B to update its cache. vfs/refresh on large directories risks losing existing entries,
while vfs/adddir / vfs/addfile would surgically insert just the new entry.

I also noticed a bug while reading the code: VFS.AddVirtual() in vfs/vfs.go:848 hardcodes false for isDir:

dir.AddVirtual(leaf, size, false) // should be: dir.AddVirtual(leaf, size, isDir)

Happy to submit a PR for both.