Before this change, if a user unmounted externally (for example, via the Finder
UI), rclone would not be aware of this and wait forever to exit -- effectively
causing a deadlock that would require Ctrl+C to terminate.
After this change, when the handler detects an external unmount, it calls a
function which allows rclone to cleanly shutdown the VFS and exit.
Before this change, writing files to an `nfsmount` via Finder on macOS would
cause critical errors, rendering `nfsmount` effectively unusable on macOS. This
change fixes the issue so that writes via Finder should be possible.
The issue was primarily caused by the handler's HandleLimit being set to -1. -1 is
the correct default for a NullAuthHandler, but not for a CachingHandler, which
interprets -1 not as "no limit" but as "no cache".
This change sets a high default of 1000000, and gives the user control over it
with a new --nfs-cache-handle-limit flag (available in both `serve nfs` and
`nfsmount`. A minimum of 5 is enforced, as any lower than this will be
insufficient to support directory listing.
Summary:
Adding a new command to serve any remote over NFS. This is only useful for new macOS versions where FUSE mounts are not available.
* Added willscot/go-nfs dependency and updated go.mod and go.sum
Test Plan:
```
go run rclone.go serve nfs --http-url https://beta.rclone.org :http:
```
Test that it is serving correctly by mounting the NFS directory.
```
mkdir nfs-test
mount -oport=58654,mountport=58654 localhost: nfs-test
```
Then we can list the mounted directory to see it is working.
```
ls nfs-test
```
