serve nfs: fix writing files via Finder on macOS - fixes #7503

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.

cmd/serve/nfs/server.go

@@ -36,7 +36,7 @@ func NewServer(ctx context.Context, vfs *vfs.VFS, opt *Options) (s *Server, err
 		ctx: ctx,
 		opt: *opt,
 	}
-	s.handler = newHandler(vfs)
+	s.handler = newHandler(vfs, opt)
 	s.listener, err = net.Listen("tcp", s.opt.ListenAddr)
 	if err != nil {
 		fs.Errorf(nil, "NFS server failed to listen: %v\n", err)