mount: register UNC mount points as WinFsp network file systems (#10943)

A \\server\share -dir was passed to WinFsp as a plain mount point, which
treats it as a directory path on an actual remote server and fails. Turn it
into the VolumePrefix option instead, so the mount registers with the WinFsp
network provider: the UNC path is then reachable from every logon session,
which a drive letter mounted from a service is not, and each user can map
their own drive letter to it.

weed/command/mount.go

@@ -192,6 +192,13 @@ var cmdMount = &Command{
 
   On OS X, it requires OSXFUSE (https://osxfuse.github.io/).
 
+  On Windows, it requires WinFsp (https://winfsp.dev/). The mount point can be
+  a drive letter (-dir=S:), a directory that does not exist yet, or a network
+  path (-dir=\\seaweedfs\share). A drive letter belongs to the logon session
+  that created it, so one mounted by a service is usually invisible to users
+  at the desktop; the network path form is reachable from every session, and
+  each user can map their own drive letter to it.
+
   RDMA Acceleration:
   For ultra-fast reads, enable RDMA acceleration with an RDMA sidecar:
     weed mount -filer=localhost:8888 -dir=/mnt/seaweedfs \

weed/command/mount_windows.go

@@ -136,6 +136,9 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 	glog.V(0).Infof("mounting %s%s to %v", *option.filer, mountRoot, dir)
 	glog.V(0).Infof("This is SeaweedFS version %s %s %s", version.Version(), runtime.GOOS, runtime.GOARCH)
 	glog.V(0).Infof("Windows mount is beta: hard links are unavailable and byte-range locks are not shared across mounts")
+	if prefix, _ := winfsp.VolumePrefix(dir); prefix != "" {
+		glog.V(0).Infof(`network mount: every session on this machine can open \%s or map a drive letter to it`, prefix)
+	}
 
 	if err := host.Serve(windowsMountPoint(dir)); err != nil {
 		glog.Errorf("%v", err)
@@ -157,6 +160,9 @@ func checkWindowsMountPoint(dir string) error {
 		}
 		return nil
 	}
+	if prefix, err := winfsp.VolumePrefix(dir); err != nil || prefix != "" {
+		return err
+	}
 	if strings.HasPrefix(dir, `\\`) {
 		return nil
 	}