cmount: use ReaddirPlus on Windows to improve directory listing performance

Before this change Windows would read a directory then immedately stat
every item in the directory.

After this change we return the stat information along with the
directory which stops so many callbacks.

cmd/cmount/fs.go

@@ -251,7 +251,14 @@ func (fsys *FS) Readdir(dirPath string,
 				fs.Errorf(dirPath, "Name too long (%d bytes) for FUSE, skipping: %s", len(name), name)
 				continue
 			}
-			fill(name, nil, 0)
+			if usingReaddirPlus {
+				// We have called host.SetCapReaddirPlus() so supply the stat information
+				var stat fuse.Stat_t
+				_ = fsys.stat(node, &stat) // not capable of returning an error
+				fill(name, &stat, 0)
+			} else {
+				fill(name, nil, 0)
+			}
 		}
 	}
 	itemsRead = len(items)

cmd/cmount/mount.go

@@ -26,6 +26,13 @@ import (
 	"github.com/rclone/rclone/vfs/vfsflags"
 )
 
+const (
+	// SetCapReaddirPlus informs the host that the hosted file system has the readdir-plus
+	// capability [Windows only]. A file system that has the readdir-plus capability can send
+	// full stat information during Readdir, thus avoiding extraneous Getattr calls.
+	usingReaddirPlus = runtime.GOOS == "windows"
+)
+
 func init() {
 	name := "cmount"
 	if runtime.GOOS == "windows" {
@@ -142,6 +149,9 @@ func mount(f fs.Fs, mountpoint string) (*vfs.VFS, <-chan error, func() error, er
 	// Create underlying FS
 	fsys := NewFS(f)
 	host := fuse.NewFileSystemHost(fsys)
+	if usingReaddirPlus {
+		host.SetCapReaddirPlus(true)
+	}
 
 	// Create options
 	options := mountOptions(f.Name()+":"+f.Root(), mountpoint)