fs: create Overlay feature flag to indicate backend wraps others

Set this automatically for any backend which implements UnWrap and
manually for combine and union which can't implement UnWrap but do
overlay other backends.

backend/combine/combine.go

@@ -299,6 +299,9 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (outFs fs
 		}
 	}
 
+	// show that we wrap other backends
+	features.Overlay = true
+
 	f.features = features
 
 	// Get common intersection of hashes

backend/union/union.go

@@ -914,6 +914,9 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		}
 	}
 
+	// show that we wrap other backends
+	features.Overlay = true
+
 	f.features = features
 
 	// Get common intersection of hashes

fs/features.go

@@ -32,6 +32,7 @@ type Features struct {
 	FilterAware             bool // can make use of filters if provided for listing
 	PartialUploads          bool // uploaded file can appear incomplete on the fs while it's being uploaded
 	NoMultiThreading        bool // set if can't have multiplethreads on one download open
+	Overlay                 bool // this wraps one or more backends to add functionality
 
 	// Purge all files in the directory specified
 	//
@@ -271,6 +272,7 @@ func (ft *Features) Fill(ctx context.Context, f Fs) *Features {
 	if do, ok := f.(Wrapper); ok {
 		ft.WrapFs = do.WrapFs
 		ft.SetWrapper = do.SetWrapper
+		ft.Overlay = true // if implement UnWrap then must be an Overlay
 	}
 	if do, ok := f.(DirCacheFlusher); ok {
 		ft.DirCacheFlush = do.DirCacheFlush
@@ -339,6 +341,9 @@ func (ft *Features) Mask(ctx context.Context, f Fs) *Features {
 	ft.SlowModTime = ft.SlowModTime && mask.SlowModTime
 	ft.SlowHash = ft.SlowHash && mask.SlowHash
 	ft.FilterAware = ft.FilterAware && mask.FilterAware
+	ft.PartialUploads = ft.PartialUploads && mask.PartialUploads
+	ft.NoMultiThreading = ft.NoMultiThreading && mask.NoMultiThreading
+	// ft.Overlay = ft.Overlay && mask.Overlay don't propagate Overlay
 
 	if mask.Purge == nil {
 		ft.Purge = nil

fstest/fstests/fstests.go

@@ -460,7 +460,7 @@ func Run(t *testing.T, opt *Opt) {
 			t.Skip("Skipping FsCheckWrap on this Fs")
 		}
 		ft := new(fs.Features).Fill(ctx, f)
-		if ft.UnWrap == nil {
+		if ft.UnWrap == nil && !f.Features().Overlay {
 			t.Skip("Not a wrapping Fs")
 		}
 		v := reflect.ValueOf(ft).Elem()