fs: Implement PartialUploads feature flag

Implement a Partialuploads feature flag to mark backends for which
uploads are not atomic.

This is set for the following backends

- local
- ftp
- sftp

See #3770

backend/ftp/ftp.go

@@ -580,6 +580,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (ff fs.Fs
 	}
 	f.features = (&fs.Features{
 		CanHaveEmptyDirectories: true,
+		PartialUploads:          true,
 	}).Fill(ctx, f)
 	// set the pool drainer timer going
 	if f.opt.IdleTimeout > 0 {

backend/local/local.go

@@ -303,6 +303,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		WriteMetadata:           true,
 		UserMetadata:            xattrSupported, // can only R/W general purpose metadata if xattrs are supported
 		FilterAware:             true,
+		PartialUploads:          true,
 	}).Fill(ctx, f)
 	if opt.FollowSymlinks {
 		f.lstat = os.Stat

backend/sftp/sftp.go

@@ -994,6 +994,7 @@ func NewFsWithConnection(ctx context.Context, f *Fs, name string, root string, m
 	f.features = (&fs.Features{
 		CanHaveEmptyDirectories: true,
 		SlowHash:                true,
+		PartialUploads:          true,
 	}).Fill(ctx, f)
 	// Make a connection and pool it to return errors early
 	c, err := f.getSftpConnection(ctx)

fs/features.go

@@ -30,6 +30,7 @@ type Features struct {
 	WriteMetadata           bool // can write metadata to objects
 	UserMetadata            bool // can read/write general purpose metadata
 	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
 
 	// Purge all files in the directory specified
 	//