fs: add ChunkWriterDoesntSeek feature flag and set it for b2

backend/b2/b2.go

@@ -505,10 +505,11 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 	}
 	f.setRoot(root)
 	f.features = (&fs.Features{
-		ReadMimeType:      true,
-		WriteMimeType:     true,
-		BucketBased:       true,
-		BucketBasedRootOK: true,
+		ReadMimeType:          true,
+		WriteMimeType:         true,
+		BucketBased:           true,
+		BucketBasedRootOK:     true,
+		ChunkWriterDoesntSeek: true,
 	}).Fill(ctx, f)
 	// Set the test flag if required
 	if opt.TestMode != "" {

fs/features.go

@@ -33,6 +33,7 @@ type Features struct {
 	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
+	ChunkWriterDoesntSeek   bool // set if the chunk writer doesn't need to read the data more than once
 
 	// Purge all files in the directory specified
 	//