Add context cancellation support to Scan

mfer/manifest.go

@@ -192,12 +192,19 @@ func (m *manifest) addFile(p string, fi fs.FileInfo, sfsIndex int) error {
 }
 
 func (m *manifest) Scan() error {
-	// FIXME scan and whatever function does the hashing should take ctx
 	for idx, sfs := range m.sourceFS {
 		if sfs == nil {
 			return errors.New("invalid source fs")
 		}
 		e := afero.Walk(sfs, "/", func(p string, info fs.FileInfo, err error) error {
+			// Check for context cancellation
+			if m.ctx != nil {
+				select {
+				case <-m.ctx.Done():
+					return m.ctx.Err()
+				default:
+				}
+			}
 			return m.addFile(p, info, idx)
 		})
 		if e != nil {