Emit unmodified change events for directories

When a file changes, emit events for the seen parent
directories to ensure that those parents are included
in change streams such as archives. The parents are
needed accurately recreate diff representation apart
from a parent (such as overlay and aufs graph drivers
in Docker).

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

fs/diff.go

@@ -222,8 +222,10 @@ func doubleWalkDiff(ctx context.Context, changeFn ChangeFunc, a, b string) (err
 		c1 = make(chan *currentPath)
 		c2 = make(chan *currentPath)
 
-		f1, f2 *currentPath
-		rmdir  string
+		f1, f2         *currentPath
+		rmdir          string
+		lastEmittedDir = string(filepath.Separator)
+		parents        []os.FileInfo
 	)
 	g.Go(func() error {
 		defer close(c1)
@@ -258,7 +260,10 @@ func doubleWalkDiff(ctx context.Context, changeFn ChangeFunc, a, b string) (err
 				continue
 			}
 
-			var f os.FileInfo
+			var (
+				f    os.FileInfo
+				emit = true
+			)
 			k, p := pathChange(f1, f2)
 			switch k {
 			case ChangeKindAdd:
@@ -294,13 +299,35 @@ func doubleWalkDiff(ctx context.Context, changeFn ChangeFunc, a, b string) (err
 				f2 = nil
 				if same {
 					if !isLinked(f) {
-						continue
+						emit = false
 					}
 					k = ChangeKindUnmodified
 				}
 			}
-			if err := changeFn(k, p, f, nil); err != nil {
-				return err
+			if emit {
+				emittedDir, emitParents := commonParents(lastEmittedDir, p, parents)
+				for _, pf := range emitParents {
+					p := filepath.Join(emittedDir, pf.Name())
+					if err := changeFn(ChangeKindUnmodified, p, pf, nil); err != nil {
+						return err
+					}
+					emittedDir = p
+				}
+
+				if err := changeFn(k, p, f, nil); err != nil {
+					return err
+				}
+
+				if f != nil && f.IsDir() {
+					lastEmittedDir = p
+				} else {
+					lastEmittedDir = emittedDir
+				}
+
+				parents = parents[:0]
+			} else if f.IsDir() {
+				lastEmittedDir, parents = commonParents(lastEmittedDir, p, parents)
+				parents = append(parents, f)
 			}
 		}
 		return nil
@@ -308,3 +335,47 @@ func doubleWalkDiff(ctx context.Context, changeFn ChangeFunc, a, b string) (err
 
 	return g.Wait()
 }
+
+func commonParents(base, updated string, dirs []os.FileInfo) (string, []os.FileInfo) {
+	if basePrefix := makePrefix(base); strings.HasPrefix(updated, basePrefix) {
+		var (
+			parents []os.FileInfo
+			last    = base
+		)
+		for _, d := range dirs {
+			next := filepath.Join(last, d.Name())
+			if strings.HasPrefix(updated, makePrefix(last)) {
+				parents = append(parents, d)
+				last = next
+			} else {
+				break
+			}
+		}
+		return base, parents
+	}
+
+	baseS := strings.Split(base, string(filepath.Separator))
+	updatedS := strings.Split(updated, string(filepath.Separator))
+	commonS := []string{string(filepath.Separator)}
+
+	min := len(baseS)
+	if len(updatedS) < min {
+		min = len(updatedS)
+	}
+	for i := 0; i < min; i++ {
+		if baseS[i] == updatedS[i] {
+			commonS = append(commonS, baseS[i])
+		} else {
+			break
+		}
+	}
+
+	return filepath.Join(commonS...), []os.FileInfo{}
+}
+
+func makePrefix(d string) string {
+	if d == "" || d[len(d)-1] != filepath.Separator {
+		return d + string(filepath.Separator)
+	}
+	return d
+}