archive.ChagesDir() - faster and better implementation

This replaces the current version with the latest version from the
dm branch. Advantages in this version:

We only scan each directory tree once, which means less i/o and less chance
of container changes inbetween the two scans causing inconsistencies.

We avoid comparing some fields for change detection:

* Inode
* size-in-blocks
  These can change during a copy operation (e.g. in the dummy backend)
  without needing to actually reflect a change in content or metadata.

* Ctime
  Any copy operation will create a new Ctime value, and there is no
  API to change it to the "source" value.

* size for directories
  The size of a directory doesn't have to be the same just because you
  recreated the same content as another director. Internal details in
  the filesystem may make these different with no "real" change.

archive/changes.go

@@ -106,107 +106,179 @@ func Changes(layers []string, rw string) ([]Change, error) {
 	return changes, nil
 }
 
-func ChangesDirs(newDir, oldDir string) ([]Change, error) {
-	var changes []Change
-	err := filepath.Walk(newDir, func(newPath string, f os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
+type FileInfo struct {
+	parent   *FileInfo
+	name     string
+	stat     syscall.Stat_t
+	children map[string]*FileInfo
+}
 
-		var newStat syscall.Stat_t
-		err = syscall.Lstat(newPath, &newStat)
-		if err != nil {
-			return err
-		}
+func (root *FileInfo) LookUp(path string) *FileInfo {
+	parent := root
+	if path == "/" {
+		return root
+	}
 
-		// Rebase path
-		relPath, err := filepath.Rel(newDir, newPath)
-		if err != nil {
-			return err
-		}
-		relPath = filepath.Join("/", relPath)
-
-		// Skip root
-		if relPath == "/" || relPath == "/.docker-id" {
-			return nil
-		}
-
-		change := Change{
-			Path: relPath,
-		}
-
-		oldPath := filepath.Join(oldDir, relPath)
-
-		var oldStat = &syscall.Stat_t{}
-		err = syscall.Lstat(oldPath, oldStat)
-		if err != nil {
-			if !os.IsNotExist(err) {
-				return err
+	pathElements := strings.Split(path, "/")
+	for _, elem := range pathElements {
+		if elem != "" {
+			child := parent.children[elem]
+			if child == nil {
+				return nil
 			}
-			oldStat = nil
+			parent = child
 		}
+	}
+	return parent
+}
 
-		if oldStat == nil {
-			change.Kind = ChangeAdd
-			changes = append(changes, change)
-		} else {
-			if oldStat.Ino != newStat.Ino ||
-				oldStat.Mode != newStat.Mode ||
+func (info *FileInfo) path() string {
+	if info.parent == nil {
+		return "/"
+	}
+	return filepath.Join(info.parent.path(), info.name)
+}
+
+func (info *FileInfo) isDir() bool {
+	return info.parent == nil || info.stat.Mode&syscall.S_IFDIR == syscall.S_IFDIR
+}
+
+func (info *FileInfo) addChanges(oldInfo *FileInfo, changes *[]Change) {
+	if oldInfo == nil {
+		// add
+		change := Change{
+			Path: info.path(),
+			Kind: ChangeAdd,
+		}
+		*changes = append(*changes, change)
+	}
+
+	// We make a copy so we can modify it to detect additions
+	// also, we only recurse on the old dir if the new info is a directory
+	// otherwise any previous delete/change is considered recursive
+	oldChildren := make(map[string]*FileInfo)
+	if oldInfo != nil && info.isDir() {
+		for k, v := range oldInfo.children {
+			oldChildren[k] = v
+		}
+	}
+
+	for name, newChild := range info.children {
+		oldChild, _ := oldChildren[name]
+		if oldChild != nil {
+			// change?
+			oldStat := &oldChild.stat
+			newStat := &newChild.stat
+			// Note: We can't compare inode or ctime or blocksize here, because these change
+			// when copying a file into a container. However, that is not generally a problem
+			// because any content change will change mtime, and any status change should
+			// be visible when actually comparing the stat fields. The only time this
+			// breaks down is if some code intentionally hides a change by setting
+			// back mtime
+			if oldStat.Mode != newStat.Mode ||
 				oldStat.Uid != newStat.Uid ||
 				oldStat.Gid != newStat.Gid ||
 				oldStat.Rdev != newStat.Rdev ||
-				oldStat.Size != newStat.Size ||
-				oldStat.Blocks != newStat.Blocks ||
-				oldStat.Mtim != newStat.Mtim ||
-				oldStat.Ctim != newStat.Ctim {
-				change.Kind = ChangeModify
-				changes = append(changes, change)
+				// Don't look at size for dirs, its not a good measure of change
+				(oldStat.Size != newStat.Size && oldStat.Mode&syscall.S_IFDIR != syscall.S_IFDIR) ||
+				oldStat.Mtim != newStat.Mtim {
+				change := Change{
+					Path: newChild.path(),
+					Kind: ChangeModify,
+				}
+				*changes = append(*changes, change)
 			}
+
+			// Remove from copy so we can detect deletions
+			delete(oldChildren, name)
 		}
 
-		return nil
-	})
-	if err != nil {
-		return nil, err
+		newChild.addChanges(oldChild, changes)
 	}
-	err = filepath.Walk(oldDir, func(oldPath string, f os.FileInfo, err error) error {
+	for _, oldChild := range oldChildren {
+		// delete
+		change := Change{
+			Path: oldChild.path(),
+			Kind: ChangeDelete,
+		}
+		*changes = append(*changes, change)
+	}
+
+}
+
+func (info *FileInfo) Changes(oldInfo *FileInfo) []Change {
+	var changes []Change
+
+	info.addChanges(oldInfo, &changes)
+
+	return changes
+}
+
+func newRootFileInfo() *FileInfo {
+	root := &FileInfo{
+		name:     "/",
+		children: make(map[string]*FileInfo),
+	}
+	return root
+}
+
+func collectFileInfo(sourceDir string) (*FileInfo, error) {
+	root := newRootFileInfo()
+
+	err := filepath.Walk(sourceDir, func(path string, f os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
 
 		// Rebase path
-		relPath, err := filepath.Rel(oldDir, oldPath)
+		relPath, err := filepath.Rel(sourceDir, path)
 		if err != nil {
 			return err
 		}
 		relPath = filepath.Join("/", relPath)
 
-		// Skip root
 		if relPath == "/" {
 			return nil
 		}
 
-		change := Change{
-			Path: relPath,
+		parent := root.LookUp(filepath.Dir(relPath))
+		if parent == nil {
+			return fmt.Errorf("collectFileInfo: Unexpectedly no parent for %s", relPath)
 		}
 
-		newPath := filepath.Join(newDir, relPath)
-
-		var newStat = &syscall.Stat_t{}
-		err = syscall.Lstat(newPath, newStat)
-		if err != nil && os.IsNotExist(err) {
-			change.Kind = ChangeDelete
-			changes = append(changes, change)
+		info := &FileInfo{
+			name:     filepath.Base(relPath),
+			children: make(map[string]*FileInfo),
+			parent:   parent,
 		}
 
+		if err := syscall.Lstat(path, &info.stat); err != nil {
+			return err
+		}
+
+		parent.children[info.name] = info
+
 		return nil
 	})
 	if err != nil {
 		return nil, err
 	}
-	return changes, nil
+	return root, nil
 }
 
+// Compare two directories and generate an array of Change objects describing the changes
+func ChangesDirs(newDir, oldDir string) ([]Change, error) {
+	oldRoot, err := collectFileInfo(oldDir)
+	if err != nil {
+		return nil, err
+	}
+	newRoot, err := collectFileInfo(newDir)
+	if err != nil {
+		return nil, err
+	}
+
+	return newRoot.Changes(oldRoot), nil
+}
 
 func ExportChanges(root, rw string) (Archive, error) {
         changes, err := ChangesDirs(root, rw)