archive: Optimize ChangesDirs on Linux

If we tear through a few layers of abstraction, we can get at the inodes
contained in a directory without having to stat all the files. This
allows us to eliminate identical files much earlier in the changelist
generation process.

Signed-off-by: Burke Libbey <burke@libbey.me>

archive/changes.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
@@ -277,78 +278,23 @@ func newRootFileInfo() *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(sourceDir, path)
-		if err != nil {
-			return err
-		}
-		relPath = filepath.Join("/", relPath)
-
-		if relPath == "/" {
-			return nil
-		}
-
-		parent := root.LookUp(filepath.Dir(relPath))
-		if parent == nil {
-			return fmt.Errorf("collectFileInfo: Unexpectedly no parent for %s", relPath)
-		}
-
-		info := &FileInfo{
-			name:     filepath.Base(relPath),
-			children: make(map[string]*FileInfo),
-			parent:   parent,
-		}
-
-		s, err := system.Lstat(path)
-		if err != nil {
-			return err
-		}
-		info.stat = s
-
-		info.capability, _ = system.Lgetxattr(path, "security.capability")
-
-		parent.children[info.name] = info
-
-		return nil
-	})
-	if err != nil {
-		return nil, err
-	}
-	return root, nil
-}
-
 // ChangesDirs compares two directories and generates an array of Change objects describing the changes.
 // If oldDir is "", then all files in newDir will be Add-Changes.
 func ChangesDirs(newDir, oldDir string) ([]Change, error) {
 	var (
 		oldRoot, newRoot *FileInfo
-		err1, err2       error
-		errs             = make(chan error, 2)
 	)
-	go func() {
-		if oldDir != "" {
-			oldRoot, err1 = collectFileInfo(oldDir)
-		}
-		errs <- err1
-	}()
-	go func() {
-		newRoot, err2 = collectFileInfo(newDir)
-		errs <- err2
-	}()
-
-	// block until both routines have returned
-	for i := 0; i < 2; i++ {
-		if err := <-errs; err != nil {
+	if oldDir == "" {
+		emptyDir, err := ioutil.TempDir("", "empty")
+		if err != nil {
 			return nil, err
 		}
+		defer os.Remove(emptyDir)
+		oldDir = emptyDir
+	}
+	oldRoot, newRoot, err := collectFileInfoForChanges(oldDir, newDir)
+	if err != nil {
+		return nil, err
 	}
 
 	return newRoot.Changes(oldRoot), nil