func (c *APIUnitsCmd) Execute(args []string) error {
	context, err := prepareCommandContext(c.Args.Dir.String())
	if err != nil {
		return err
	}

	var unitSlice []unit.SourceUnit
	unitSuffix := buildstore.DataTypeSuffix(unit.SourceUnit{})
	foundUnit := false
	w := fs.WalkFS(".", context.commitFS)
	for w.Step() {
		unitFile := w.Path()
		if strings.HasSuffix(unitFile, unitSuffix) {
			var unit unit.SourceUnit
			foundUnit = true
			f, err := context.commitFS.Open(unitFile)
			if err != nil {
				return err
			}
			defer f.Close()
			if err := json.NewDecoder(f).Decode(&unit); err != nil {
				return fmt.Errorf("%s: %s", unitFile, err)
			}
			unitSlice = append(unitSlice, unit)
		}
	}

	if foundUnit == false {
		return errors.New("No source units found. Try running `src config` first.")
	}

	return json.NewEncoder(os.Stdout).Encode(unitSlice)
}

// ListRepoPaths implements RepoPaths.
func (defaultRepoPaths) ListRepoPaths(vfs rwvfs.WalkableFileSystem, after string, max int) ([][]string, error) {
	var paths [][]string
	w := fs.WalkFS(".", rwvfs.Walkable(vfs))
	for w.Step() {
		if err := w.Err(); err != nil {
			return nil, err
		}
		fi := w.Stat()
		if w.Path() >= after && fi.Mode().IsDir() {
			if fi.Name() == SrclibStoreDir {
				w.SkipDir()
				paths = append(paths, strings.Split(filepath.ToSlash(w.Path()), "/"))
				if max != 0 && len(paths) >= max {
					break
				}
				continue
			}
			if fi.Name() != "." && strings.HasPrefix(fi.Name(), ".") {
				w.SkipDir()
				continue
			}
		}
	}
	return paths, nil
}

// RemoveAll removes a tree recursively.
func RemoveAll(path string, vfs rwvfs.WalkableFileSystem) error {
	w := fs.WalkFS(path, vfs)

	remove := func(par *parallel.Run, path string) {
		par.Do(func() error { return vfs.Remove(path) })
	}

	var dirs []string // remove dirs after removing all files
	filesPar := parallel.NewRun(20)
	for w.Step() {
		if err := w.Err(); err != nil {
			return err
		}
		if w.Stat().IsDir() {
			dirs = append(dirs, w.Path())
		} else {
			remove(filesPar, w.Path())
		}
	}

	if err := filesPar.Wait(); err != nil {
		return err
	}

	dirsPar := parallel.NewRun(20)
	sort.Sort(sort.Reverse(sort.StringSlice(dirs))) // reverse so we delete leaf dirs first
	for _, dir := range dirs {
		remove(dirsPar, dir)
	}
	return dirsPar.Wait()
}

// ListRepoPaths implements RepoPaths.
func (defaultRepoPaths) ListRepoPaths(vfs rwvfs.WalkableFileSystem, after string, max int) ([][]string, error) {
	var paths [][]string
	w := fs.WalkFS(".", rwvfs.Walkable(vfs))
	for w.Step() {
		if err := w.Err(); err != nil {
			return nil, err
		}
		fi := w.Stat()
		if w.Path() >= after && fi.Mode().IsDir() {
			if fi.Name() == SrclibStoreDir {
				w.SkipDir()
				// NOTE: This assumes that the vfs's path
				// separator is "/", which is not true in general.
				paths = append(paths, strings.Split(w.Path(), "/"))
				if max != 0 && len(paths) >= max {
					break
				}
				continue
			}
			if fi.Name() != "." && strings.HasPrefix(fi.Name(), ".") {
				w.SkipDir()
				continue
			}
		}
	}
	return paths, nil
}

func (c *BuildDataFetchCmd) Execute(args []string) error {
	localFS, localRepoLabel, err := c.getLocalFileSystem()
	if err != nil {
		return err
	}

	remoteFS, remoteRepoLabel, repoRevSpec, err := c.getRemoteFileSystem()
	if err != nil {
		return err
	}

	// Use uncached API client because the .srclib-cache already
	// caches it, and we want to be able to stream large files.
	//
	// TODO(sqs): this uncached client isn't authed because it doesn't
	// have the other API client's http.Client or http.RoundTripper
	cl := newAPIClientWithAuth(false)
	remoteFS, err = cl.BuildData.FileSystem(repoRevSpec)
	if err != nil {
		return err
	}

	if GlobalOpt.Verbose {
		log.Printf("Fetching remote build files for %s to %s...", remoteRepoLabel, localRepoLabel)
	}

	// TODO(sqs): check if file exists in local cache and don't fetch it if it does and if it is identical

	par := parallel.NewRun(8)
	w := fs.WalkFS(".", rwvfs.Walkable(remoteFS))
	for w.Step() {
		path := w.Path()
		if err := w.Err(); err != nil {
			if path == "." {
				log.Printf("# No build data to pull from %s", remoteRepoLabel)
				return nil
			}
			return fmt.Errorf("walking remote dir tree: %s", err)
		}
		fi := w.Stat()
		if fi == nil {
			continue
		}
		if !fi.Mode().IsRegular() {
			continue
		}
		par.Do(func() error {
			return fetchFile(remoteFS, localFS, path, fi, c.DryRun)
		})
	}
	if err := par.Wait(); err != nil {
		return fmt.Errorf("error fetching: %s", err)
	}
	return nil
}

// StatAllRecursive recursively stats all files and dirs in fs,
// starting at path and descending. The Name methods of the returned
// FileInfos returns their full path, not just their filename.
func StatAllRecursive(path string, wfs WalkableFileSystem) ([]os.FileInfo, error) {
	var fis []os.FileInfo
	w := fs.WalkFS(path, wfs)
	for w.Step() {
		if err := w.Err(); err != nil {
			return nil, err
		}
		fis = append(fis, treeFileInfo{w.Path(), w.Stat()})
	}
	return fis, nil
}

func getSourceUnits(commitFS rwvfs.WalkableFileSystem, repo *Repo) []string {
	var unitFiles []string
	unitSuffix := buildstore.DataTypeSuffix(unit.SourceUnit{})
	w := fs.WalkFS(".", commitFS)
	for w.Step() {
		if strings.HasSuffix(w.Path(), unitSuffix) {
			unitFiles = append(unitFiles, w.Path())
		}
	}
	return unitFiles
}

// Glob returns the names of all files under prefix matching pattern or nil if
// there is no matching file. The syntax of patterns is the same as in
// path/filepath.Match.
func Glob(wfs WalkableFileSystem, prefix, pattern string) (matches []string, err error) {
	walker := fs.WalkFS(filepath.Clean(prefix), wfs)
	for walker.Step() {
		path := walker.Path()
		matched, err := filepath.Match(pattern, path)
		if err != nil {
			return nil, err
		}
		if matched {
			matches = append(matches, path)
		}
	}
	return
}

func (s *fsTreeStore) unitFilenames() ([]string, error) {
	var files []string
	w := fs.WalkFS(".", rwvfs.Walkable(s.fs))
	for w.Step() {
		if err := w.Err(); err != nil {
			return nil, err
		}
		fi := w.Stat()
		if fi.Mode().IsRegular() && strings.HasSuffix(fi.Name(), unitFileSuffix) {
			files = append(files, filepath.ToSlash(w.Path()))
		}
	}
	return files, nil
}

func (r *FileRepo) Walk(walkFn func(file string)) error {
	walker := fs.WalkFS(".", r.fs)
	for walker.Step() {
		if err := walker.Err(); err != nil {
			return err
		}
		path := walker.Path()
		if walker.Stat().IsDir() || filepath.Ext(path) != fileExtension {
			continue
		}
		walkFn(path[:len(path)-len(fileExtension)])
	}
	return nil
}

// ReadCached reads a Tree's configuration from all of its source unit
// definition files (which may either be in a local VFS rooted at a
// .srclib-cache/<COMMITID> dir, or a remote VFS). It does not read
// the Srcfile; the Srcfile's directives are already accounted for in
// the cached source unit definition files.
//
// bdfs should be a VFS obtained from a call to
// (buildstore.RepoBuildStore).Commit.
func ReadCached(bdfs vfs.FileSystem) (*Tree, error) {
	if _, err := bdfs.Lstat("."); os.IsNotExist(err) {
		return nil, fmt.Errorf("build cache dir does not exist (did you run `srclib config` to create it)?")
	} else if err != nil {
		return nil, err
	}

	// Collect all **/*.unit.json files.
	var unitFiles []string
	unitSuffix := buildstore.DataTypeSuffix(unit.SourceUnit{})
	w := fs.WalkFS(".", rwvfs.Walkable(rwvfs.ReadOnly(bdfs)))
	for w.Step() {
		if err := w.Err(); err != nil {
			return nil, err
		}
		if path := w.Path(); strings.HasSuffix(path, unitSuffix) {
			unitFiles = append(unitFiles, path)
		}
	}

	// Parse units
	sort.Strings(unitFiles)
	units := make([]*unit.SourceUnit, len(unitFiles))
	par := parallel.NewRun(runtime.GOMAXPROCS(0))
	for i_, unitFile_ := range unitFiles {
		i, unitFile := i_, unitFile_
		par.Acquire()
		go func() {
			defer par.Release()
			f, err := bdfs.Open(unitFile)
			if err != nil {
				par.Error(err)
				return
			}
			if err := json.NewDecoder(f).Decode(&units[i]); err != nil {
				f.Close()
				par.Error(err)
				return
			}
			if err := f.Close(); err != nil {
				par.Error(err)
				return
			}
		}()
	}
	if err := par.Wait(); err != nil {
		return nil, err
	}
	return &Tree{SourceUnits: units}, nil
}

func TestMap_Walk2(t *testing.T) {
	m := map[string]string{"a/b/c/d": "a"}
	mapFS := Map(m)

	var names []string
	w := fs.WalkFS(".", Walkable(Sub(mapFS, "a/b")))
	for w.Step() {
		if err := w.Err(); err != nil {
			t.Fatalf("walk path %q: %s", w.Path(), err)
		}
		names = append(names, w.Path())
	}

	wantNames := []string{".", "c", "c/d"}
	sort.Strings(names)
	sort.Strings(wantNames)
	if !reflect.DeepEqual(names, wantNames) {
		t.Errorf("got entry names %v, want %v", names, wantNames)
	}
}

func TestMap_Walk(t *testing.T) {
	m := map[string]string{"a": "a", "b/c": "c", "b/x/y/z": "z"}
	mapFS := rwvfs.Map(m)

	var names []string
	w := fs.WalkFS(".", rwvfs.Walkable(mapFS))
	for w.Step() {
		if err := w.Err(); err != nil {
			t.Fatalf("walk path %q: %s", w.Path(), err)
		}
		names = append(names, w.Path())
	}

	wantNames := []string{".", "a", "b", "b/c", "b/x", "b/x/y", "b/x/y/z"}
	sort.Strings(names)
	sort.Strings(wantNames)
	if !reflect.DeepEqual(names, wantNames) {
		t.Errorf("got entry names %v, want %v", names, wantNames)
	}
}

// getSourceUnitsWithFile gets a list of all source units that contain
// the given file.
func getSourceUnitsWithFile(buildStore buildstore.RepoBuildStore, repo *Repo, filename string) ([]*unit.SourceUnit, error) {
	filename = filepath.Clean(filename)

	// TODO(sqs): This whole lookup is totally inefficient. The storage format
	// is not optimized for lookups.
	commitFS := buildStore.Commit(repo.CommitID)
	unitFiles := getSourceUnits(commitFS, repo)

	// Find all source unit definition files.
	unitSuffix := buildstore.DataTypeSuffix(unit.SourceUnit{})
	w := fs.WalkFS(".", commitFS)
	for w.Step() {
		if strings.HasSuffix(w.Path(), unitSuffix) {
			unitFiles = append(unitFiles, w.Path())
		}
	}

	// Find which source units the file belongs to.
	var units []*unit.SourceUnit
	for _, unitFile := range unitFiles {
		var u *unit.SourceUnit
		f, err := commitFS.Open(unitFile)
		if err != nil {
			return nil, err
		}
		defer f.Close()
		if err := json.NewDecoder(f).Decode(&u); err != nil {
			return nil, fmt.Errorf("%s: %s", unitFile, err)
		}
		for _, f2 := range u.Files {
			if filepath.Clean(f2) == filename {
				units = append(units, u)
				break
			}
		}
	}

	return units, nil
}

func (r *FormRepo) List() ([]Form, error) {
	forms := make([]Form, 0)
	walker := fs.WalkFS(".", r.fs)
	for walker.Step() {
		if err := walker.Err(); err != nil {
			return nil, err
		}
		if !walker.Stat().IsDir() {
			continue
		}
		form, err := r.Fields(walker.Path())
		switch err {
		case ErrNotFound: // Ignore
		case nil:
			if len(form.Fields) > 0 {
				forms = append(forms, *form)
			}
		default:
			return nil, err
		}
	}
	return forms, nil
}

func (c *BuildDataUploadCmd) Execute(args []string) error {
	localFS, localRepoLabel, err := c.getLocalFileSystem()
	if err != nil {
		return err
	}

	remoteFS, remoteRepoLabel, _, err := c.getRemoteFileSystem()
	if err != nil {
		return err
	}

	if GlobalOpt.Verbose {
		log.Printf("Uploading build files from %s to %s...", localRepoLabel, remoteRepoLabel)
	}

	// TODO(sqs): check if file exists remotely and don't upload it if it does and if it is identical

	par := parallel.NewRun(8)
	w := fs.WalkFS(".", rwvfs.Walkable(localFS))
	for w.Step() {
		if err := w.Err(); err != nil {
			return err
		}
		fi := w.Stat()
		if fi == nil {
			continue
		}
		if !fi.Mode().IsRegular() {
			continue
		}
		path := w.Path()
		par.Do(func() error {
			return uploadFile(localFS, remoteFS, path, fi, c.DryRun)
		})
	}
	return par.Wait()
}

// Walk returns a new Walker rooted at root.
func (c *Client) Walk(root string) *fs.Walker {
	return fs.WalkFS(root, c)
}