fs: added Directory.Child() method to look up children by name without necessarily loading all of them

fs/cachefs/cachefs.go

@@ -21,6 +21,15 @@ type directory struct {
 	fs.Directory
 }
 
+func (d *directory) Child(ctx context.Context, name string) (fs.Entry, error) {
+	e, err := d.Directory.Child(ctx, name)
+	if err != nil {
+		return nil, err
+	}
+
+	return wrapWithContext(e, d.ctx), nil
+}
+
 func (d *directory) Readdir(ctx context.Context) (fs.Entries, error) {
 	entries, err := d.ctx.cacher.Readdir(ctx, d.Directory)
 	if err != nil {

fs/entry.go

@@ -2,6 +2,7 @@ package fs
 
 import (
 	"context"
+	"errors"
 	"io"
 	"os"
 	"sort"
@@ -40,10 +41,30 @@ type File interface {
 // Directory represents contents of a directory.
 type Directory interface {
 	Entry
+	Child(ctx context.Context, name string) (Entry, error)
 	Readdir(ctx context.Context) (Entries, error)
 	Summary() *DirectorySummary
 }
 
+// ErrEntryNotFound is returned when an entry is not found.
+var ErrEntryNotFound = errors.New("entry not found")
+
+// ReadDirAndFindChild reads all entries from a directory and returns one by name.
+// This is a convenience function that may be helpful in implementations of Directory.Child()
+func ReadDirAndFindChild(ctx context.Context, d Directory, name string) (Entry, error) {
+	children, err := d.Readdir(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	e := children.FindByName(name)
+	if e == nil {
+		return nil, ErrEntryNotFound
+	}
+
+	return e, nil
+}
+
 // DirectorySummary represents summary information about a directory.
 type DirectorySummary struct {
 	TotalFileSize    int64     `json:"size"`

fs/localfs/local_fs.go

@@ -101,6 +101,21 @@ func (fsd *filesystemDirectory) Summary() *fs.DirectorySummary {
 	return nil
 }
 
+func (fsd *filesystemDirectory) Child(ctx context.Context, name string) (fs.Entry, error) {
+	fullPath := fsd.fullPath()
+
+	st, err := os.Stat(filepath.Join(fullPath, name))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fs.ErrEntryNotFound
+		}
+
+		return nil, errors.Wrap(err, "unable to get child")
+	}
+
+	return entryFromChildFileInfo(st, fullPath)
+}
+
 func (fsd *filesystemDirectory) Readdir(ctx context.Context) (fs.Entries, error) {
 	fullPath := fsd.fullPath()
 

fs/localfs/local_fs_test.go

@@ -97,6 +97,47 @@ func TestFiles(t *testing.T) {
 			t.Logf("e[%v] = %v %v %v", i, e.Name(), e.Size(), e.Mode())
 		}
 	}
+
+	verifyChild(t, dir)
+}
+
+func verifyChild(t *testing.T, dir fs.Directory) {
+	ctx := context.Background()
+
+	child, err := dir.Child(ctx, "f3")
+	if err != nil {
+		t.Errorf("child error: %v", err)
+	}
+
+	if _, err = dir.Child(ctx, "f4"); err != fs.ErrEntryNotFound {
+		t.Errorf("unexpected child error: %v", err)
+	}
+
+	if got, want := child.Name(), "f3"; got != want {
+		t.Errorf("unexpected child name: %v, want %v", got, want)
+	}
+
+	if got, want := child.Size(), int64(3); got != want {
+		t.Errorf("unexpected child size: %v, want %v", got, want)
+	}
+
+	if _, err = fs.ReadDirAndFindChild(ctx, dir, "f4"); err != fs.ErrEntryNotFound {
+		t.Errorf("unexpected child error: %v", err)
+	}
+
+	// read child again, this time using ReadAndFindChild
+	child2, err := fs.ReadDirAndFindChild(ctx, dir, "f3")
+	if err != nil {
+		t.Errorf("child2 error: %v", err)
+	}
+
+	if got, want := child2.Name(), "f3"; got != want {
+		t.Errorf("unexpected child2 name: %v, want %v", got, want)
+	}
+
+	if got, want := child2.Size(), int64(3); got != want {
+		t.Errorf("unexpected child2 size: %v, want %v", got, want)
+	}
 }
 
 func assertNoError(t *testing.T, err error) {

fs/loggingfs/loggingfs.go

@@ -22,6 +22,19 @@ type loggingDirectory struct {
 	fs.Directory
 }
 
+func (ld *loggingDirectory) Child(ctx context.Context, name string) (fs.Entry, error) {
+	t0 := time.Now()
+	entry, err := ld.Directory.Child(ctx, name)
+	dt := time.Since(t0)
+	ld.options.printf(ld.options.prefix+"Child(%v) took %v and returned %v", ld.relativePath, dt, err)
+
+	if err != nil {
+		return nil, err
+	}
+
+	return wrapWithOptions(entry, ld.options, ld.relativePath+"/"+entry.Name()), nil
+}
+
 func (ld *loggingDirectory) Readdir(ctx context.Context) (fs.Entries, error) {
 	t0 := time.Now()
 	entries, err := ld.Directory.Readdir(ctx)

internal/mockfs/mockfs.go

@@ -173,6 +173,10 @@ func (imd *Directory) FailReaddir(err error) {
 	imd.readdirError = err
 }
 
+func (imd *Directory) Child(ctx context.Context, name string) (fs.Entry, error) {
+	return fs.ReadDirAndFindChild(ctx, imd, name)
+}
+
 // Readdir gets the contents of a directory.
 func (imd *Directory) Readdir(ctx context.Context) (fs.Entries, error) {
 	if imd.readdirError != nil {

snapshot/snapshotfs/all_sources.go

@@ -47,6 +47,10 @@ func (s *repositoryAllSources) Sys() interface{} {
 	return nil
 }
 
+func (s *repositoryAllSources) Child(ctx context.Context, name string) (fs.Entry, error) {
+	return fs.ReadDirAndFindChild(ctx, s, name)
+}
+
 func (s *repositoryAllSources) Readdir(ctx context.Context) (fs.Entries, error) {
 	srcs, err := snapshot.ListSources(ctx, s.rep)
 	if err != nil {

snapshot/snapshotfs/repofs.go

@@ -79,6 +79,10 @@ func (rd *repositoryDirectory) Summary() *fs.DirectorySummary {
 	return rd.summary
 }
 
+func (rd *repositoryDirectory) Child(ctx context.Context, name string) (fs.Entry, error) {
+	return fs.ReadDirAndFindChild(ctx, rd, name)
+}
+
 func (rd *repositoryDirectory) Readdir(ctx context.Context) (fs.Entries, error) {
 	r, err := rd.repo.Objects.Open(ctx, rd.metadata.ObjectID)
 	if err != nil {

snapshot/snapshotfs/source_directories.go

@@ -47,6 +47,10 @@ func (s *sourceDirectories) Owner() fs.OwnerInfo {
 	return fs.OwnerInfo{}
 }
 
+func (s *sourceDirectories) Child(ctx context.Context, name string) (fs.Entry, error) {
+	return fs.ReadDirAndFindChild(ctx, s, name)
+}
+
 func (s *sourceDirectories) Readdir(ctx context.Context) (fs.Entries, error) {
 	sources, err := snapshot.ListSources(ctx, s.rep)
 	if err != nil {
@@ -67,3 +71,5 @@ func (s *sourceDirectories) Readdir(ctx context.Context) (fs.Entries, error) {
 
 	return result, nil
 }
+
+var _ fs.Directory = (*sourceDirectories)(nil)

snapshot/snapshotfs/source_snapshots.go

@@ -56,6 +56,10 @@ func (s *sourceSnapshots) Summary() *fs.DirectorySummary {
 	return nil
 }
 
+func (s *sourceSnapshots) Child(ctx context.Context, name string) (fs.Entry, error) {
+	return fs.ReadDirAndFindChild(ctx, s, name)
+}
+
 func (s *sourceSnapshots) Readdir(ctx context.Context) (fs.Entries, error) {
 	manifests, err := snapshot.ListSnapshots(ctx, s.rep, s.src)
 	if err != nil {
@@ -94,3 +98,5 @@ func (s *sourceSnapshots) Readdir(ctx context.Context) (fs.Entries, error) {
 
 	return result, nil
 }
+
+var _ fs.Directory = (*sourceSnapshots)(nil)