Add a DirEntryInfo interface that is a subset of os.FileInfo

ioutil.ReadDir returns []os.FileInfo, which contains information on
each entry in the directory that is only available by calling
os.Lstat on the entry.  Finder only the name and type (regular,
directory or symlink) of the files, which on Linux kernels >= 2.6.4
is available in the return values of syscall.Getdents.

In preparation for using syscall.Getdents, switch filesystem.ReadDir
to return an interface that only contains the information that will
be available from syscall.Getdents.

Bug: 70897635
Test: m checkbuild
Change-Id: Id2749d709a0f7b5a61abedde68549d4bf208a568

finder/finder.go

@@ -1384,7 +1384,7 @@ func (f *Finder) listDirSync(dir *pathMap) {
 		f.onFsError(path, err)
 		// if listing the contents of the directory fails (presumably due to
 		// permission denied), then treat the directory as empty
-		children = []os.FileInfo{}
+		children = nil
 	}
 
 	var subdirs []string

finder/fs/fs.go

@@ -51,7 +51,7 @@ type FileSystem interface {
 	// getting information about files
 	Open(name string) (file io.ReadCloser, err error)
 	Lstat(path string) (stats os.FileInfo, err error)
-	ReadDir(path string) (contents []os.FileInfo, err error)
+	ReadDir(path string) (contents []DirEntryInfo, err error)
 
 	InodeNumber(info os.FileInfo) (number uint64, err error)
 	DeviceNumber(info os.FileInfo) (number uint64, err error)
@@ -67,18 +67,39 @@ type FileSystem interface {
 	ViewId() (id string) // Some unique id of the user accessing the filesystem
 }
 
+// DentryInfo is a subset of the functionality available through os.FileInfo that might be able
+// to be gleaned through only a syscall.Getdents without requiring a syscall.Lstat of every file.
+type DirEntryInfo interface {
+	Name() string
+	Mode() os.FileMode // the file type encoded as an os.FileMode
+	IsDir() bool
+}
+
+var _ DirEntryInfo = os.FileInfo(nil)
+
 // osFs implements FileSystem using the local disk.
 type osFs struct{}
 
+var _ FileSystem = (*osFs)(nil)
+
 func (osFs) Open(name string) (io.ReadCloser, error) { return os.Open(name) }
 
 func (osFs) Lstat(path string) (stats os.FileInfo, err error) {
 	return os.Lstat(path)
 }
 
-func (osFs) ReadDir(path string) (contents []os.FileInfo, err error) {
-	return ioutil.ReadDir(path)
+func (osFs) ReadDir(path string) (contents []DirEntryInfo, err error) {
+	entries, err := ioutil.ReadDir(path)
+	if err != nil {
+		return nil, err
+	}
+	for _, entry := range entries {
+		contents = append(contents, entry)
+	}
+
+	return contents, nil
 }
+
 func (osFs) Rename(oldPath string, newPath string) error {
 	return os.Rename(oldPath, newPath)
 }
@@ -154,6 +175,8 @@ type MockFs struct {
 	aggregatesLock sync.Mutex
 }
 
+var _ FileSystem = (*MockFs)(nil)
+
 type mockInode struct {
 	modTime     time.Time
 	permTime    time.Time
@@ -475,7 +498,7 @@ func (m *MockFs) PermTime(info os.FileInfo) (when time.Time, err error) {
 		fmt.Errorf("%v is not a mockFileInfo", info)
 }
 
-func (m *MockFs) ReadDir(path string) (contents []os.FileInfo, err error) {
+func (m *MockFs) ReadDir(path string) (contents []DirEntryInfo, err error) {
 	// update aggregates
 	m.aggregatesLock.Lock()
 	m.ReadDirCalls = append(m.ReadDirCalls, path)
@@ -486,7 +509,7 @@ func (m *MockFs) ReadDir(path string) (contents []os.FileInfo, err error) {
 	if err != nil {
 		return nil, err
 	}
-	results := []os.FileInfo{}
+	results := []DirEntryInfo{}
 	dir, err := m.getDir(path, false)
 	if err != nil {
 		return nil, err