vfs: Make file handles compatible with OS

* Implement directory handles
  * Unify OpenFile
  * Add all the methods to match *os.File
  * Add StatParent and Rename methods to VFS

vfs/dir_handle.go

@@ -0,0 +1,91 @@
+package vfs
+
+import (
+	"io"
+	"os"
+)
+
+// DirHandle represents an open directory
+type DirHandle struct {
+	baseHandle
+	d   *Dir
+	fis []os.FileInfo // where Readdir got to
+}
+
+// newDirHandle opens a directory for read
+func newDirHandle(d *Dir) *DirHandle {
+	return &DirHandle{
+		d: d,
+	}
+}
+
+// Stat returns info about the current directory
+func (fh *DirHandle) Stat() (fi os.FileInfo, err error) {
+	return fh.d, nil
+}
+
+// Readdir reads the contents of the directory associated with file and returns
+// a slice of up to n FileInfo values, as would be returned by Lstat, in
+// directory order. Subsequent calls on the same file will yield further
+// FileInfos.
+//
+// If n > 0, Readdir returns at most n FileInfo structures. In this case, if
+// Readdir returns an empty slice, it will return a non-nil error explaining
+// why. At the end of a directory, the error is io.EOF.
+//
+// If n <= 0, Readdir returns all the FileInfo from the directory in a single
+// slice. In this case, if Readdir succeeds (reads all the way to the end of
+// the directory), it returns the slice and a nil error. If it encounters an
+// error before the end of the directory, Readdir returns the FileInfo read
+// until that point and a non-nil error.
+func (fh *DirHandle) Readdir(n int) (fis []os.FileInfo, err error) {
+	if fh.fis == nil {
+		nodes, err := fh.d.ReadDirAll()
+		if err != nil {
+			return nil, err
+		}
+		fh.fis = []os.FileInfo{}
+		for _, node := range nodes {
+			fh.fis = append(fh.fis, node)
+		}
+	}
+	nn := len(fh.fis)
+	if n > 0 {
+		if nn == 0 {
+			return nil, io.EOF
+		}
+		if nn > n {
+			nn = n
+		}
+	}
+	fis, fh.fis = fh.fis[:nn], fh.fis[nn:]
+	return fis, nil
+}
+
+// Readdirnames reads and returns a slice of names from the directory f.
+//
+// If n > 0, Readdirnames returns at most n names. In this case, if
+// Readdirnames returns an empty slice, it will return a non-nil error
+// explaining why. At the end of a directory, the error is io.EOF.
+//
+// If n <= 0, Readdirnames returns all the names from the directory in a single
+// slice. In this case, if Readdirnames succeeds (reads all the way to the end
+// of the directory), it returns the slice and a nil error. If it encounters an
+// error before the end of the directory, Readdirnames returns the names read
+// until that point and a non-nil error.
+func (fh *DirHandle) Readdirnames(n int) (names []string, err error) {
+	nodes, err := fh.Readdir(n)
+	if err != nil {
+		return nil, err
+	}
+	for _, node := range nodes {
+		names = append(names, node.Name())
+	}
+	return names, nil
+}
+
+// Close closes the handle
+func (fh *DirHandle) Close() (err error) {
+	fh.fis = nil
+	return nil
+}