Fix MemMapFs.Readdir to use files buffer

To match behavior of os.File.Readdir, MemMapFs.Readdir should use
files buffer instead of retrieving currently stored files on each
call.

Fixes #261

mem/file.go

@@ -31,6 +31,7 @@ type File struct {
 	// atomic requires 64-bit alignment for struct field access
 	at           int64
 	readDirCount int64
+	dirBuf       []*FileData
 	closed       bool
 	readOnly     bool
 	fileData     *FileData
@@ -150,7 +151,10 @@ func (f *File) Readdir(count int) (res []os.FileInfo, err error) {
 	var outLength int64
 
 	f.fileData.Lock()
-	files := f.fileData.memDir.Files()[f.readDirCount:]
+	if f.dirBuf == nil {
+		f.dirBuf = f.fileData.memDir.Files()
+	}
+	files := f.dirBuf[f.readDirCount:]
 	if count > 0 {
 		if len(files) < count {
 			outLength = int64(len(files))

mem/file_test.go

@@ -2,6 +2,7 @@ package mem
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"testing"
 	"time"
@@ -206,6 +207,49 @@ func TestFileDataSizeRace(t *testing.T) {
 	}
 }
 
+func TestFileReaddirBuffer(t *testing.T) {
+	dir := CreateDir("dir")
+
+	const testFiles = 5
+	for i := 0; i < testFiles; i++ {
+		fd := CreateFile(fmt.Sprintf("%d.txt", i))
+		AddToMemDir(dir, fd)
+	}
+
+	f := NewFileHandle(dir)
+	defer f.Close()
+	// Read part of all files
+	wantNames := 3
+	names, err := f.Readdirnames(wantNames)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(names) != wantNames {
+		t.Fatalf("got %d names %v, want %d", len(names), names, wantNames)
+	}
+
+	// Remove half of files
+	for _, fd := range dir.memDir.Files()[:testFiles/2] {
+		RemoveFromMemDir(dir, fd)
+	}
+
+	// Try to read more files than remaining
+	wantNames = testFiles - len(names)
+	names, err = f.Readdirnames(wantNames + 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(names) != wantNames {
+		t.Fatalf("got %d names %v, want %d", len(names), names, wantNames)
+	}
+
+	// End of directory
+	_, err = f.Readdirnames(testFiles + 1)
+	if err != io.EOF {
+		t.Fatal(err)
+	}
+}
+
 func TestFileReadAtSeekOffset(t *testing.T) {
 	t.Parallel()