Fix amount of files read in UnionFile.Readdir

To match behavior of os.File.Readdir, argument c in UnionFile.Readdir
should control amount of files read, not the upper bound of buffered
files.

Fixes #259

composite_test.go

@@ -477,7 +477,8 @@ func TestUnionFileReaddirAskForTooMany(t *testing.T) {
 	base := &MemMapFs{}
 	overlay := &MemMapFs{}
 
-	for i := 0; i < 5; i++ {
+	const testFiles = 5
+	for i := 0; i < testFiles; i++ {
 		WriteFile(base, fmt.Sprintf("file%d.txt", i), []byte("afero"), 0777)
 	}
 
@@ -490,13 +491,24 @@ func TestUnionFileReaddirAskForTooMany(t *testing.T) {
 
 	defer f.Close()
 
-	names, err := f.Readdirnames(6)
+	// 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)
+	}
 
-	if len(names) != 5 {
-		t.Fatal(names)
+	// 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

unionFile.go

@@ -186,25 +186,22 @@ func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 		}
 		f.files = append(f.files, merged...)
 	}
+	files := f.files[f.off:]
 
-	if c <= 0 && len(f.files) == 0 {
-		return f.files, nil
+	if c <= 0 {
+		return files, nil
 	}
 
-	if f.off >= len(f.files) {
+	if len(files) == 0 {
 		return nil, io.EOF
 	}
 
-	if c <= 0 {
-		return f.files[f.off:], nil
-	}
-
-	if c > len(f.files) {
-		c = len(f.files)
+	if c > len(files) {
+		c = len(files)
 	}
 
 	defer func() { f.off += c }()
-	return f.files[f.off:c], nil
+	return files[:c], nil
 }
 
 func (f *UnionFile) Readdirnames(c int) ([]string, error) {