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) {