Fix missing bounds checking in UnionFile.Readdir

It will now return io.EOF at the end of the directory view.

Fixes #194

composite_test.go

@@ -3,6 +3,7 @@ package afero
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -401,3 +402,72 @@ func TestCacheOnReadFsNotInLayer(t *testing.T) {
 	}
 	fh.Close()
 }
+
+// #194
+func TestUniontFileReaddirEmpty(t *testing.T) {
+	osFs := NewOsFs()
+
+	base := NewMemMapFs()
+	overlay := NewMemMapFs()
+	ufs := &CopyOnWriteFs{base: base, layer: overlay}
+	mem := NewMemMapFs()
+
+	// The OS file will return io.EOF on end of directory.
+	for _, fs := range []Fs{osFs, ufs, mem} {
+		baseDir, err := TempDir(fs, "", "empty-dir")
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		f, err := fs.Open(baseDir)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		names, err := f.Readdirnames(1)
+		if err != io.EOF {
+			t.Fatal(err)
+		}
+
+		if len(names) != 0 {
+			t.Fatal("should be empty")
+		}
+
+		f.Close()
+
+		fs.RemoveAll(baseDir)
+	}
+}
+
+func TestUniontFileReaddirAskForTooMany(t *testing.T) {
+	base := &MemMapFs{}
+	overlay := &MemMapFs{}
+
+	for i := 0; i < 5; i++ {
+		WriteFile(base, fmt.Sprintf("file%d.txt", i), []byte("afero"), 0777)
+	}
+
+	ufs := &CopyOnWriteFs{base: base, layer: overlay}
+
+	f, err := ufs.Open("")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer f.Close()
+
+	names, err := f.Readdirnames(6)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(names) != 5 {
+		t.Fatal(names)
+	}
+
+	// End of directory
+	_, err = f.Readdirnames(3)
+	if err != io.EOF {
+		t.Fatal(err)
+	}
+}

unionFile.go

@@ -156,6 +156,7 @@ var defaultUnionMergeDirsFn = func(lofi, bofi []os.FileInfo) ([]os.FileInfo, err
 
 // Readdir will weave the two directories together and
 // return a single view of the overlayed directories
+// At the end of the directory view, the error is io.EOF.
 func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 	var merge DirsMerger = f.Merger
 	if merge == nil {
@@ -185,9 +186,19 @@ func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 		}
 		f.files = append(f.files, merged...)
 	}
+
+	if f.off >= len(f.files) {
+		return nil, io.EOF
+	}
+
 	if c == -1 {
 		return f.files[f.off:], nil
 	}
+
+	if c > len(f.files) {
+		c = len(f.files)
+	}
+
 	defer func() { f.off += c }()
 	return f.files[f.off:c], nil
 }