Fixes #352.

Add tests:
- ``*mem.File` implementation of `io.ReaderAt` must return non-nil error when reading less than length of give slice.
- `MemMapFs` must pass `fstest.TestFS` tests for non-empty files.

Fixes the bug.

iofs_test.go

@@ -29,7 +29,15 @@ func TestIOFS(t *testing.T) {
 			t.Fatal("OpenFile (O_CREATE) failed:", err)
 		}
 
-		f.Close()
+		_, err = f.WriteString("a")
+		if err != nil {
+			t.Fatal("WriteString failed:", err)
+		}
+
+		err = f.Close()
+		if err != nil {
+			t.Fatal("Close failed:", err)
+		}
 
 		if err := fstest.TestFS(NewIOFS(mmfs), "dir1/dir2/test.txt"); err != nil {
 			t.Error(err)

mem/file.go

@@ -210,6 +210,11 @@ func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
 	atomic.StoreInt64(&f.at, off)
 	n, err = f.Read(b)
 	atomic.StoreInt64(&f.at, prev)
+	if int64(n) == int64(len(f.fileData.data))-off {
+		if n < len(b) {
+			err = io.EOF
+		}
+	}
 	return
 }
 

mem/file_test.go

@@ -232,7 +232,7 @@ func TestFileReadAtSeekOffset(t *testing.T) {
 		t.Fatal("expected 0")
 	}
 
-	b := make([]byte, 4)
+	b := make([]byte, 4, 5)
 	n, err := f.ReadAt(b, 0)
 	if err != nil {
 		t.Fatal(err)
@@ -244,6 +244,17 @@ func TestFileReadAtSeekOffset(t *testing.T) {
 		t.Fail()
 	}
 
+	n, err = f.ReadAt(b[:cap(b)], 0)
+	if err != io.EOF {
+		t.Fatal(err)
+	}
+	if n != 4 {
+		t.Fail()
+	}
+	if string(b) != "TEST" {
+		t.Fail()
+	}
+
 	offsetAfterReadAt, err := f.Seek(0, io.SeekCurrent)
 	if err != nil {
 		t.Fatal(err)