Report io.EOF from UnionFile.ReadAt

composite_test.go

@@ -515,3 +515,28 @@ func TestUnionFileReaddirAskForTooMany(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestUnionFileReadAtReportsEOF(t *testing.T) {
+	base := NewMemMapFs()
+	layer := NewMemMapFs()
+	for _, fs := range []Fs{base, layer} {
+		if err := WriteFile(fs, "data", []byte("0123456789"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	f, err := NewCacheOnReadFs(base, layer, 0).OpenFile("data", os.O_RDWR, 0o644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	// io.ReaderAt: a short read must report why fewer bytes came back.
+	buf := make([]byte, 8)
+	if n, err := f.ReadAt(buf, 6); n != 4 || err != io.EOF {
+		t.Errorf("ReadAt short read = (%d, %v), want (4, EOF)", n, err)
+	}
+	if n, err := f.ReadAt(buf, 10); n != 0 || err != io.EOF {
+		t.Errorf("ReadAt at EOF = (%d, %v), want (0, EOF)", n, err)
+	}
+}

unionFile.go

@@ -71,7 +71,11 @@ func (f *UnionFile) ReadAt(s []byte, o int64) (int, error) {
 	if f.Layer != nil {
 		n, err := f.Layer.ReadAt(s, o)
 		if (err == nil || err == io.EOF) && f.Base != nil {
-			_, err = f.Base.Seek(o+int64(n), io.SeekStart)
+			// only overwrite err in case the seek fails: we need to
+			// report an eventual io.EOF to the caller
+			if _, seekErr := f.Base.Seek(o+int64(n), io.SeekStart); seekErr != nil {
+				err = seekErr
+			}
 		}
 		return n, err
 	}