Use %w to wrap errors (#1175)

stackless/writer_test.go

@@ -73,7 +73,7 @@ func testWriter(newWriter NewWriterFunc, newReader func(io.Reader) io.Reader) er
 
 	for i := 0; i < 5; i++ {
 		if err := testWriterReuse(w, dstW, newReader); err != nil {
-			return fmt.Errorf("unexpected error when re-using writer on iteration %d: %s", i, err)
+			return fmt.Errorf("unexpected error when re-using writer on iteration %d: %w", i, err)
 		}
 		dstW = &bytes.Buffer{}
 		w.Reset(dstW)
@@ -89,7 +89,7 @@ func testWriterReuse(w Writer, r io.Reader, newReader func(io.Reader) io.Reader)
 		fmt.Fprintf(mw, "foobar %d\n", i)
 		if i%13 == 0 {
 			if err := w.Flush(); err != nil {
-				return fmt.Errorf("error on flush: %s", err)
+				return fmt.Errorf("error on flush: %w", err)
 			}
 		}
 	}
@@ -98,7 +98,7 @@ func testWriterReuse(w Writer, r io.Reader, newReader func(io.Reader) io.Reader)
 	zr := newReader(r)
 	data, err := ioutil.ReadAll(zr)
 	if err != nil {
-		return fmt.Errorf("unexpected error: %s, data=%q", err, data)
+		return fmt.Errorf("unexpected error: %w, data=%q", err, data)
 	}
 
 	wantData := wantW.Bytes()
@@ -120,7 +120,7 @@ func testConcurrent(testFunc func() error, concurrency int) error {
 		select {
 		case err := <-ch:
 			if err != nil {
-				return fmt.Errorf("unexpected error on goroutine %d: %s", i, err)
+				return fmt.Errorf("unexpected error on goroutine %d: %w", i, err)
 			}
 		case <-time.After(time.Second):
 			return fmt.Errorf("timeout on goroutine %d", i)