Write errors are not returned from WriteTo for short compressed files

[laurynasl-uber]

It seems that write errors from Decoder.WriteTo are not returned when the compressed input is short. This causes io.Copy calls to wrongly return success, even though a write has failed. See the test case below for an example.

type failingWriter struct {
	err error
}

func (f failingWriter) Write(_ []byte) (n int, err error) {
	return 0, f.err
}

func TestErrorWriter(t *testing.T) {
	input := make([]byte, 100)
	cmp := bytes.Buffer{}
	w, err := NewWriter(&cmp)
	if err != nil {
		t.Fatal(err)
	}
	defer w.Close()
	_, _ = rand.Read(input)
	_, err = w.Write(input)
	if err != nil {
		t.Fatal(err)
	}
	wantErr := fmt.Errorf("i'm a failure")
	zr, err := NewReader(&cmp)
	defer zr.Close()
	out := failingWriter{err: wantErr}
	_, err = zr.WriteTo(out)
	if !errors.Is(err, wantErr) {
		t.Errorf("error: wanted: %v, got: %v", wantErr, err)
	}
}

Test output:

=== RUN   TestErrorWriter
    decoder_test.go:195: error: wanted: i'm a failure, got: <nil>
--- FAIL: TestErrorWriter (0.00s)

[klauspost]

I am AFC and will be for a few weeks. Just to confirm, you are using the latest version? I seem to recall something like this a while back.

[laurynasl-uber]

Yes, the test output included is from running on latest master

[laurynasl-uber]

Thanks!