Test at which point output writers are getting closed

Tests work as expected, so this is not the bug we're trying to find.
But having these tests in place won’t hurt.

#40

cmd/qrank-builder/item_signals_writer_test.go

@@ -12,7 +12,8 @@ import (
 
 func TestItemSignalsWriter(t *testing.T) {
 	var buf bytes.Buffer
-	w := NewItemSignalsWriter(NopWriteCloser(&buf))
+	writer := TestingWriteCloser(&buf)
+	w := NewItemSignalsWriter(writer)
 	for _, s := range []ItemSignals{
 		ItemSignals{72, 1, 2, 3, 4, 5},
 		ItemSignals{72, 3, 3, 3, 3, 3},
@@ -22,9 +23,16 @@ func TestItemSignalsWriter(t *testing.T) {
 			t.Error(err)
 		}
 	}
+	if writer.closed {
+		t.Error("ItemSignalsWriter has prematurely closed its output writer")
+	}
 	if err := w.Close(); err != nil {
 		t.Error(err)
 	}
+	if !writer.closed {
+		t.Error("ItemSignalsWriter.Close() should close its output writer")
+	}
+
 	got := strings.Split(strings.TrimSuffix(string(buf.Bytes()), "\n"), "\n")
 	want := []string{
 		"item,pageviews_52w,wikitext_bytes,claims,identifiers,sitelinks",

cmd/qrank-builder/pagesignals_test.go

@@ -160,7 +160,8 @@ func storeFakePageSignals(id string, content string, s3 *FakeS3, t *testing.T) {
 func TestPageSignalMerger(t *testing.T) {
 	logger = log.New(&bytes.Buffer{}, "", log.Lshortfile)
 	var buf strings.Builder
-	m := NewPageSignalMerger(NopWriteCloser(&buf))
+	writer := TestingWriteCloser(&buf)
+	m := NewPageSignalMerger(writer)
 	for _, line := range []string{
 		"11,s=1111111",
 		"22,Q72",
@@ -171,9 +172,15 @@ func TestPageSignalMerger(t *testing.T) {
 			t.Error(err)
 		}
 	}
+	if writer.closed {
+		t.Errorf("PageSignalMerger.Close() closed output writer prematurely")
+	}
 	if err := m.Close(); err != nil {
 		t.Error(err)
 	}
+	if !writer.closed {
+		t.Errorf("PageSignalMerger.Close() should close output writer")
+	}
 	got := strings.Split(strings.TrimSuffix(buf.String(), "\n"), "\n")
 	want := []string{
 		"22,Q72,830167",

cmd/qrank-builder/s3_test.go

@@ -174,6 +174,27 @@ func (s3 *FakeS3) FPutObject(ctx context.Context, bucketName, objectName, filePa
 	return info, nil
 }
 
+type testingWriteCloser struct {
+	writer io.Writer
+	closed bool
+}
+
+func TestingWriteCloser(w io.Writer) *testingWriteCloser {
+	return &testingWriteCloser{writer: w}
+}
+
+func (t *testingWriteCloser) Write(p []byte) (int, error) {
+	if t.closed {
+		return 0, fmt.Errorf("already closed")
+	}
+	return t.writer.Write(p)
+}
+
+func (t *testingWriteCloser) Close() error {
+	t.closed = true
+	return nil
+}
+
 // NopWriteCloser returns a WriteCloser with a no-op Close method wrapping the
 // provided Writer w. Like io.ReadCloser but for writing.
 func NopWriteCloser(w io.Writer) io.WriteCloser {