Adds an exported function to flush internal reader (#1248)

Author: quentin-cha

metric/metricexport/reader.go

@@ -130,7 +130,7 @@ func (ir *IntervalReader) Start() error {
 		reportingInterval = ir.ReportingInterval
 	}
 
-	if ir.done != nil {
+	if ir.quit != nil {
 		return errAlreadyStarted
 	}
 	ir.timer = time.NewTicker(reportingInterval)
@@ -172,6 +172,19 @@ func (ir *IntervalReader) Stop() {
 	ir.quit = nil
 }
 
+// Flush flushes the metrics if IntervalReader is stopped, otherwise no-op.
+func (ir *IntervalReader) Flush() {
+	ir.mu.Lock()
+	defer ir.mu.Unlock()
+
+	// No-op if IntervalReader is not stopped
+	if ir.quit != nil {
+		return
+	}
+
+	ir.reader.ReadAndExport(ir.exporter)
+}
+
 // ReadAndExport reads metrics from all producer registered with
 // producer manager and then exports them using provided exporter.
 func (r *Reader) ReadAndExport(exporter Exporter) {

metric/metricexport/reader_test.go

@@ -117,6 +117,69 @@ func TestManualReadForIntervalReader(t *testing.T) {
 	resetExporter(exporter1)
 }
 
+func TestFlushNoOpForIntervalReader(t *testing.T) {
+	ir1 = createAndStart(exporter1, duration1, t)
+
+	gaugeEntry.Set(1)
+
+	// since IR is not stopped, flush does nothing
+	ir1.Flush()
+
+	// expect no data points
+	checkExportedCount(exporter1, 0, t)
+	checkExportedMetricDesc(exporter1, "active_request", t)
+	ir1.Stop()
+	resetExporter(exporter1)
+}
+
+func TestFlushAllowMultipleForIntervalReader(t *testing.T) {
+	ir1 = createAndStart(exporter1, duration1, t)
+
+	gaugeEntry.Set(1)
+
+	ir1.Stop()
+	ir1.Flush()
+
+	// metric is still coming in
+	gaugeEntry.Add(1)
+
+	// one more flush after IR stopped
+	ir1.Flush()
+
+	// expect 2 data point, one from each flush
+	checkExportedCount(exporter1, 2, t)
+	checkExportedValues(exporter1, []int64{1, 2}, t)
+	checkExportedMetricDesc(exporter1, "active_request", t)
+
+	resetExporter(exporter1)
+}
+
+func TestFlushRestartForIntervalReader(t *testing.T) {
+	ir1 = createAndStart(exporter1, duration1, t)
+
+	gaugeEntry.Set(1)
+	ir1.Stop()
+	ir1.Flush()
+
+	// restart the IR
+	err := ir1.Start()
+	if err != nil {
+		t.Fatalf("error starting reader %v\n", err)
+	}
+
+	gaugeEntry.Add(1)
+
+	ir1.Stop()
+	ir1.Flush()
+
+	// expect 2 data point, one from each flush
+	checkExportedCount(exporter1, 2, t)
+	checkExportedValues(exporter1, []int64{1, 2}, t)
+	checkExportedMetricDesc(exporter1, "active_request", t)
+
+	resetExporter(exporter1)
+}
+
 func TestProducerWithIntervalReaderStop(t *testing.T) {
 	ir1 = createAndStart(exporter1, duration1, t)
 	ir1.Stop()
@@ -166,7 +229,10 @@ func TestIntervalReaderMultipleStop(t *testing.T) {
 
 func TestIntervalReaderMultipleStart(t *testing.T) {
 	ir1 = createAndStart(exporter1, duration1, t)
-	ir1.Start()
+	err := ir1.Start()
+	if err == nil {
+		t.Fatalf("expected error but got nil\n")
+	}
 
 	gaugeEntry.Add(1)