ddtrace/tracer: add interchangeable writer to logTraceWriter (#939)

This change adds an interchangeable writer to logTraceWriter, to facilitate tests and reduce noise.

ddtrace/tracer/tracer_test.go

@@ -60,6 +60,14 @@ loop:
 	}
 }
 
+// setLogWriter sets the io.Writer that any new logTraceWriter will write to and returns a function
+// which will return the io.Writer to its original value.
+func setLogWriter(w io.Writer) func() {
+	tmp := logWriter
+	logWriter = w
+	return func() { logWriter = tmp }
+}
+
 // TestTracerCleanStop does frenetic testing in a scenario where the tracer is started
 // and stopped in parallel with spans being created.
 func TestTracerCleanStop(t *testing.T) {
@@ -89,10 +97,12 @@ func TestTracerCleanStop(t *testing.T) {
 		}()
 	}
 
+	defer setLogWriter(ioutil.Discard)()
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
 		for i := 0; i < n; i++ {
+			// Lambda mode is used to avoid the startup cost associated with agent discovery.
 			Start(withTransport(transport), WithLambdaMode(true))
 			time.Sleep(time.Millisecond)
 			Start(withTransport(transport), WithLambdaMode(true), WithSampler(NewRateSampler(0.99)))

ddtrace/tracer/writer.go

@@ -104,6 +104,9 @@ func (h *agentTraceWriter) flush() {
 	h.payload = newPayload()
 }
 
+// logWriter specifies the output target of the logTraceWriter; replaced in tests.
+var logWriter io.Writer = os.Stdout
+
 // logTraceWriter encodes traces into a format understood by the Datadog Forwarder
 // (https://github.com/DataDog/datadog-serverless-functions/tree/master/aws/logs_monitoring)
 // and writes them to os.Stdout. This is used to send traces from an AWS Lambda environment.
@@ -117,7 +120,7 @@ type logTraceWriter struct {
 func newLogTraceWriter(c *config) *logTraceWriter {
 	w := &logTraceWriter{
 		config: c,
-		w:      os.Stdout,
+		w:      logWriter,
 	}
 	w.resetBuffer()
 	return w