fix flaky metrics test

main.go

@@ -52,7 +52,7 @@ func main() {
 	http.HandleFunc("/status", Status)
 
 	handler.InitHandler()
-	metrics.SetupPrometheus()
+	metrics.SetupPrometheus(9090)
 	log.Print("Listening on port 8080")
 	log.Fatal(http.ListenAndServe(":8080", nil))
 }

metrics/metrics.go

@@ -1,6 +1,7 @@
 package metrics
 
 import (
+	"fmt"
 	"net"
 	"net/http"
 	"net/http/pprof"
@@ -81,7 +82,7 @@ func init() {
 }
 
 // SetupPrometheus sets up and runs a webserver to export prometheus metrics.
-func SetupPrometheus() *http.Server {
+func SetupPrometheus(port int) *http.Server {
 	// Define a custom serve mux for prometheus to listen on a separate port.
 	// We listen on a separate port so we can forward this port on the host VM.
 	// We cannot forward port 8080 because it is used by AppEngine.
@@ -96,15 +97,18 @@ func SetupPrometheus() *http.Server {
 	mux.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
 	mux.HandleFunc("/debug/pprof/trace", pprof.Trace)
 
+	// Don't ListenAndServe because we want to be able to GET as soon as this function returns.
+	// Listen synchronously.
+	addr := fmt.Sprintf(":%d", port)
+	listener, err := net.Listen("tcp", addr)
+	rtx.Must(err, "Could not open listening socket for Prometheus metrics")
+	port = listener.Addr().(*net.TCPAddr).Port
+
 	server := &http.Server{
-		Addr:    ":9090",
+		Addr:    fmt.Sprintf(":%d", port),
 		Handler: mux,
 	}
 
-	// Don't ListenAndServe because we want to be able to GET as soon as this function returns.
-	// Listen synchronously.
-	listener, err := net.Listen("tcp", server.Addr)
-	rtx.Must(err, "Could not open listening socket for Prometheus metrics")
 	// Serve asynchronously.
 	go server.Serve(listener.(*net.TCPListener))
 

metrics/metrics_test.go

@@ -3,6 +3,7 @@ package metrics_test
 import (
 	"bytes"
 	"io/ioutil"
+	"log"
 	"net/http"
 	"testing"
 
@@ -11,16 +12,17 @@ import (
 )
 
 func TestPrometheusMetrics(t *testing.T) {
-	server := metrics.SetupPrometheus()
+	server := metrics.SetupPrometheus(0)
 	defer server.Shutdown(nil)
+	log.Println(server.Addr)
 
-	metricReader, err := http.Get("http://localhost:9090/metrics")
+	metricReader, err := http.Get("http://localhost" + server.Addr + "/metrics")
 	if err != nil || metricReader == nil {
-		t.Errorf("Could not GET metrics: %v", err)
+		t.Fatalf("Could not GET metrics: %v", err)
 	}
 	metricBytes, err := ioutil.ReadAll(metricReader.Body)
 	if err != nil {
-		t.Errorf("Could not read metrics: %v", err)
+		t.Fatalf("Could not read metrics: %v", err)
 	}
 	metricsLinter := promlint.New(bytes.NewBuffer(metricBytes))
 	problems, err := metricsLinter.Lint()