internal/metrics: fix detection of GCE environment

appengine.IsAppEngine mistakenly returns false, as it relies on the app
being built with the appenginevm build constraint. This is not set,
as we're supplying our own Docker image.

The playground is always running either locally or on appengine. We can
rely on metadata.OnGCE being correct.

Other calls that are made to the appengine library either use the
metadata server or environment variables, and do not rely on a
constraint being set. They also panic if the metadata server is
unavailable.

For golang/go#44822

Change-Id: I1de5fda4bfd1e909f1fd54cc44770525827a9c1a
Reviewed-on: https://go-review.googlesource.com/c/playground/+/303469
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: toothrot

internal/metrics/service.go

@@ -145,7 +145,10 @@ func GCEResource(jobName string) (*MonitoredResource, error) {
 //
 // The resource will be in StackDrvier's gae_instance type.
 func GAEResource(ctx context.Context) (*MonitoredResource, error) {
-	if !appengine.IsAppEngine() {
+	// appengine.IsAppEngine is confusingly false as we're using a custom
+	// container and building without the appenginevm build constraint.
+	// Check metadata.OnGCE instead.
+	if !metadata.OnGCE() {
 		return nil, fmt.Errorf("not running on appengine")
 	}
 	projID, err := metadata.ProjectID()

main.go

@@ -13,6 +13,7 @@ import (
 
 	"cloud.google.com/go/compute/metadata"
 	"cloud.google.com/go/datastore"
+	"golang.org/x/playground/internal/metrics"
 )
 
 var log = newStdLogger()
@@ -44,7 +45,7 @@ func main() {
 		}
 		s.log = log
 		return nil
-	})
+	}, enableMetrics)
 	if err != nil {
 		log.Fatalf("Error creating server: %v", err)
 	}
@@ -71,6 +72,21 @@ func main() {
 	log.Fatalf("Error listening on :%v: %v", port, http.ListenAndServe(":"+port, s))
 }
 
+func enableMetrics(s *server) error {
+	gr, err := metrics.GAEResource(context.Background())
+	if err != nil {
+		s.log.Printf("metrics.GAEResource() = _, %q", err)
+	}
+	ms, err := metrics.NewService(gr, views)
+	if err != nil {
+		s.log.Printf("Failed to initialize metrics: metrics.NewService() = _, %q. (not on GCP?)", err)
+	}
+	if ms != nil && !metadata.OnGCE() {
+		s.mux.Handle("/metrics", ms)
+	}
+	return nil
+}
+
 func projectID() string {
 	id, err := metadata.ProjectID()
 	if err != nil && os.Getenv("GAE_INSTANCE") != "" {

server.go

@@ -5,15 +5,12 @@
 package main
 
 import (
-	"context"
 	"fmt"
 	"net/http"
 	"os"
 	"strings"
 	"time"
 
-	"cloud.google.com/go/compute/metadata"
-	"golang.org/x/playground/internal/metrics"
 	"golang.org/x/tools/godoc/static"
 )
 
@@ -46,17 +43,6 @@ func newServer(options ...func(s *server) error) (*server, error) {
 			s.modtime = fi.ModTime()
 		}
 	}
-	gr, err := metrics.GAEResource(context.Background())
-	if err != nil {
-		s.log.Printf("metrics.GaeService() = _, %q", err)
-	}
-	ms, err := metrics.NewService(gr, views)
-	if err != nil {
-		s.log.Printf("Failed to initialize metrics: metrics.NewService() = _, %q. (not on GCP?)", err)
-	}
-	if ms != nil && !metadata.OnGCE() {
-		s.mux.Handle("/statusz", ms)
-	}
 	s.init()
 	return s, nil
 }