playground: only build binaries for health check

This change avoids cascading sandbox backend failures to the frontend
instances.

The timeout TODO is safe to remove as we now pass in the request context
for cancellation, and sandboxBuild has its own build timeout.

Updates golang/go#25224
Updates golang/go#38530

Change-Id: If892f86bad08c55429b6ebab768b83c5d7621cf1
Reviewed-on: https://go-review.googlesource.com/c/playground/+/229677
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: toothrot

sandbox.go

@@ -593,17 +593,20 @@ func playgroundGoproxy() string {
 	return "https://proxy.golang.org"
 }
 
-func (s *server) healthCheck() error {
-	ctx := context.Background() // TODO: cap it to some reasonable timeout
-	resp, err := compileAndRun(ctx, &request{Body: healthProg})
+// healthCheck attempts to build a binary from the source in healthProg.
+// It returns any error returned from sandboxBuild, or nil if none is returned.
+func (s *server) healthCheck(ctx context.Context) error {
+	tmpDir, err := ioutil.TempDir("", "sandbox")
 	if err != nil {
-		return err
+		return fmt.Errorf("error creating temp directory: %v", err)
 	}
-	if resp.Errors != "" {
-		return fmt.Errorf("compile error: %v", resp.Errors)
+	defer os.RemoveAll(tmpDir)
+	br, err := sandboxBuild(ctx, tmpDir, []byte(healthProg), false)
+	if err != nil {
+		return err
 	}
-	if len(resp.Events) != 1 || resp.Events[0].Message != "ok" {
-		return fmt.Errorf("unexpected output: %v", resp.Events)
+	if br.errorMessage != "" {
+		return errors.New(br.errorMessage)
 	}
 	return nil
 }

server.go

@@ -75,7 +75,7 @@ func handleFavicon(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *server) handleHealthCheck(w http.ResponseWriter, r *http.Request) {
-	if err := s.healthCheck(); err != nil {
+	if err := s.healthCheck(r.Context()); err != nil {
 		http.Error(w, "Health check failed: "+err.Error(), http.StatusInternalServerError)
 		return
 	}

tests.go

@@ -37,7 +37,7 @@ func (s *server) test() {
 }
 
 func (s *server) runTests() {
-	if err := s.healthCheck(); err != nil {
+	if err := s.healthCheck(context.Background()); err != nil {
 		stdlog.Fatal(err)
 	}