fix(dev): fix frontend dev server startup race

go.work.sum

@@ -1,41 +1,82 @@
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v3/pkg/application/application_dev.go

@@ -3,6 +3,8 @@
 package application
 
 import (
+	"context"
+	"fmt"
 	"net/http"
 	"time"
 
@@ -11,30 +13,55 @@ import (
 
 var devMode = false
 
+const (
+	frontendDevServerRetryInterval = 500 * time.Millisecond
+	frontendDevServerProbeTimeout  = 2 * time.Second
+)
+
+func waitForFrontendDevServer(ctx context.Context, client *http.Client, frontendURL string, retry func()) error {
+	request, err := http.NewRequest(http.MethodGet, frontendURL, nil)
+	if err != nil {
+		return fmt.Errorf("invalid frontend dev server URL: %w", err)
+	}
+
+	for {
+		response, err := client.Do(request.Clone(ctx))
+		if err == nil {
+			response.Body.Close()
+			return nil
+		}
+
+		timer := time.NewTimer(frontendDevServerRetryInterval)
+		select {
+		case <-ctx.Done():
+			timer.Stop()
+			return ctx.Err()
+		case <-timer.C:
+			if retry != nil {
+				retry()
+			}
+		}
+	}
+}
+
 func (a *App) preRun() error {
 	// Check for frontend server url
 	frontendURL := assetserver.GetDevServerURL()
 	if frontendURL != "" {
 		devMode = true
-		// We want to check if the frontend server is running by trying to http get the url
-		// and if it is not, we wait 500ms and try again for a maximum of 10 times. If it is
-		// still not available, we return an error.
-		// This is to allow the frontend server to start up before the backend server.
-		client := http.Client{}
+		client := &http.Client{Timeout: frontendDevServerProbeTimeout}
 		a.Logger.Info("Waiting for frontend dev server to start...", "url", frontendURL)
-		for i := 0; i < 10; i++ {
-			_, err := client.Get(frontendURL)
-			if err == nil {
-				a.Logger.Info("Connected to frontend dev server!")
-				return nil
-			}
-			// Wait 500ms
-			time.Sleep(500 * time.Millisecond)
-			if i%2 == 0 {
+		retries := 0
+		err := waitForFrontendDevServer(a.Context(), client, frontendURL, func() {
+			retries++
+			if retries%2 == 1 {
 				a.Logger.Info("Retrying...")
 			}
+		})
+		if err != nil {
+			return fmt.Errorf("unable to connect to frontend server at FRONTEND_DEVSERVER_URL=%q: %w", frontendURL, err)
 		}
-		a.fatal("unable to connect to frontend server. Please check it is running - FRONTEND_DEVSERVER_URL='%s'", frontendURL)
+		a.Logger.Info("Connected to frontend dev server!")
 	}
 	return nil
 }

v3/pkg/application/application_dev_test.go

@@ -0,0 +1,82 @@
+//go:build !production
+
+package application
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+)
+
+type roundTripFunc func(*http.Request) (*http.Response, error)
+
+func (fn roundTripFunc) RoundTrip(request *http.Request) (*http.Response, error) {
+	return fn(request)
+}
+
+type trackingReadCloser struct {
+	closed bool
+}
+
+func (*trackingReadCloser) Read([]byte) (int, error) { return 0, io.EOF }
+
+func (body *trackingReadCloser) Close() error {
+	body.closed = true
+	return nil
+}
+
+func TestWaitForFrontendDevServerRetriesUntilReady(t *testing.T) {
+	body := &trackingReadCloser{}
+	attempts := 0
+	client := &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		attempts++
+		if attempts < 3 {
+			return nil, errors.New("server is still starting")
+		}
+		return &http.Response{
+			StatusCode: http.StatusServiceUnavailable,
+			Body:       body,
+			Header:     make(http.Header),
+		}, nil
+	})}
+
+	retries := 0
+	err := waitForFrontendDevServer(context.Background(), client, "http://localhost:9245", func() {
+		retries++
+	})
+	if err != nil {
+		t.Fatalf("waitForFrontendDevServer returned an error: %v", err)
+	}
+	if attempts != 3 {
+		t.Fatalf("attempt count = %d, want 3", attempts)
+	}
+	if retries != 2 {
+		t.Fatalf("retry count = %d, want 2", retries)
+	}
+	if !body.closed {
+		t.Fatal("successful probe response body was not closed")
+	}
+}
+
+func TestWaitForFrontendDevServerStopsWhenCancelled(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	client := &http.Client{Transport: roundTripFunc(func(request *http.Request) (*http.Response, error) {
+		cancel()
+		return nil, errors.New("server is unavailable")
+	})}
+
+	err := waitForFrontendDevServer(ctx, client, "http://localhost:9245", nil)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("waitForFrontendDevServer error = %v, want context.Canceled", err)
+	}
+}
+
+func TestWaitForFrontendDevServerRejectsInvalidURL(t *testing.T) {
+	err := waitForFrontendDevServer(context.Background(), http.DefaultClient, "://not-a-url", nil)
+	if err == nil || !strings.Contains(err.Error(), "invalid frontend dev server URL") {
+		t.Fatalf("waitForFrontendDevServer error = %v, want invalid URL error", err)
+	}
+}