Make browser sidecar on-demand (#44)

backend/internal/latest/browser.go

@@ -178,6 +178,36 @@ func (f *BrowserFetcher) Image(ctx context.Context, imageID string) ([]byte, str
 // the poller answers with a 403 and its ordinary cooldown.
 var errChallengeHeld = errors.New("challenge held")
 
+// errBrowserInterrupted distinguishes a remote Chrome restart from the
+// caller's own deadline. chromedp reports both as context.Canceled.
+var errBrowserInterrupted = errors.New("browser interrupted")
+
+func classifyBrowserError(ctx context.Context, browserLost bool, err error) error {
+	if err == nil || ctx.Err() != nil {
+		return err
+	}
+	if !browserLost {
+		return err
+	}
+	if !errors.Is(err, context.Canceled) {
+		return err
+	}
+	return fmt.Errorf("%w: %w", errBrowserInterrupted, err)
+}
+
+func browserConnectionLost(ctx context.Context) bool {
+	c := chromedp.FromContext(ctx)
+	if c == nil || c.Browser == nil {
+		return true
+	}
+	select {
+	case <-c.Browser.LostConnection:
+		return true
+	default:
+		return false
+	}
+}
+
 // challengePollInterval paces re-reads while a challenge solves itself.
 const challengePollInterval = 2 * time.Second
 
@@ -203,6 +233,7 @@ func (f *BrowserFetcher) run(ctx context.Context, target string, read chromedp.A
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
+	callerCtx := ctx
 	ctx, cancel := context.WithTimeout(ctx, challengeTimeout)
 	defer cancel()
 	tabCtx, cancelTab := chromedp.NewContext(f.allocCtx)
@@ -219,20 +250,26 @@ func (f *BrowserFetcher) run(ctx context.Context, target string, read chromedp.A
 		chromedp.Navigate(target),
 		chromedp.WaitReady("body", chromedp.ByQuery),
 	); err != nil {
-		return err
+		return classifyBrowserError(callerCtx, browserConnectionLost(tabCtx), err)
 	}
-
 	var lastErr error
 	for {
 		// The challenge reloads the page when it passes, which tears down the
 		// execution context mid-read. That is a retry, not a failure.
 		if err := chromedp.Run(tabCtx, read); err != nil {
+			err = classifyBrowserError(callerCtx, browserConnectionLost(tabCtx), err)
+			if errors.Is(err, errBrowserInterrupted) {
+				return err
+			}
 			lastErr = err
 		} else if done() {
 			return nil
 		}
 		select {
 		case <-ctx.Done():
+			if err := callerCtx.Err(); err != nil {
+				return err
+			}
 			if lastErr != nil {
 				return fmt.Errorf("%w (last read: %v)", errChallengeHeld, lastErr)
 			}