Don't block traffic to old instances too early during search isolate …

…renewal. (#7045)

CHANGELOG.md

@@ -2,6 +2,7 @@ Important changes to data models, configuration, and migrations between each
 AppEngine version, listed here to ease deployment and troubleshooting.
 
 ## Next Release (replace with git tag when deployed)
+ * Fix: search isolate renewal doesn't block ongoing traffic.
 
 ## `20230914t133300-all`
  * Bumped runtimeVersion to `2023.09.12`.

app/lib/service/entrypoint/_isolate.dart

@@ -64,10 +64,13 @@ class IsolateRunner {
     required Duration wait,
   }) async {
     final isolatesToClose = [..._isolates];
+
+    await start(count);
+    // prevent traffic to hit the old instances
     for (final i in isolatesToClose) {
       i.markedForRenew = true;
     }
-    await start(count);
+
     await Future.delayed(wait);
     for (final i in isolatesToClose) {
       await i.close();

app/lib/service/entrypoint/_messages.dart

@@ -222,4 +222,7 @@ class IsolateRequestException implements Exception {
   final String message;
 
   IsolateRequestException(this.message);
+
+  @override
+  String toString() => 'IsolateRequestException: $message';
 }