Restart the task scheduling scan with the initially larger window every 6 hours.

Author: isoos

app/lib/task/backend.dart

@@ -289,8 +289,14 @@ class TaskBackend {
     // Map from package to updated that has been seen.
     final seen = <String, DateTime>{};
 
+    // The scan process will give up control after 6 hours and the restart
+    // will trigger the 3-days period scan at the beginning of the loop.
+    final stopAfter = clock.fromNow(hours: 6);
+
     // In theory 30 minutes overlap should be enough. In practice we should
-    // allow an ample room for missed windows, and 3 days seems to be large enough.
+    // allow an ample room for missed windows, and 3 days seems to be large enough
+    // for the initial scan. Subsequent scans will run with a shorter (5-minutes)
+    // window.
     var since = clock.ago(days: 3);
     while (claim.valid && !abort.isCompleted) {
       // Look at all packages changed in [since]
@@ -326,6 +332,11 @@ class TaskBackend {
       // be relevant to the next iteration.
       seen.removeWhere((_, updated) => updated.isBefore(since));
 
+      // Do not start waiting if after that we would be over the stop threshold.
+      if (clock.fromNow(minutes: 10).isAfter(stopAfter)) {
+        break;
+      }
+
       // Wait until aborted or 10 minutes before scanning again!
       await abort.future.timeout(Duration(minutes: 10), onTimeout: () => null);
     }