[GitHub] [arrow] westonpace commented on a diff in pull request #14524: ARROW-17509: [C++] Simplify async scheduler by removing the need to call End

[GitBox <gi...@apache.org>]

westonpace commented on code in PR #14524:
URL: https://github.com/apache/arrow/pull/14524#discussion_r1008434063


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cpp/src/arrow/util/async_util.cc:
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@@ -108,30 +108,37 @@ class AlreadyFailedScheduler : public AsyncTaskScheduler {
     std::ignore = std::move(finish_callback_)(failure_reason_);
   }
   bool AddTask(std::unique_ptr<Task> task) override { return false; }
-  void End() override {
-    Status::UnknownError("Do not call End on a sub-scheduler.").Abort();
-  }
-  Future<> OnFinished() const override {
-    Status::UnknownError(
-        "You should not rely on sub-scheduler's OnFinished.  Use a "
-        "finished callback when creating the sub-scheduler instead")
-        .Abort();
-  }
-  std::shared_ptr<AsyncTaskScheduler> MakeSubScheduler(
+  std::unique_ptr<AsyncTaskScheduler::Holder> MakeSubScheduler(
+      FnOnce<Status(AsyncTaskScheduler*)> initial_task,

Review Comment:
   Yes.  `initial_task` is always run synchronously as part of the call to `MakeSubScheduler` (so it should be safe, e.g. to use `[&]`).  Furthermore, when you add a task to a scheduler that does not have a throttle, the task submitter portion will always run synchronously as part of the call to `AddTask`.  I'll update the docs to make this clearer.



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