Add force option to mailbox.put

[ntkme]

This PR adds an option to force overwrite the message already in Mailbox. This is necessary for dart-sass to properly shutdown. As dart-sass is being shutting down, we send a zero length message to the worker isolate to indicate that the main isolate is planning to shutdown, that no further work shall be done. Once the worker isolate received the message and detected that the length is zero, it will break out of the loop and stop the work, in order to prevent it from running another mailbox.take and getting stuck.

https://github.com/sass/dart-sass/blob/d4b19397c493adb6e518ed5344913aa2615c11f6/lib/src/embedded/reusable_isolate.dart#L119-L129

  /// Shuts down the isolate.
  void kill() {
    _isolate.kill();
    _receivePort.close();

    // If the isolate is blocking on [Mailbox.take], it won't even process a
    // kill event, so we send an empty message to make sure it wakes up.
    try {
      _mailbox.put(Uint8List(0));
    } on StateError catch (_) {}
  }

This currently causes an issue that dart vm would lock up if sending of the empty message fails due to Mailbox already has content, and thus causes the worker isolate to not shutting down properly if it gets a non-empty message and quickly loop again to wait on the next message before the kill event is processed: sass/dart-sass#2278

I've tested locally that this PR together with the following patch in dart-sass fixes the issue: sass/dart-sass#2279

diff --git a/lib/src/embedded/reusable_isolate.dart b/lib/src/embedded/reusable_isolate.dart
index 0ed9eec8..304cb3de 100644
--- a/lib/src/embedded/reusable_isolate.dart
+++ b/lib/src/embedded/reusable_isolate.dart
@@ -123,9 +123,7 @@ class ReusableIsolate {
 
     // If the isolate is blocking on [Mailbox.take], it won't even process a
     // kill event, so we send an empty message to make sure it wakes up.
-    try {
-      _mailbox.put(Uint8List(0));
-    } on StateError catch (_) {}
+    _mailbox.put(Uint8List(0), force: true);
   }
 }
 

[ntkme]

An alternative implementation to a force put is to add a mailbox.close method which change the mailbox to a new state _stateClosed and notify. mailbox.take will throw a StateError if the mailbox changed to closed state. In dart-sass we just interrupt the loop on StateError, instead of based on an empty message. As long as we have reliable way to deliver the signal that we need to close regardless of whether mailbox is currently filled or not, we will be able to address the problem.

@mraleph @nex3 I've put up PR #26 as an alternative. Any preference on the implementation?

[nex3]

I have no particular preference.

[ntkme]

I think #26 might be slightly better, as the logic to free the buffer of dropped message looks a bit messy in this PR.

[ntkme]

Closing in favor of #26.