more refactor and dhtrecord multiple-open support

packages/async_tools/lib/src/single_future.dart

@@ -0,0 +1,42 @@
+import 'dart:async';
+
+import 'async_tag_lock.dart';
+
+AsyncTagLock<Object> _keys = AsyncTagLock();
+
+// Process a single future at a time per tag
+//
+// The closure function is called to produce the future that is to be executed.
+// If a future with a particular tag is still executing, the onBusy callback
+// is called.
+// When a tagged singleFuture finishes executing, the onDone callback is called.
+// If an unhandled exception happens in the closure future, the onError callback
+// is called.
+void singleFuture<T>(Object tag, Future<T> Function() closure,
+    {void Function()? onBusy,
+    void Function(T)? onDone,
+    void Function(Object e, StackTrace? st)? onError}) {
+  if (!_keys.tryLock(tag)) {
+    if (onBusy != null) {
+      onBusy();
+    }
+    return;
+  }
+  unawaited(() async {
+    try {
+      final out = await closure();
+      if (onDone != null) {
+        onDone(out);
+      }
+      // ignore: avoid_catches_without_on_clauses
+    } catch (e, sp) {
+      if (onError != null) {
+        onError(e, sp);
+      } else {
+        rethrow;
+      }
+    } finally {
+      _keys.unlockTag(tag);
+    }
+  }());
+}