[das] allow interleaved requests in the message scheduler

Includes a new `allowOverlappingHandlers` flag to support re-disablement as discussed w/ Danny.

Bug: #60440
Change-Id: I007f0276bf8cdbfd418ea5dacb251db3eef8dd3d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419340
Reviewed-by: Brian Wilkerson <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analysis_server/lib/src/server/message_scheduler.dart

@@ -75,6 +75,9 @@ sealed class MessageObject {}
 /// and the Diagnostic server. The [MessageScheduler] acts as a hub for all
 /// incoming messages and forwards the messages to the appropriate handlers.
 final class MessageScheduler {
+  /// A flag to allow disabling overlapping message handlers.
+  static bool allowOverlappingHandlers = true;
+
   /// The [AnalysisServer] associated with the scheduler.
   late final AnalysisServer server;
 
@@ -278,7 +281,20 @@ final class MessageScheduler {
               completer,
             );
         }
-        await completer.future;
+
+        // Blocking here with an await on the future was intended to prevent unwanted
+        // interleaving but was found to cause a significant performance regression.
+        // For more context see: https://github.com/dart-lang/sdk/issues/60440.
+        // To re-disable interleaving, set [allowOverlappingHandlers] to `false`.
+        if (!allowOverlappingHandlers) {
+          await completer.future;
+        }
+
+        // NOTE that this message is not accurate if [allowOverlappingHandlers] is `true`
+        // as in that case we're not blocking anymore and the future may not be complete.
+        // TODO(pq): if not awaited, consider adding a `then` so we can track when the future completes
+        // but note that we may see some flakiness in tests as message handling gets
+        // non-deterministically interleaved.
         testView?.messageLog.add(
           '  Complete ${message.runtimeType}: ${message.toString()}',
         );

pkg/analysis_server/test/integration/server/message_scheduler_test.dart

@@ -68,6 +68,8 @@ Exit process messages loop
   }
 
   Future<void> test_multipleRequests() async {
+    if (MessageScheduler.allowOverlappingHandlers) return;
+
     var futures = <Future<void>>[];
     futures.add(setRoots(included: [workspaceRootPath], excluded: []));
     var request = ExecutionCreateContextParams(
@@ -105,6 +107,8 @@ class LspServerMessageSchedulerTest extends RefactorCodeActionsTest {
   }
 
   Future<void> test_documentChange() async {
+    if (MessageScheduler.allowOverlappingHandlers) return;
+
     const content = '''
 void f() {
   print('Test!');
@@ -178,6 +182,8 @@ Exit process messages loop
   }
 
   Future<void> test_duplicateRequests() async {
+    if (MessageScheduler.allowOverlappingHandlers) return;
+
     const content = '''
 class B {
   @^
@@ -254,6 +260,8 @@ Exit process messages loop
   }
 
   Future<void> test_multipleRequests() async {
+    if (MessageScheduler.allowOverlappingHandlers) return;
+
     const content = '''
 void main() {
   print('Hello world!!');
@@ -290,6 +298,8 @@ Exit process messages loop
   }
 
   Future<void> test_response() async {
+    if (MessageScheduler.allowOverlappingHandlers) return;
+
     const content = '''
 void f() {
   print('Test!');