Keep a newer queue change over a queue reply it overtook

A QueueEvent arriving in the same socket read as a QueueSyncRequest reply is
applied before the reply, which then wrote the older queue over it. Skip the
reply when a QueueEvent was applied while it was in flight.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

plugins/research_sync/instance.ts

@@ -26,6 +26,7 @@ type IpcQueue = { queue: unknown, player?: string };
 export default async function(context: InstancePluginContext) {
 	const { instance, logger, plugin } = context;
 	let syncStarted = false;
+	let queueEventsApplied = 0;
 
 	// Commands are sent one at a time so they execute in order
 	let rconQueue: Promise<unknown> = Promise.resolve();
@@ -50,7 +51,13 @@ export default async function(context: InstancePluginContext) {
 
 	// queue null asks for the cluster queue without seeding it
 	async function followQueue(queue: string[] | null, player: string | null) {
+		let applied = queueEventsApplied;
 		let result = await instance.sendTo("controller", new QueueSyncRequest(queue, player));
+		// A QueueEvent handled meanwhile is at least as new as this reply, since the controller broadcasts every
+		// queue change in order with its replies. Arriving in the same read, it is applied before the reply.
+		if (queueEventsApplied !== applied) {
+			return;
+		}
 		await applyQueue({ queue: result });
 	}
 
@@ -116,6 +123,7 @@ export default async function(context: InstancePluginContext) {
 		if (event.player !== null) {
 			data.player = event.player;
 		}
+		queueEventsApplied += 1;
 		await applyQueue(data);
 	});
 

plugins/research_sync/test/instance.js

@@ -117,6 +117,17 @@ describe("research_sync instance", function() {
 			instance.sendTo = async () => { throw new Error("gone"); };
 			await instance.hooks.controllerConnectionEvent.invoke("connect");
 		});
+		it("should not let the reply overwrite a newer QueueEvent delivered with it", async function() {
+			// As if the reply and the broadcast came in one socket read: the event is handled before the
+			// reply resolves the request's await
+			instance.sendTo = async (dst, message) => {
+				sent.push(message);
+				mock.getHandler(instance, QueueEvent)(new QueueEvent(["new"], null, "B"));
+				return ["old"];
+			};
+			await instance.hooks.controllerConnectionEvent.invoke("connect");
+			assert.equal(lastCommand(), applyCommand({ queue: ["new"], instance: "B" }));
+		});
 	});
 
 	it("should forward queue changes from the game", async function() {