fix(mutex): prioritize waiting exclusive locks over newly added shared tasks

app/server/core/__tests__/repository-mutex.test.ts

@@ -0,0 +1,38 @@
+import { test, describe, expect } from "bun:test";
+import { repoMutex } from "../repository-mutex";
+
+describe("RepositoryMutex", () => {
+	test("should prioritize waiting exclusive locks over new shared locks", async () => {
+		const repoId = "test-repo";
+		const results: string[] = [];
+
+		const releaseShared1 = await repoMutex.acquireShared(repoId, "backup-1");
+		results.push("acquired-shared-1");
+
+		const exclusivePromise = repoMutex.acquireExclusive(repoId, "unlock").then((release) => {
+			results.push("acquired-exclusive");
+			return release;
+		});
+
+		const shared2Promise = repoMutex.acquireShared(repoId, "backup-2").then((release) => {
+			results.push("acquired-shared-2");
+			return release;
+		});
+
+		await new Promise((resolve) => setTimeout(resolve, 50));
+
+		expect(results).toEqual(["acquired-shared-1"]);
+
+		releaseShared1();
+
+		const releaseExclusive = await exclusivePromise;
+		expect(results).toEqual(["acquired-shared-1", "acquired-exclusive"]);
+
+		releaseExclusive();
+
+		const releaseShared2 = await shared2Promise;
+		expect(results).toEqual(["acquired-shared-1", "acquired-exclusive", "acquired-shared-2"]);
+
+		releaseShared2();
+	});
+});

app/server/core/repository-mutex.ts

@@ -49,7 +49,9 @@ class RepositoryMutex {
 	async acquireShared(repositoryId: string, operation: string): Promise<() => void> {
 		const state = this.getOrCreateState(repositoryId);
 
-		if (!state.exclusiveHolder) {
+		const hasExclusiveInQueue = state.waitQueue.some((item) => item.type === "exclusive");
+
+		if (!state.exclusiveHolder && !hasExclusiveInQueue) {
 			const lockId = this.generateLockId();
 			state.sharedHolders.set(lockId, {
 				id: lockId,
@@ -60,7 +62,7 @@ class RepositoryMutex {
 		}
 
 		logger.debug(
-			`[Mutex] Waiting for shared lock on repo ${repositoryId}: ${operation} (exclusive held by: ${state.exclusiveHolder.operation})`,
+			`[Mutex] Waiting for shared lock on repo ${repositoryId}: ${operation} (exclusive held by: ${state.exclusiveHolder?.operation ?? "none"}, queue: ${state.waitQueue.length})`,
 		);
 		const lockId = await new Promise<string>((resolve) => {
 			state.waitQueue.push({ type: "shared", operation, resolve });