fix(kysely-adapter): allow concurrent queries on the D1 dialect (#11490)

Co-authored-by: Taesu <bytaesu@gmail.com>

.changeset/fix-d1-concurrent-queries.md

@@ -0,0 +1,5 @@
+---
+"@better-auth/kysely-adapter": patch
+---
+
+Allow concurrent D1 queries on a shared auth instance with Kysely 0.29, avoiding cross-request I/O errors.

e2e/smoke/test/fixtures/cloudflare/package.json

@@ -3,11 +3,13 @@
   "private": true,
   "dependencies": {
     "@better-auth/core": "workspace:*",
+    "@better-auth/kysely-adapter": "workspace:*",
     "@better-auth/sso": "workspace:*",
     "better-auth": "workspace:*",
     "hono": "^4.13.7"
   },
   "devDependencies": {
+    "kysely-029": "npm:kysely@0.29.6",
     "vitest": "catalog:vitest",
     "wrangler": "4.123.0"
   },

e2e/smoke/test/fixtures/cloudflare/src/index.ts

@@ -1,3 +1,4 @@
+import { env } from "cloudflare:workers";
 import type { AuthEndpointContext } from "@better-auth/core/context";
 import {
 	getCurrentAdapter,
@@ -9,12 +10,48 @@ import type {
 	DBAdapter,
 	DBTransactionAdapter,
 } from "@better-auth/core/db/adapter";
+import { createKyselyAdapter } from "@better-auth/kysely-adapter";
 import { getMigrations } from "better-auth/db/migration";
 import { Hono } from "hono";
 import { auth } from "./auth";
 
 const app = new Hono();
 
+let slowQueryStarted = false;
+let releaseSlowQuery = false;
+const delayedDatabase = {
+	prepare(query: string) {
+		const statement = env.DB.prepare(query);
+		return {
+			bind(...parameters: unknown[]) {
+				const bound = statement.bind(...parameters);
+				return {
+					async all() {
+						if (query.includes("slow_marker")) {
+							slowQueryStarted = true;
+							try {
+								while (!releaseSlowQuery) {
+									await new Promise((resolve) => setTimeout(resolve, 10));
+								}
+							} finally {
+								slowQueryStarted = false;
+								releaseSlowQuery = false;
+							}
+						}
+						return bound.all();
+					},
+				};
+			},
+		};
+	},
+	batch: (...statements: D1PreparedStatement[]) => env.DB.batch(statements),
+	exec: (query: string) => env.DB.exec(query),
+} as unknown as D1Database;
+const { kysely: concurrencyDatabase } = await createKyselyAdapter({
+	database: delayedDatabase,
+});
+if (!concurrencyDatabase) throw new Error("D1 test database was not created");
+
 app.all("/api/auth/*", (c) => auth.handler(c.req.raw));
 
 app.get("/_test/session", async (c) => {
@@ -24,6 +61,26 @@ app.get("/_test/session", async (c) => {
 	return c.json(session);
 });
 
+app.get("/_test/d1-concurrency/:query", async (c) => {
+	if (c.req.param("query") === "status") {
+		return c.json({ slowQueryStarted });
+	}
+	if (c.req.param("query") === "release") {
+		releaseSlowQuery = true;
+		return c.body(null, 204);
+	}
+
+	const slow = c.req.param("query") === "slow";
+	const query = concurrencyDatabase.selectFrom("sqlite_master");
+	await (slow
+		? query.select("name as slow_marker")
+		: query.select("name as fast_marker")
+	)
+		.limit(1)
+		.execute();
+	return c.body(null, 204);
+});
+
 app.post("/_test/migrate", async (c) => {
 	const { runMigrations } = await getMigrations(auth.options);
 	await runMigrations();

e2e/smoke/test/fixtures/cloudflare/test/d1-concurrency.test.ts

@@ -0,0 +1,50 @@
+import { afterAll, afterEach, beforeAll, expect, it, vi } from "vitest";
+import { createTestHarness } from "wrangler";
+
+const server = createTestHarness({
+	workers: [{ configPath: "./wrangler.json" }],
+});
+
+beforeAll(async () => {
+	await server.listen();
+});
+
+afterEach(({ task }) => {
+	if (task.result?.state === "fail") server.debug();
+});
+
+afterAll(async () => {
+	await server.close();
+});
+
+/**
+ * @see https://github.com/better-auth/better-auth/issues/11483
+ */
+it("lets independent D1 requests complete while another query is pending", async () => {
+	const slowRequest = server.fetch(
+		"http://localhost:8787/_test/d1-concurrency/slow",
+	);
+	let slowStatus: number;
+	try {
+		await vi.waitFor(
+			async () => {
+				const response = await server.fetch(
+					"http://localhost:8787/_test/d1-concurrency/status",
+				);
+				expect(await response.json()).toEqual({ slowQueryStarted: true });
+			},
+			{ timeout: 5000 },
+		);
+
+		const fastResponse = await server.fetch(
+			"http://localhost:8787/_test/d1-concurrency/fast",
+			{ signal: AbortSignal.timeout(5000) },
+		);
+		expect(fastResponse.status).toBe(204);
+	} finally {
+		await server.fetch("http://localhost:8787/_test/d1-concurrency/release");
+		slowStatus = (await slowRequest).status;
+	}
+
+	expect(slowStatus).toBe(204);
+}, 15_000);

e2e/smoke/test/fixtures/cloudflare/wrangler.json

@@ -5,6 +5,9 @@
   "name": "cloudflare",
   "upload_source_maps": true,
   "compatibility_flags": ["nodejs_compat"],
+  "alias": {
+    "kysely": "kysely-029"
+  },
   "d1_databases": [
     {
       "binding": "DB",

packages/kysely-adapter/src/d1-sqlite-dialect.ts

@@ -81,7 +81,11 @@ export function createD1IndexIntrospector(
 	};
 }
 
-class D1SqliteAdapter extends SqliteAdapter {}
+class D1SqliteAdapter extends SqliteAdapter {
+	get supportsMultipleConnections(): boolean {
+		return true;
+	}
+}
 
 /**
  * Config for the D1 SQLite dialect.

pnpm-lock.yaml

@@ -887,6 +887,9 @@ importers:
       '@better-auth/core':
         specifier: workspace:*
         version: link:../../../../../packages/core
+      '@better-auth/kysely-adapter':
+        specifier: workspace:*
+        version: link:../../../../../packages/kysely-adapter
       '@better-auth/sso':
         specifier: workspace:*
         version: link:../../../../../packages/sso
@@ -897,6 +900,9 @@ importers:
         specifier: ^4.13.7
         version: 4.13.7
     devDependencies:
+      kysely-029:
+        specifier: npm:kysely@0.29.6
+        version: kysely@0.29.6
       vitest:
         specifier: catalog:vitest
         version: 5.0.0(@opentelemetry/api@1.9.1)(@types/node@25.9.4)(@vitest/coverage-istanbul@5.0.0)(@vitest/ui@5.0.0)(happy-dom@20.8.9)(msw@2.14.6(@types/node@25.9.4)(typescript@6.0.3))(vite@8.2.0(@types/node@25.9.4)(esbuild@0.28.1)(jiti@2.7.0)(terser@5.46.1)(tsx@4.21.0)(yaml@2.9.0))
@@ -11275,6 +11281,10 @@ packages:
     resolution: {integrity: sha512-VHtBdW6XB/pgoTSqraM3UAa2rYoYdNXqnNPpX+8XXP+cwYbVEFuAp3HyPt1vpNfU9l7Y2kpUrA9QDPsy8uUqOQ==}
     engines: {node: '>=22.0.0'}
 
+  kysely@0.29.6:
+    resolution: {integrity: sha512-hHaB8C/rfzDDtr/t8YZwxAuPJTT0zHyaPoVzcXwDYhYNAgH/4sIfVhi/XLLIY+bL/FqaIJnjATDbi8ObSELmxg==}
+    engines: {node: '>=22.0.0'}
+
   lan-network@0.1.7:
     resolution: {integrity: sha512-mnIlAEMu4OyEvUNdzco9xpuB9YVcPkQec+QsgycBCtPZvEqWPCDPfbAE4OJMdBBWpZWtpCn1xw9jJYlwjWI5zQ==}
     hasBin: true
@@ -27524,6 +27534,8 @@ snapshots:
 
   kysely@0.29.3: {}
 
+  kysely@0.29.6: {}
+
   lan-network@0.1.7: {}
 
   launch-editor@2.14.1: