fix: retry login across deploy cutovers

src/lib/auth/sign-in-retry.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it, vi } from "vitest";
+import { signInWithTransientRetry } from "./sign-in-retry";
+
+describe("signInWithTransientRetry", () => {
+	it("retries thrown transport errors with a short backoff", async () => {
+		const transportError = new SyntaxError("unexpected character in JSON");
+		const signIn = vi
+			.fn<() => Promise<{ ok: boolean }>>()
+			.mockRejectedValueOnce(transportError)
+			.mockRejectedValueOnce(transportError)
+			.mockResolvedValue({ ok: true });
+		const wait = vi.fn(async () => undefined);
+
+		await expect(signInWithTransientRetry(signIn, wait)).resolves.toEqual({
+			ok: true,
+		});
+		expect(signIn).toHaveBeenCalledTimes(3);
+		expect(wait).toHaveBeenNthCalledWith(1, 400);
+		expect(wait).toHaveBeenNthCalledWith(2, 800);
+	});
+
+	it("returns ordinary auth responses without retrying", async () => {
+		const response = { ok: false, error: "CredentialsSignin" };
+		const signIn = vi.fn(async () => response);
+		const wait = vi.fn(async () => undefined);
+
+		await expect(signInWithTransientRetry(signIn, wait)).resolves.toBe(
+			response,
+		);
+		expect(signIn).toHaveBeenCalledOnce();
+		expect(wait).not.toHaveBeenCalled();
+	});
+
+	it("rethrows the last transport error after four attempts", async () => {
+		const transportError = new Error("temporary upstream failure");
+		const signIn = vi
+			.fn<() => Promise<never>>()
+			.mockRejectedValue(transportError);
+		const wait = vi.fn(async () => undefined);
+
+		await expect(signInWithTransientRetry(signIn, wait)).rejects.toBe(
+			transportError,
+		);
+		expect(signIn).toHaveBeenCalledTimes(4);
+		expect(wait).toHaveBeenCalledTimes(3);
+	});
+});