xo-cash-utils/test/sse-session/async-push-iterator.test.ts

import { expect, test, vi } from 'vitest';

import { AsyncPushIterator } from '../../source/sse-session/async-push-iterator.ts';

/**
 * Collects every value from the iterator into an array.
 *
 * @param iterator - Iterator under test.
 */
const collectAll = async <T>(iterator: AsyncPushIterator<T>): Promise<T[]> => {
    const results: T[] = [];

    for await (const value of iterator) {
        results.push(value);
    }

    return results;
};

/**
 * Tests that values pushed while a consumer is already waiting are delivered in order.
 */
const testPushComposedPushAndConsume = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    const result = (): Promise<number[]> => collectAll(iterator);

    iterator.push(1);
    iterator.push(2);
    iterator.push(3);
    iterator.close();

    await expect(result()).resolves.toEqual([ 1, 2, 3 ]);
};

/**
 * Tests that values pushed before `for await...of` starts are buffered and yielded
 * once the consumer begins reading.
 */
const testPushComposedBuffersValuesPushedBeforeLoopStarts = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.push(1);
    iterator.push(2);
    iterator.push(3);

    const result = collectAll(iterator);

    iterator.close();

    await expect(result).resolves.toEqual([ 1, 2, 3 ]);
};

/**
 * Tests that the iterator completes with no values when nothing was pushed.
 */
const testPushComposedResolvesWithNoValues = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    const result = async (): Promise<number[]> => collectAll(iterator);

    iterator.close();

    await expect(result()).resolves.toEqual([]);
};

/**
 * Tests that values pushed after {@link AsyncPushIterator.close} are ignored.
 */
const testPushComposedIgnoresValuesAfterClose = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    const result = async (): Promise<number[]> => collectAll(iterator);

    iterator.push(1);
    iterator.push(2);
    iterator.push(3);
    iterator.close();
    iterator.push(4);

    await expect(result()).resolves.toEqual([ 1, 2, 3 ]);
};

/**
 * Tests that only one async consumer can read from the composed ReadableStream at a time.
 *
 * Unlike the hand-rolled async-push-iterator, the second consumer fails with a
 * stream lock error rather than TooManyAsyncIteratorsError.
 */
const testPushComposedRejectsMultipleConsumers = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    const failureFlag = vi.fn();

    const successfulIterator = (): Promise<number[]> => collectAll(iterator);

    const failedIterator = async (): Promise<void> => {
        try {
            /* eslint-disable-next-line */
            for await (const _value of iterator) {
            }
        } catch (error) {
            failureFlag();
        }
    };

    const promises = [ successfulIterator(), failedIterator() ];

    iterator.close();

    await Promise.all(promises);

    expect(failureFlag).toHaveBeenCalledOnce();
};

/**
 * Tests that closing before iteration starts lets the loop finish immediately.
 */
const testPushComposedResolvesWhenClosedBeforeLoop = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.close();

    await expect(collectAll(iterator)).resolves.toEqual([]);
};

/**
 * Tests that breaking out of `for await...of` early does not cancel the stream.
 *
 * {@link AsyncPushIterator} uses `preventCancel: true` so producers can keep pushing
 * and a later consumer can read the remaining values.
 */
const testPushComposedAllowsPushingAfterEarlyBreak = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.push(1);

    const firstPass: number[] = [];

    for await (const value of iterator) {
        firstPass.push(value);
        break;
    }

    iterator.push(2);
    iterator.push(3);
    iterator.close();

    const secondPass = await collectAll(iterator);

    expect(firstPass).toEqual([ 1 ]);
    expect(secondPass).toEqual([ 2, 3 ]);
};

/**
 * Tests that the iterator rejects after {@link AsyncPushIterator.error} is called.
 */
const testPushIteratorRejectsAfterError = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.error(new Error('Stream has been closed for a test'));

    await expect(collectAll(iterator)).rejects.toThrow('Stream has been closed for a test');
};

/**
 * Tests that the iterator closes the stream when error() is called.
 */
const testPushIteratorClosesWhenErrorIsCalled = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.error(new Error('Stream has been closed for a test'));
    expect(iterator.closed).toBe(true);
};

/**
 * Tests that subsequent calls to error() are ignored.
 */
const testPushIteratorIgnoresSubsequentErrorCalls = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    iterator.error(new Error('Stream has been closed for a test'));
    expect(iterator.closed).toBe(true);

    expect(iterator.error(new Error('Second error'))).toBe(undefined);
};

/**
 * Tests that a consumer can check if the iterator is closed.
 */
const testPushIteratorCanCheckIfClosed = async (): Promise<void> => {
    const iterator = new AsyncPushIterator<number>();

    expect(iterator.closed).toBe(false);

    iterator.close();
    expect(iterator.closed).toBe(true);
};

const runTests = async (): Promise<void> => {
    test('AsyncPushIterator: pushes and consumes values', testPushComposedPushAndConsume);
    test('AsyncPushIterator: buffers values pushed before the for-await loop starts', testPushComposedBuffersValuesPushedBeforeLoopStarts);
    test('AsyncPushIterator: resolves with no values when nothing was pushed', testPushComposedResolvesWithNoValues);
    test('AsyncPushIterator: ignores values pushed after close', testPushComposedIgnoresValuesAfterClose);
    test('AsyncPushIterator: rejects multiple consumers', testPushComposedRejectsMultipleConsumers);
    test('AsyncPushIterator: resolves immediately when closed before the loop starts', testPushComposedResolvesWhenClosedBeforeLoop);
    test('AsyncPushIterator: keeps the stream open after an early break', testPushComposedAllowsPushingAfterEarlyBreak);
    test('AsyncPushIterator: rejects after error', testPushIteratorRejectsAfterError);
    test('AsyncPushIterator: closes the stream when error() is called', testPushIteratorClosesWhenErrorIsCalled);
    test('AsyncPushIterator: ignores subsequent error() calls', testPushIteratorIgnoresSubsequentErrorCalls);
    test('AsyncPushIterator: can check if closed', testPushIteratorCanCheckIfClosed);
};

await runTests();