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Composed push iterator

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This commit is contained in:
Harvey Zuccon
2026-07-24 04:51:58 +00:00
committed by Kuldeep
parent 22db675196
commit c93ed7c09c
4 changed files with 350 additions and 30 deletions
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60
package-lock.json generated
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{

105
source/sse-session/async-push-iterator.ts Normal file
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@@ -0,0 +1,105 @@
/**
* An async iterable queue that bridges push-based producers and pull-based consumers.
*
* Composes an internal {@link ReadableStream} instead of extending it, so producers
* call {@link push} while consumers use standard async iteration (`for await...of`).
*
* ```ts
* const messages = new AsyncPushIterator<SSEvent>();
*
* // Producer (elsewhere)
* messages.push(event);
*
* // Consumer
* for await (const event of messages) {
* handle(event);
* }
* ```
*
* {@link Symbol.asyncIterator} returns `stream.values({ preventCancel: true })` so
* breaking out of `for await...of` does not cancel the underlying stream. That
* matters for long-lived sessions where the producer keeps pushing after a consumer
* stops reading early (for example, test helpers that only collect a fixed count).
*/
export class AsyncPushIterator<T> {
/** ReadableStream backing the async iterator returned from {@link Symbol.asyncIterator}. */
#stream: ReadableStream<T>;
/** Controller used to enqueue values and close the stream from {@link push} and {@link close}. */
#controller: ReadableStreamDefaultController<T> | undefined;
/** When true, no more values are accepted and iteration eventually completes. */
#closed = false;
public constructor() {
// `start`'s `this` is the underlying source object when using a plain method.
// An arrow function captures the class instance so the controller is stored here.
this.#stream = new ReadableStream({
start: (controller: ReadableStreamDefaultController<T>): void => {
this.#controller = controller;
},
});
}
/**
* Flag indicating if the iterator is closed.
*/
public get closed(): boolean {
return this.#closed;
}
/**
* Enqueues a value for the consumer.
*
* After {@link close}, pushes are silently dropped.
*
* @param value - The next value to yield from the iterator.
*/
push(value: T): void {
if (this.#closed) return;
this.#controller?.enqueue(value);
}
/**
* Causes any future interactions with the associated stream to error with {@link error}.
* Calling this will also clear the pending values immediately, so iterators that were listening will not receive them.
*
* @param error - The error to throw from the stream.
*/
error(error: Error): void {
if (this.#closed) return;
this.#closed = true;
this.#controller?.error(error);
}
/**
* Ends the stream.
*
* Marks the iterator closed so future {@link push} calls are ignored.
* Buffered values are still yielded before iteration completes.
*/
close(): void {
this.#closed = true;
try {
this.#controller?.close();
} catch {
// The reader may already have released or cancelled the stream.
}
}
/**
* Returns an async iterator over the composed stream.
*
* Uses `preventCancel: true` so early `break` from `for await...of` does not
* close the stream and block later pushes.
*
* Because values are discarded after being read, only a single consumer is supported.
* Additional consumers will receive a stream lock error. Unread values will be preserved until {@link close} is called.
*/
[Symbol.asyncIterator](): AsyncIterableIterator<T> {
return this.#stream.values({ preventCancel: true });
}
}

1
source/sse-session/index.ts
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@@ -1 +1,2 @@
export * from './async-push-iterator.ts';
export * from './sse-event-parser.ts';

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test/sse-session/async-push-iterator.test.ts Normal file
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@@ -0,0 +1,214 @@
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();