/
pool.ts
80 lines (78 loc) · 2.83 KB
/
pool.ts
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// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
export const ERROR_WHILE_MAPPING_MESSAGE = "Threw while mapping.";
/**
* pooledMap transforms values from an (async) iterable into another async
* iterable. The transforms are done concurrently, with a max concurrency
* defined by the poolLimit.
*
* If an error is thrown from `iterableFn`, no new transformations will begin.
* All currently executing transformations are allowed to finish and still
* yielded on success. After that, the rejections among them are gathered and
* thrown by the iterator in an `AggregateError`.
*
* @param poolLimit The maximum count of items being processed concurrently.
* @param array The input array for mapping.
* @param iteratorFn The function to call for every item of the array.
*/
export function pooledMap<T, R>(
poolLimit: number,
array: Iterable<T> | AsyncIterable<T>,
iteratorFn: (data: T) => Promise<R>,
): AsyncIterableIterator<R> {
// Create the async iterable that is returned from this function.
const res = new TransformStream<Promise<R>, R>({
async transform(
p: Promise<R>,
controller: TransformStreamDefaultController<R>,
) {
try {
const s = await p;
controller.enqueue(s);
} catch (e) {
if (
e instanceof AggregateError &&
e.message == ERROR_WHILE_MAPPING_MESSAGE
) {
controller.error(e as unknown);
}
}
},
});
// Start processing items from the iterator
(async () => {
const writer = res.writable.getWriter();
const executing: Array<Promise<unknown>> = [];
try {
for await (const item of array) {
const p = Promise.resolve().then(() => iteratorFn(item));
// Only write on success. If we `writer.write()` a rejected promise,
// that will end the iteration. We don't want that yet. Instead let it
// fail the race, taking us to the catch block where all currently
// executing jobs are allowed to finish and all rejections among them
// can be reported together.
writer.write(p);
const e: Promise<unknown> = p.then(() =>
executing.splice(executing.indexOf(e), 1)
);
executing.push(e);
if (executing.length >= poolLimit) {
await Promise.race(executing);
}
}
// Wait until all ongoing events have processed, then close the writer.
await Promise.all(executing);
writer.close();
} catch {
const errors = [];
for (const result of await Promise.allSettled(executing)) {
if (result.status == "rejected") {
errors.push(result.reason);
}
}
writer.write(Promise.reject(
new AggregateError(errors, ERROR_WHILE_MAPPING_MESSAGE),
)).catch(() => {});
}
})();
return res.readable[Symbol.asyncIterator]();
}