1. add promiseEmitter to preserveOrder: false Promise.race to address case of: infinite concurrency + async iterable producing >1 element

2. use `!isSyncIterator` as shortcut for `isPromiseLike(next)` (`next` is promise iff iterator is async)
3. add `trySpawn` to the `returnValue === pMapSkip && preserveOrder && (promise mapping next input iterable element is pending` branch
4. add tests for changes (1) and (3)
5. tests `rangeAround` helper
6. extra `pMapSkip` tests
7. test for #76

Author: graham-fisher-ii

index.js

@@ -193,28 +193,61 @@ export function pMapIterable(
 
 	return {
 		async * [Symbol.asyncIterator]() {
-			const iterator = iterable[Symbol.asyncIterator] === undefined ? iterable[Symbol.iterator]() : iterable[Symbol.asyncIterator]();
+			const isSyncIterator = iterable[Symbol.asyncIterator] === undefined;
+			const iterator = isSyncIterator ? iterable[Symbol.iterator]() : iterable[Symbol.asyncIterator]();
 
 			const promises = [];
 			const promisesIndexFromInputIndex = {};
 			const inputIndexFromPromisesIndex = [];
 			let runningMappersCount = 0;
 			let isDone = false;
 			let inputIndex = 0;
-			let outputIndex = 0; // Only used when `preserveOrder: false`
+			let outputIndex = 0; // Only used when `preserveOrder: true`
+
+			// This event emitter prevents the race conditions that arises when:
+			//  - `preserveOrder: false`
+			//  - `promises` are added after `Promise.race` is invoked, since `Promise.race` only races the promises that existed in its input array at call time
+			// More specifically, this occurs when (in addition to `preserveOrder: false`):
+			//  - `concurrency === Number.PositiveInfinity && Number.PositiveInfinity === backpressure`
+			//    - this forces us to forgo eagerly filling the `promises` pool to avoid infinite recursion
+			//    - IMO this is the root of this problem, and a problem in and of itself: we should consider requiring a finite concurrency & backpressure
+			//    - given the inability to eagerly filing the `promises` pool with infinite concurrency & backpressure, there are some situations in which specifying
+			//      a finite concurrency & backpressure will be faster than specifying the otherwise faster-sounding infinite concurrency & backpressure
+			//  - an async iterator input iterable
+			//    - `mapNext` can't `trySpawn` until it `await`s its `next`, since the input iterable might be done
+			//    - the initial `trySpawn` thus ends when the execution of `mapNext` is suspended to `await next`
+			//  - the input iterable produces more than one element
+			//    - the (single) running `mapNext`'s `trySpawn` _will necessarily_ (since concurrency and backpressure are infinite)
+			//      start another `mapNext` promise that `trySpawn` adds to `promises`
+			//    - this additional promise does not partake in the already-running `nextPromise`, because its underlying `Promise.race` began without it,
+			//      when the initial `trySpawn` returned and `nextPromise` was invoked from the main loop
+			const promiseEmitter = new EventTarget(); // Only used when `preserveOrder: false`
+			const promiseEmitterEvent = 'promiseFulfilled';
 
 			const nextPromise = preserveOrder
 				// Treat `promises` as a queue
 				? () => {
-					// May be undefined bc of `pMapSkip`s
+					// May be `undefined` bc of `pMapSkip`s
 					while (promisesIndexFromInputIndex[outputIndex] === undefined) {
 						outputIndex += 1;
 					}
 
 					return promises[promisesIndexFromInputIndex[outputIndex++]];
 				}
 				// Treat `promises` as a pool (order doesn't matter)
-				: () => Promise.race(promises);
+				: () => Promise.race([
+					// Ensures correctness in the case that mappers resolve between the time that one `await nextPromise()` resolves and the next `nextPromise` call is made
+					// (these promises would otherwise be lost if an event emitter is not listening - the `promises` pool buffers resolved promises to be processed)
+					// (I wonder if it may be actually be possible to convert the `preserveOrder: false` case to _exclusively_ event-based,
+					//  but such a solution may get messy since we'd want to `yield` from a callback, likely requiring a resolved promises buffer anyway...)
+					Promise.race(promises),
+					// Ensures correctness in the case that more promises are added to `promises` after the initial `nextPromise` call is made
+					// (these additional promises are not be included in the above `Promise.race`)
+					// (see comment above `promiseEmitter` declaration for details on when this can occur)
+					new Promise(resolve => {
+						promiseEmitter.addEventListener(promiseEmitterEvent, r => resolve(r.detail), {once: true});
+					}),
+				]);
 
 			function popPromise(inputIndex) {
 				// Swap the fulfilled promise with the last element to avoid an O(n) shift to the `promises` array
@@ -239,7 +272,7 @@ export function pMapIterable(
 				let next;
 				try {
 					next = iterator.next();
-					if (isPromiseLike(next)) {
+					if (!isSyncIterator) { // `!isSyncIterator` iff `isPromiseLike(next)`, but former is already computed
 						// Optimization: if our concurrency and/or backpressure is bounded (so that we won't infinitely recurse),
 						// and we need to `await` the next `iterator` element, we first eagerly spawn more `mapNext` promises,
 						// so that these promises can begin `await`ing their respective `iterator` elements (if needed) and `mapper` results in parallel.
@@ -250,6 +283,7 @@ export function pMapIterable(
 						// However, the time needed to `await` and ignore these `done` promises is presumed to be small relative to the time needed to perform common
 						// `async` operations like disk reads, network requests, etc.
 						// Overall, this can reduce the total time taken to process all elements.
+						// Potential TODO: in the `concurrency === Number.POSITIVE_INFINITY` case, we could potentially still optimize here by eagerly spawning some # of promises.
 						if (backpressure !== Number.POSITIVE_INFINITY) {
 							// Spawn if still below concurrency and backpressure limit
 							trySpawn();
@@ -291,12 +325,15 @@ export function pMapIterable(
 				if (returnValue === pMapSkip) {
 					// If `preserveOrder: true`, resolve to the next inputIndex's promise, in case we are already being `await`ed
 					// NOTE: no chance that `myInputIndex + 1`-spawning code is waiting to be executed in another part of the event loop,
-					// but currently `promisesIndexFromInputIndex[myInputIndex + 1] === undefined` (so that we incorrectly `mapNext` and
-					// this potentially-currently-awaited promise resolves to the result of mapping a later element than a different member of
-					// `promises`, i.e. `promises` resolve out of order), because all `trySpawn`/`mapNext` calls execute the bookkeeping synchronously,
-					// before any `await`s.
+					// but currently `promisesIndexFromInputIndex[myInputIndex + 1] === undefined` (so that we incorrectly skip this `if` condition and
+					// instead call `mapNext`, causing this potentially-currently-awaited promise to resolve to the result of mapping an element
+					// of the input iterable that was produced later `myInputIndex + 1`, i.e., no chance `promises` resolve out of order, because:
+					// all `trySpawn`/`mapNext` calls execute their bookkeeping synchronously, before any `await`s, so we cannot observe an intermediate
+					// state in which input the promise mapping iterable element `myInputIndex + 1` has not been recorded in the `promisesIndexFromInputIndex` ledger.
 					if (preserveOrder && promisesIndexFromInputIndex[myInputIndex + 1] !== undefined) {
 						popPromise(myInputIndex);
+						// Spawn if still below backpressure limit and just dropped below concurrency limit
+						trySpawn();
 						return promises[promisesIndexFromInputIndex[myInputIndex + 1]];
 					}
 
@@ -321,24 +358,26 @@ export function pMapIterable(
 				// Reserve index in `promises` array: we don't actually have the promise to save yet,
 				// but we don't want recursive `trySpawn` calls to use this same index.
 				// This is safe (i.e., the empty slot won't be `await`ed) because we replace the value immediately,
-				// without yielding to the event loop, so no consumers (namely `getAndRemoveFromPoolNextPromise`)
+				// without yielding to the event loop, so no consumers (namely `nextPromise`)
 				// can observe the intermediate state.
 				const promisesIndex = promises.length++;
 				promises[promisesIndex] = mapNext(promisesIndex);
+				promises[promisesIndex].then(p => promiseEmitter.dispatchEvent(new CustomEvent(promiseEmitterEvent, {detail: p})));
 			}
 
+			// Bootstrap `promises`
 			trySpawn();
 
 			while (promises.length > 0) {
-				const {result: {error, done, value}, inputIndex} = await nextPromise();// eslint-disable-line no-await-in-loop
+				const {result: {error, done, value}, inputIndex} = await nextPromise(); // eslint-disable-line no-await-in-loop
 				popPromise(inputIndex);
 
 				if (error) {
 					throw error;
 				}
 
 				if (done) {
-					// When `preserveOrder: false`, ignore to consume any remaining pending promises in the pool
+					// When `preserveOrder: false`, `continue` to consume any remaining pending promises in the pool
 					if (!preserveOrder) {
 						continue;
 					}

test.js

@@ -102,6 +102,10 @@ class ThrowingIterator {
 	}
 }
 
+function rangeAround(expected) {
+	return {start: expected - 5, end: expected + 50};
+}
+
 test('main', async t => {
 	const end = timeSpan();
 	t.deepEqual(await pMap(sharedInput, mapper), [10, 20, 30]);
@@ -622,9 +626,9 @@ test('pMapIterable - concurrency: 2', async t => {
 
 	assertInRange(t, times.get(10), {start: 0, end: 50});
 	assertInRange(t, times.get(20), {start: 0, end: 50});
-	assertInRange(t, times.get(30), {start: 195, end: 250});
-	assertInRange(t, times.get(40), {start: 295, end: 350});
-	assertInRange(t, times.get(50), {start: 295, end: 350});
+	assertInRange(t, times.get(30), rangeAround(200));
+	assertInRange(t, times.get(40), rangeAround(300));
+	assertInRange(t, times.get(50), rangeAround(300));
 });
 
 test('pMapIterable - backpressure', async t => {
@@ -716,6 +720,18 @@ test('pMapIterable - complex pMapSkip pattern - concurrency 2 - preserveOrder: f
 	t.assert(result.length === 8);
 });
 
+test('pMapIterable - pMapSkip + preserveOrder: true + next input mapping promise pending - eagerly spawns next promise', async t => {
+	const end = timeSpan();
+	const testData = [
+		[pMapSkip, 100],
+		[2, 200],
+		[3, 100], // Ensure 3 is spawned when pMapSkip ends (otherwise, overall runtime will be 300 ms)
+	];
+	const result = await collectAsyncIterable(pMapIterable(testData, mapper, {preserveOrder: true, concurrency: 2}));
+	assertInRange(t, end(), rangeAround(200));
+	t.deepEqual(result, [2, 3]);
+});
+
 test('pMapIterable - async iterable input', async t => {
 	const result = await collectAsyncIterable(pMapIterable(new AsyncTestData(sharedInput), mapper));
 	t.deepEqual(result, [10, 20, 30]);
@@ -759,26 +775,47 @@ function * promiseGenerator() {
 	})();
 }
 
+// Differs from `AsyncTestData` because it is not a generator:
+// each `next` yields a promise that does not wait for previous `next` promises to finish.
+const asyncIterableDoingWorkOnEachNext = (start, stop) => {
+	let i = start;
+	return {
+		[Symbol.asyncIterator]() {
+			return {
+				async next() {
+					const me = i++;
+					if (me > stop) {
+						return {done: true};
+					}
+
+					await delay(100);
+					return {done: false, value: me};
+				},
+			};
+		},
+	};
+};
+
 test('pMapIterable - eager spawn when input iterable returns promise', async t => {
 	const end = timeSpan();
-	await collectAsyncIterable(pMapIterable(promiseGenerator(), value => delay(100, {value}), {concurrency: 3}));
-	assertInRange(t, end(), {start: 195, end: 250});
+	await collectAsyncIterable(pMapIterable(asyncIterableDoingWorkOnEachNext(1, 3), value => value, /* value => delay(100, {value}), */{concurrency: 5}));
+	assertInRange(t, end(), rangeAround(100));
 });
 
 test('pMapIterable - eager spawn when input iterable returns promise incurs little overhead', async t => {
 	const end = timeSpan();
 	await collectAsyncIterable(pMapIterable(promiseGenerator(), value => delay(100, {value}), {concurrency: 100}));
-	assertInRange(t, end(), {start: 195, end: 250});
+	assertInRange(t, end(), rangeAround(200));
 });
 
 test('pMapIterable - preserveOrder: false - yields mappings as they resolve', async t => {
 	const end = timeSpan();
 	const result = await collectAsyncIterable(pMapIterable(sharedInput, mapper, {preserveOrder: false}));
 	t.deepEqual(result, [30, 20, 10]);
-	assertInRange(t, end(), {start: 295, end: 350});
+	assertInRange(t, end(), rangeAround(300));
 });
 
-test('pMapIterable - preserveOrder: false - more complex example', async t => {
+test('pMapIterable - preserveOrder: false - more complex example - sync iterable and bounded concurrency', async t => {
 	t.deepEqual(await collectAsyncIterable(pMapIterable([
 		[1, 200],
 		[2, 100],
@@ -789,6 +826,38 @@ test('pMapIterable - preserveOrder: false - more complex example', async t => {
 	], mapper, {concurrency: 3, preserveOrder: false})), [2, 3, 1, 5, 6, 4]);
 });
 
+test('pMapIterable - preserveOrder: false - more complex example - async iterable and unbounded concurrency', async t => {
+	const testData = [
+		[1, 200],
+		[2, 125],
+		[3, 150],
+		[4, 200],
+		[5, 100],
+		[6, 75],
+	];
+	async function * asyncIterable() {
+		yield * testData;
+	}
+
+	t.deepEqual(await collectAsyncIterable(pMapIterable(asyncIterable(), mapper, {concurrency: Number.POSITIVE_INFINITY, preserveOrder: false})), testData.toSorted(([_aId, aMs], [_bId, bMs]) => aMs - bMs).map(([id, _ms]) => id));
+});
+
+test('pMapIterable - preserveOrder: false - more complex example - sync promise-returning iterable and unbounded concurrency', async t => {
+	const testData = [
+		[1, 200],
+		[2, 125],
+		[3, 150],
+		[4, 225],
+		[5, 100],
+		[6, 75],
+	];
+	function * syncPromiseReturningIterable() {
+		yield * testData.map(d => Promise.resolve(d));
+	}
+
+	t.deepEqual(await collectAsyncIterable(pMapIterable(syncPromiseReturningIterable(), mapper, {concurrency: Number.POSITIVE_INFINITY, preserveOrder: false})), testData.toSorted(([_aId, aMs], [_bId, bMs]) => aMs - bMs).map(([id, _ms]) => id));
+});
+
 test('pMapIterable - preserveOrder: false - concurrency: 2', async t => {
 	const input = [100, 200, 10, 36, 13, 45];
 	const times = new Map();
@@ -799,12 +868,12 @@ test('pMapIterable - preserveOrder: false - concurrency: 2', async t => {
 		return delay(value, {value});
 	}, {concurrency: 2, backpressure: Number.POSITIVE_INFINITY, preserveOrder: false})), [100, 10, 36, 13, 200, 45]);
 
-	assertInRange(t, times.get(100), {start: 0, end: 50});
-	assertInRange(t, times.get(200), {start: 0, end: 50});
-	assertInRange(t, times.get(10), {start: times.get(100) + 100 - 5, end: times.get(100) + 100 + 50});
-	assertInRange(t, times.get(36), {start: times.get(10) + 10 - 5, end: times.get(10) + 10 + 50});
-	assertInRange(t, times.get(13), {start: times.get(36) + 36 - 5, end: times.get(36) + 36 + 50});
-	assertInRange(t, times.get(45), {start: times.get(13) + 13 - 5, end: times.get(13) + 13 + 50});
+	assertInRange(t, times.get(100), rangeAround(0));
+	assertInRange(t, times.get(200), rangeAround(0));
+	assertInRange(t, times.get(10), rangeAround(times.get(100) + 100));
+	assertInRange(t, times.get(36), rangeAround(times.get(10) + 10));
+	assertInRange(t, times.get(13), rangeAround(times.get(36) + 36));
+	assertInRange(t, times.get(45), rangeAround(times.get(13) + 13));
 });
 
 test('pMapIterable - preserveOrder: false - backpressure', async t => {
@@ -846,3 +915,27 @@ test('pMapIterable - preserveOrder: false - throws first error to settle', async
 		}, 10],
 	], mapper, {preserveOrder: false, concurrency: 2})), {message: 'bar'});
 });
+
+test('pMapIterable - {concurrency: 1, backpressure: 2} => no concurrent mappers (#76)', async t => {
+	const theLog = [];
+	const log = message => theLog.push(message);
+	const startLog = n => `${n}: mapper start`;
+	const endLog = n => `${n}: mapper end`;
+
+	async function * source() {
+		yield 1;
+		yield 2;
+		yield 3;
+	}
+
+	await collectAsyncIterable(pMapIterable(source(), async n => {
+		log(startLog(n));
+		await delay(100);
+		log(endLog(n));
+	}, {
+		concurrency: 1,
+		backpressure: 2,
+	}));
+	t.deepEqual(theLog, [startLog(1), endLog(1), startLog(2), endLog(2), startLog(3), endLog(3)]);
+});
+