fix: memory leak w/slow sources in parallel-merge and in write-to-stream (#5)

@dko-slapdash reported a issue with Promise.race() where even when the race promise resolves the callbacks it left on the unresolved promises are not garbage collected. This change no longer calls `Promise.race()` more than once on any promise by removing it completely.

Closes #4

Author: reconbot

lib/parallel-merge.ts

@@ -1,6 +1,6 @@
 /// <reference lib="esnext.asynciterable" />
 import { getIterator } from './get-iterator'
-import { AnyIterable, UnArrayAnyIterable } from './types'
+import { AnyIterable, UnArrayAnyIterable, NullOrFunction } from './types'
 
 export async function* parallelMerge<I extends Array<AnyIterable<any>>>(
   ...iterables: I
@@ -9,6 +9,35 @@ export async function* parallelMerge<I extends Array<AnyIterable<any>>>(
   const concurrentWork = new Set()
   const values = new Map()
 
+  let lastError = null
+  let errCb: NullOrFunction = null
+  let valueCb: NullOrFunction = null
+
+  const notifyError = err => {
+    lastError = err
+    if (errCb) {
+      errCb(err)
+    }
+  }
+
+  const notifyDone = value => {
+    if (valueCb) {
+      valueCb(value)
+    }
+  }
+
+  const waitForQueue = () =>
+    new Promise((resolve, reject) => {
+      if (lastError) {
+        reject(lastError)
+      }
+      if (values.size > 0) {
+        return resolve()
+      }
+      valueCb = resolve
+      errCb = reject
+    })
+
   const queueNext = input => {
     const nextVal = Promise.resolve(input.next()).then(async ({ done, value }) => {
       if (!done) {
@@ -17,17 +46,21 @@ export async function* parallelMerge<I extends Array<AnyIterable<any>>>(
       concurrentWork.delete(nextVal)
     })
     concurrentWork.add(nextVal)
+    nextVal.then(notifyDone, notifyError)
   }
 
   for (const input of inputs) {
     queueNext(input)
   }
 
   while (true) {
-    if (concurrentWork.size === 0) {
+    // We technically don't have to check `values.size` as the for loop should have emptied it
+    // However I haven't yet found specs verifying that behavior, only tests
+    // the guard in waitForQueue() checking for values is in place for the same reason
+    if (concurrentWork.size === 0 && values.size === 0) {
       return
     }
-    await Promise.race(concurrentWork)
+    await waitForQueue()
     for (const [input, value] of values) {
       values.delete(input)
       yield value

lib/types.ts

@@ -3,3 +3,4 @@ export type Iterableish<T> = Iterable<T> | Iterator<T> | AsyncIterable<T> | Asyn
 export type AnyIterable<T> = Iterable<T> | AsyncIterable<T>
 export type FlatMapValue<B> = B | AnyIterable<B> | undefined | null | Promise<B | AnyIterable<B> | undefined | null>
 export type UnArrayAnyIterable<A extends Array<AnyIterable<any>>> = A extends Array<AnyIterable<infer T>> ? T : never
+export type NullOrFunction = null | ((anything?: any) => void)

lib/write-to-stream.ts

@@ -1,5 +1,5 @@
 /// <reference lib="esnext.asynciterable" />
-import { AnyIterable } from './types'
+import { AnyIterable, NullOrFunction } from './types'
 
 interface IWritable {
   once: any
@@ -14,28 +14,52 @@ function once(event: string, stream: IWritable): Promise<any> {
 }
 
 async function _writeToStream(stream: IWritable, iterable: AnyIterable<any>): Promise<void> {
-  let errorListener
-  let error
-  const errorPromise = new Promise((resolve, reject) => {
-    errorListener = err => {
-      error = err
-      reject(err)
+  let lastError = null
+  let errCb: NullOrFunction = null
+  let drainCb: NullOrFunction = null
+
+  const notifyError = err => {
+    lastError = err
+    if (errCb) {
+      errCb(err)
     }
-    stream.once('error', errorListener)
-  }) as Promise<void>
+  }
+
+  const notifyDrain = () => {
+    if (drainCb) {
+      drainCb()
+    }
+  }
+
+  const cleanup = () => {
+    stream.removeListener('error', notifyError)
+    stream.removeListener('drain', notifyDrain)
+  }
+
+  stream.once('error', notifyError)
+
+  const waitForDrain = () =>
+    new Promise((resolve, reject) => {
+      if (lastError) {
+        return reject(lastError)
+      }
+      stream.once('drain', notifyDrain)
+      drainCb = resolve
+      errCb = reject
+    })
 
   for await (const value of iterable) {
     if (stream.write(value) === false) {
-      await Promise.race([errorPromise, once('drain', stream)])
+      await waitForDrain()
     }
-    if (error) {
-      return errorPromise
+    if (lastError) {
+      break
     }
   }
 
-  stream.removeListener('error', errorListener)
-  if (error) {
-    return errorPromise
+  cleanup()
+  if (lastError) {
+    throw lastError
   }
 }