Add ReadableStream.from(asyncIterable)

This static method takes an async iterable and returns a ReadableStream pulling chunks from that async iterable. Sync iterables (including arrays and generators) are also supported, since GetIterator() already has all the necessary handling to adapt a sync iterator into an async iterator.

Closes #1018.

index.bs

@@ -45,6 +45,7 @@ urlPrefix: https://tc39.es/ecma262/; spec: ECMASCRIPT
   text: Number type; url: #sec-ecmascript-language-types-number-type
   text: Data Block; url: #sec-data-blocks
  type: abstract-op
+  text: Call; url: #sec-call
   text: CloneArrayBuffer; url: #sec-clonearraybuffer
   text: CopyDataBlockBytes; url: #sec-copydatablockbytes
   text: CreateArrayFromList; url: #sec-createarrayfromlist
@@ -53,9 +54,14 @@ urlPrefix: https://tc39.es/ecma262/; spec: ECMASCRIPT
   text: Construct; url: #sec-construct
   text: DetachArrayBuffer; url: #sec-detacharraybuffer
   text: Get; url: #sec-get-o-p
+  text: GetIterator; url: #sec-getiterator
+  text: GetMethod; url: #sec-getmethod
   text: GetV; url: #sec-getv
   text: IsDetachedBuffer; url: #sec-isdetachedbuffer
   text: IsInteger; url: #sec-isinteger
+  text: IteratorComplete; url: #sec-iteratorcomplete
+  text: IteratorNext; url: #sec-iteratornext
+  text: IteratorValue; url: #sec-iteratorvalue
   text: OrdinaryObjectCreate; url: #sec-ordinaryobjectcreate
   text: SameValue; url: #sec-samevalue
   text: Type; url: #sec-ecmascript-data-types-and-values
@@ -478,6 +484,8 @@ The Web IDL definition for the {{ReadableStream}} class is given as follows:
 interface ReadableStream {
   constructor(optional object underlyingSource, optional QueuingStrategy strategy = {});
 
+  static ReadableStream from(any asyncIterable);
+
   readonly attribute boolean locked;
 
   Promise<undefined> cancel(optional any reason);
@@ -808,6 +816,13 @@ option. If {{UnderlyingSource/type}} is set to undefined (including via omission
      |underlyingSource|, |underlyingSourceDict|, |highWaterMark|, |sizeAlgorithm|).
 </div>
 
+<div algorithm>
+ The static <dfn id="rs-from" method for="ReadableStream">from(|asyncIterable|)</dfn> method steps
+ are:
+
+ 1. Return ? [$ReadableStreamFromIterable$](|asyncIterable|).
+</div>
+
 <div algorithm>
  The <dfn id="rs-locked" attribute for="ReadableStream">locked</dfn> getter steps are:
 
@@ -2095,6 +2110,49 @@ The following abstract operations operate on {{ReadableStream}} instances at a h
  1. Return true.
 </div>
 
+<div algorithm>
+ <dfn abstract-op lt="ReadableStreamFromIterable" id="readable-stream-from-iterable">
+ ReadableStreamFromIterable(|asyncIterable|)</dfn> performs the following steps:
+
+ 1. Let |stream| be undefined.
+ 1. Let |iteratorRecord| be ? [$GetIterator$](|asyncIterable|, async).
+ 1. Let |startAlgorithm| be an algorithm that returns undefined.
+ 1. Let |pullAlgorithm| be the following steps:
+  1. Let |nextResult| be [$IteratorNext$](|iteratorRecord|).
+  1. If |nextResult| is an abrupt completion, return [=a promise rejected with=]
+     |nextResult|.\[[Value]].
+  1. Let |nextPromise| be [=a promise resolved with=] |nextResult|.\[[Value]].
+  1. Return the result of [=reacting=] to |nextPromise| with the following fulfillment steps,
+     given |iterResult|:
+   1. If [$Type$](|iterResult|) is not Object, throw a {{TypeError}}.
+   1. Let |done| be ? [$IteratorComplete$](|iterResult|).
+   1. If |done| is true:
+    1. Perform ! [$ReadableStreamDefaultControllerClose$](|stream|.[=ReadableStream/[[controller]]=]).
+   1. Otherwise:
+    1. Let |value| be ? [$IteratorValue$](|iterResult|).
+    1. Perform ! [$ReadableStreamDefaultControllerEnqueue$](|stream|.[=ReadableStream/[[controller]]=],
+       |value|).
+       <!-- TODO (future): If we allow changing the queuing strategy, this Enqueue might throw.
+             We'll then need to catch the error and close the async iterator. -->
+ 1. Let |cancelAlgorithm| be the following steps, given |reason|:
+  1. Let |iterator| be |iteratorRecord|.\[[Iterator]].
+  1. Let |returnMethod| be [$GetMethod$](|iterator|, "`return`").
+  1. If |returnMethod| is an abrupt completion, return [=a promise rejected with=]
+     |returnMethod|.\[[Value]].
+  1. If |returnMethod|.\[[Value]] is undefined, return [=a promise resolved with=] undefined.
+  1. Let |returnResult| be [$Call$](|returnMethod|.\[[Value]], |iterator|, « |reason| »).
+  1. If |returnResult| is an abrupt completion, return [=a promise rejected with=]
+     |returnResult|.\[[Value]].
+  1. Let |returnPromise| be [=a promise resolved with=] |returnResult|.\[[Value]].
+  1. Return the result of [=reacting=] to |returnPromise| with the following fulfillment steps,
+     given |iterResult|:
+   1. If [$Type$](|iterResult|) is not Object, throw a {{TypeError}}.
+   1. Return undefined.
+ 1. Set |stream| to ! [$CreateReadableStream$](|startAlgorithm|, |pullAlgorithm|, |cancelAlgorithm|,
+    0).
+ 1. Return |stream|.
+</div>
+
 <div algorithm="ReadableStreamPipeTo">
  <dfn abstract-op lt="ReadableStreamPipeTo"
  id="readable-stream-pipe-to">ReadableStreamPipeTo(|source|, |dest|, |preventClose|, |preventAbort|,

reference-implementation/lib/ReadableStream-impl.js

@@ -151,6 +151,10 @@ exports.implementation = class ReadableStreamImpl {
     aos.ReadableStreamDefaultReaderRelease(reader);
     return promiseResolvedWith(undefined);
   }
+
+  static from(asyncIterable) {
+    return aos.ReadableStreamFromIterable(asyncIterable);
+  }
 };
 
 // See pipeTo()/pipeThrough() for why this is needed.

reference-implementation/lib/ReadableStream.webidl

@@ -2,6 +2,8 @@
 interface ReadableStream {
   constructor(optional object underlyingSource, optional QueuingStrategy strategy = {});
 
+  static ReadableStream from(any asyncIterable);
+
   readonly attribute boolean locked;
 
   Promise<undefined> cancel(optional any reason);

reference-implementation/lib/abstract-ops/ecmascript.js

@@ -3,6 +3,8 @@ const assert = require('assert');
 
 const isFakeDetached = Symbol('is "detached" for our purposes');
 
+exports.typeIsObject = x => (typeof x === 'object' && x !== null) || typeof x === 'function';
+
 exports.CreateArrayFromList = elements => {
   // We use arrays to represent lists, so this is basically a no-op.
   // Do a slice though just in case we happen to depend on the unique-ness.
@@ -39,3 +41,84 @@ exports.CanTransferArrayBuffer = O => {
 exports.IsDetachedBuffer = O => {
   return isFakeDetached in O;
 };
+
+exports.Call = (F, V, args = []) => {
+  if (typeof F !== 'function') {
+    throw new TypeError('Argument is not a function');
+  }
+
+  return Reflect.apply(F, V, args);
+};
+
+exports.GetMethod = (V, P) => {
+  const func = V[P];
+  if (func === undefined || func === null) {
+    return undefined;
+  }
+  if (typeof func !== 'function') {
+    throw new TypeError(`${P} is not a function`);
+  }
+  return func;
+};
+
+exports.CreateAsyncFromSyncIterator = syncIteratorRecord => {
+  // Instead of re-implementing CreateAsyncFromSyncIterator and %AsyncFromSyncIteratorPrototype%,
+  // we use yield* inside an async generator function to achieve the same result.
+
+  // Wrap the sync iterator inside a sync iterable, so we can use it with yield*.
+  const syncIterable = {
+    [Symbol.iterator]: () => syncIteratorRecord.iterator
+  };
+  // Create an async generator function and immediately invoke it.
+  const asyncIterator = (async function* () {
+    return yield* syncIterable;
+  }());
+  // Return as an async iterator record.
+  const nextMethod = asyncIterator.next;
+  return { iterator: asyncIterator, nextMethod, done: false };
+};
+
+exports.GetIterator = (obj, hint = 'sync', method) => {
+  assert(hint === 'sync' || hint === 'async');
+  if (method === undefined) {
+    if (hint === 'async') {
+      method = exports.GetMethod(obj, Symbol.asyncIterator);
+      if (method === undefined) {
+        const syncMethod = exports.GetMethod(obj, Symbol.iterator);
+        const syncIteratorRecord = exports.GetIterator(obj, 'sync', syncMethod);
+        return exports.CreateAsyncFromSyncIterator(syncIteratorRecord);
+      }
+    } else {
+      method = exports.GetMethod(obj, Symbol.iterator);
+    }
+  }
+  const iterator = exports.Call(method, obj);
+  if (!exports.typeIsObject(iterator)) {
+    throw new TypeError('The iterator method must return an object');
+  }
+  const nextMethod = iterator.next;
+  return { iterator, nextMethod, done: false };
+};
+
+exports.IteratorNext = (iteratorRecord, value) => {
+  let result;
+  if (value === undefined) {
+    result = exports.Call(iteratorRecord.nextMethod, iteratorRecord.iterator);
+  } else {
+    result = exports.Call(iteratorRecord.nextMethod, iteratorRecord.iterator, [value]);
+  }
+  if (!exports.typeIsObject(result)) {
+    throw new TypeError('The iterator.next() method must return an object');
+  }
+  return result;
+};
+
+exports.IteratorComplete = iterResult => {
+  assert(exports.typeIsObject(iterResult));
+  return Boolean(iterResult.done);
+};
+
+exports.IteratorValue = iterResult => {
+  assert(exports.typeIsObject(iterResult));
+  return iterResult.value;
+};

reference-implementation/lib/abstract-ops/readable-streams.js

@@ -4,8 +4,8 @@ const assert = require('assert');
 const { promiseResolvedWith, promiseRejectedWith, newPromise, resolvePromise, rejectPromise, uponPromise,
         setPromiseIsHandledToTrue, waitForAllPromise, transformPromiseWith, uponFulfillment, uponRejection } =
   require('../helpers/webidl.js');
-const { CanTransferArrayBuffer, CopyDataBlockBytes, CreateArrayFromList, IsDetachedBuffer, TransferArrayBuffer } =
-  require('./ecmascript.js');
+const { CanTransferArrayBuffer, Call, CopyDataBlockBytes, CreateArrayFromList, GetIterator, GetMethod, IsDetachedBuffer,
+        IteratorComplete, IteratorNext, IteratorValue, TransferArrayBuffer, typeIsObject } = require('./ecmascript.js');
 const { CloneAsUint8Array, IsNonNegativeNumber } = require('./miscellaneous.js');
 const { EnqueueValueWithSize, ResetQueue } = require('./queue-with-sizes.js');
 const { AcquireWritableStreamDefaultWriter, IsWritableStreamLocked, WritableStreamAbort,
@@ -55,6 +55,7 @@ Object.assign(exports, {
   ReadableStreamDefaultControllerHasBackpressure,
   ReadableStreamDefaultReaderRead,
   ReadableStreamDefaultReaderRelease,
+  ReadableStreamFromIterable,
   ReadableStreamGetNumReadRequests,
   ReadableStreamHasDefaultReader,
   ReadableStreamPipeTo,
@@ -1879,3 +1880,61 @@ function SetUpReadableByteStreamControllerFromUnderlyingSource(
     stream, controller, startAlgorithm, pullAlgorithm, cancelAlgorithm, highWaterMark, autoAllocateChunkSize
   );
 }
+
+function ReadableStreamFromIterable(asyncIterable) {
+  let stream;
+  const iteratorRecord = GetIterator(asyncIterable, 'async');
+
+  const startAlgorithm = () => undefined;
+
+  function pullAlgorithm() {
+    let nextResult;
+    try {
+      nextResult = IteratorNext(iteratorRecord);
+    } catch (e) {
+      return promiseRejectedWith(e);
+    }
+    const nextPromise = promiseResolvedWith(nextResult);
+    return transformPromiseWith(nextPromise, iterResult => {
+      if (!typeIsObject(iterResult)) {
+        throw new TypeError('The promise returned by the iterator.next() method must fulfill with an object');
+      }
+      const done = IteratorComplete(iterResult);
+      if (done === true) {
+        ReadableStreamDefaultControllerClose(stream._controller);
+      } else {
+        const value = IteratorValue(iterResult);
+        ReadableStreamDefaultControllerEnqueue(stream._controller, value);
+      }
+    });
+  }
+
+  function cancelAlgorithm(reason) {
+    const iterator = iteratorRecord.iterator;
+    let returnMethod;
+    try {
+      returnMethod = GetMethod(iterator, 'return');
+    } catch (e) {
+      return promiseRejectedWith(e);
+    }
+    if (returnMethod === undefined) {
+      return promiseResolvedWith(undefined);
+    }
+    let returnResult;
+    try {
+      returnResult = Call(returnMethod, iterator, [reason]);
+    } catch (e) {
+      return promiseRejectedWith(e);
+    }
+    const returnPromise = promiseResolvedWith(returnResult);
+    return transformPromiseWith(returnPromise, iterResult => {
+      if (!typeIsObject(iterResult)) {
+        throw new TypeError('The promise returned by the iterator.return() method must fulfill with an object');
+      }
+      return undefined;
+    });
+  }
+
+  stream = CreateReadableStream(startAlgorithm, pullAlgorithm, cancelAlgorithm, 0);
+  return stream;
+}