stream: add a non-destroying iterator to Readable

doc/api/stream.md

@@ -1506,13 +1506,67 @@ async function print(readable) {
 print(fs.createReadStream('file')).catch(console.error);
 ```
 
-If the loop terminates with a `break` or a `throw`, the stream will be
-destroyed. In other terms, iterating over a stream will consume the stream
+If the loop terminates with a `break`, `return` or a `throw`, the stream will
+be destroyed. In other terms, iterating over a stream will consume the stream
 fully. The stream will be read in chunks of size equal to the `highWaterMark`
 option. In the code example above, data will be in a single chunk if the file
 has less then 64KB of data because no `highWaterMark` option is provided to
 [`fs.createReadStream()`][].
 
+##### `readable.iterator([options])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `destroyOnReturn` {boolean} When set to `false`, calling `return` on the
+    async iterator, or exiting a `for await...of` iteration using a `break`,
+    `return` or `throw` will not destroy the stream. **Default:** `true`.
+  * `destroyOnError` {boolean} When set to `false`, if the stream emits an
+    error while it's being iterated, the iterator will not destroy the stream.
+    **Default:** `true`.
+* Returns: {AsyncIterator} to consume the stream.
+
+The iterator created by this method gives users the option to cancel the
+destruction of the stream if the `for await...of` loop is exited by `return`,
+`break` or `throw`, or if the iterator should destroy the stream if the stream
+emitted an error during iteration.
+
+```js
+const { Readable } = require('stream');
+
+async function printIterator(readable) {
+  for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
+    console.log(chunk); // 1
+    break;
+  }
+
+  console.log(readable.destroyed); // false
+
+  for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
+    console.log(chunk); // Will print 2 and then 3
+  }
+
+  console.log(readable.destroyed); // True, stream was totally consumed
+}
+
+async function printSymbolAsyncIterator(readable) {
+  for await (const chunk of readable) {
+    console.log(chunk); // 1
+    break;
+  }
+
+  console.log(readable.destroyed); // true
+}
+
+async function showBoth() {
+  await printIterator(Readable.from([1, 2, 3]));
+  await printSymbolAsyncIterator(Readable.from([1, 2, 3]));
+}
+
+showBoth();
+```
+
 ### Duplex and transform streams
 
 #### Class: `stream.Duplex`

lib/internal/streams/readable.js

@@ -62,6 +62,7 @@ const {
   ERR_METHOD_NOT_IMPLEMENTED,
   ERR_STREAM_UNSHIFT_AFTER_END_EVENT
 } = require('internal/errors').codes;
+const { validateObject } = require('internal/validators');
 
 const kPaused = Symbol('kPaused');
 
@@ -1062,8 +1063,17 @@ Readable.prototype.wrap = function(stream) {
 };
 
 Readable.prototype[SymbolAsyncIterator] = function() {
-  let stream = this;
+  return streamToAsyncIterator(this);
+};
 
+Readable.prototype.iterator = function(options) {
+  if (options !== undefined) {
+    validateObject(options, 'options');
+  }
+  return streamToAsyncIterator(this, options);
+};
+
+function streamToAsyncIterator(stream, options) {
   if (typeof stream.read !== 'function') {
     // v1 stream
     const src = stream;
@@ -1076,14 +1086,20 @@ Readable.prototype[SymbolAsyncIterator] = function() {
     }).wrap(src);
   }
 
-  const iter = createAsyncIterator(stream);
+  const iter = createAsyncIterator(stream, options);
   iter.stream = stream;
   return iter;
-};
+}
 
-async function* createAsyncIterator(stream) {
+async function* createAsyncIterator(stream, options) {
   let callback = nop;
 
+  const opts = {
+    destroyOnReturn: true,
+    destroyOnError: true,
+    ...options,
+  };
+
   function next(resolve) {
     if (this === stream) {
       callback();
@@ -1116,6 +1132,7 @@ async function* createAsyncIterator(stream) {
       next.call(this);
     });
 
+  let errorThrown = false;
   try {
     while (true) {
       const chunk = stream.destroyed ? null : stream.read();
@@ -1132,12 +1149,17 @@ async function* createAsyncIterator(stream) {
       }
     }
   } catch (err) {
-    destroyImpl.destroyer(stream, err);
+    if (opts.destroyOnError) {
+      destroyImpl.destroyer(stream, err);
+    }
+    errorThrown = true;
     throw err;
   } finally {
-    if (state.autoDestroy || !endEmitted) {
-      // TODO(ronag): ERR_PREMATURE_CLOSE?
-      destroyImpl.destroyer(stream, null);
+    if (!errorThrown && opts.destroyOnReturn) {
+      if (state.autoDestroy || !endEmitted) {
+        // TODO(ronag): ERR_PREMATURE_CLOSE?
+        destroyImpl.destroyer(stream, null);
+      }
     }
   }
 }

test/parallel/test-stream-readable-async-iterators.js

@@ -693,6 +693,122 @@ async function tests() {
   });
 }
 
+// AsyncIterator non-destroying iterator
+{
+  function createReadable() {
+    return Readable.from((async function* () {
+      await Promise.resolve();
+      yield 5;
+      await Promise.resolve();
+      yield 7;
+      await Promise.resolve();
+    })());
+  }
+
+  function createErrorReadable() {
+    const opts = { read() { throw new Error('inner'); } };
+    return new Readable(opts);
+  }
+
+  // Check default destroys on return
+  (async function() {
+    const readable = createReadable();
+    for await (const chunk of readable.iterator()) {
+      assert.strictEqual(chunk, 5);
+      break;
+    }
+
+    assert.ok(readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check explicit destroying on return
+  (async function() {
+    const readable = createReadable();
+    for await (const chunk of readable.iterator({ destroyOnReturn: true })) {
+      assert.strictEqual(chunk, 5);
+      break;
+    }
+
+    assert.ok(readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check default destroys on error
+  (async function() {
+    const readable = createErrorReadable();
+    try {
+      // eslint-disable-next-line no-unused-vars
+      for await (const chunk of readable) { }
+      assert.fail('should have thrown');
+    } catch (err) {
+      assert.strictEqual(err.message, 'inner');
+    }
+
+    assert.ok(readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check explicit destroys on error
+  (async function() {
+    const readable = createErrorReadable();
+    const opts = { destroyOnError: true, destroyOnReturn: false };
+    try {
+      // eslint-disable-next-line no-unused-vars
+      for await (const chunk of readable.iterator(opts)) { }
+      assert.fail('should have thrown');
+    } catch (err) {
+      assert.strictEqual(err.message, 'inner');
+    }
+
+    assert.ok(readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check explicit non-destroy with return true
+  (async function() {
+    const readable = createErrorReadable();
+    const opts = { destroyOnError: false, destroyOnReturn: true };
+    try {
+      // eslint-disable-next-line no-unused-vars
+      for await (const chunk of readable.iterator(opts)) { }
+      assert.fail('should have thrown');
+    } catch (err) {
+      assert.strictEqual(err.message, 'inner');
+    }
+
+    assert.ok(!readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check explicit non-destroy with return true
+  (async function() {
+    const readable = createReadable();
+    const opts = { destroyOnReturn: false };
+    for await (const chunk of readable.iterator(opts)) {
+      assert.strictEqual(chunk, 5);
+      break;
+    }
+
+    assert.ok(!readable.destroyed);
+
+    for await (const chunk of readable.iterator(opts)) {
+      assert.strictEqual(chunk, 7);
+    }
+
+    assert.ok(readable.destroyed);
+  })().then(common.mustCall());
+
+  // Check non-object options.
+  {
+    const readable = createReadable();
+    assert.throws(
+      () => readable.iterator(42),
+      {
+        code: 'ERR_INVALID_ARG_TYPE',
+        name: 'TypeError',
+        message: 'The "options" argument must be of type object. Received ' +
+                 'type number (42)',
+      }
+    );
+  }
+}
+
 {
   let _req;
   const server = http.createServer((request, response) => {