Leaving a generator via an exception causes it to close

src/generator.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 "use strict";
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Function = global.Function;
 
 // ----------------------------------------------------------------------------
 
 
 // Generator functions and objects are specified by ES6, sections 15.19.3 and
 // 15.19.4.
 
 function GeneratorObjectNext(value) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['[Generator].prototype.next', this]);
   }
 
   if (DEBUG_IS_ACTIVE) %DebugPrepareStepInIfStepping(this);
-  return %_GeneratorNext(this, value);
+  try {
+    return %_GeneratorNext(this, value);
+  } catch (e) {
+    %GeneratorClose(this);
+    throw e;
+  }
 }
 
 function GeneratorObjectThrow(exn) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError('incompatible_method_receiver',
                         ['[Generator].prototype.throw', this]);
   }
 
-  return %_GeneratorThrow(this, exn);
+  try {
+    return %_GeneratorThrow(this, exn);
+  } catch (e) {

[rossberg, 2014/11/12 11:56:27]

Wait, won't this catch the very throw above?

[wingo, 2014/11/12 12:03:02]

It could, if the generator itself doesn't catch it.   But the generator could
catch it and return a value.

+    %GeneratorClose(this);
+    throw e;
+  }
 }
 
 function GeneratorObjectIterator() {
   return this;
 }
 
 function GeneratorFunctionPrototypeConstructor(x) {
   if (%_IsConstructCall()) {
     throw MakeTypeError('not_constructor', ['GeneratorFunctionPrototype']);
   }
@@ skipping 34 matching lines @@
       symbolToStringTag, "Generator", DONT_ENUM | READ_ONLY);
   %InternalSetPrototype(GeneratorFunctionPrototype, $Function.prototype);
   %SetCode(GeneratorFunctionPrototype, GeneratorFunctionPrototypeConstructor);
   %AddNamedProperty(GeneratorFunctionPrototype, "constructor",
       GeneratorFunction, DONT_ENUM | DONT_DELETE | READ_ONLY);
   %InternalSetPrototype(GeneratorFunction, $Function);
   %SetCode(GeneratorFunction, GeneratorFunctionConstructor);
 }
 
 SetUpGenerators();