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src: update v8::Object::GetPropertyNames() usage #23660

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Oct 17, 2018
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src: update v8::Object::GetPropertyNames() usage #23660

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7 changes: 6 additions & 1 deletion src/node_contextify.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -508,7 +508,12 @@ void ContextifyContext::PropertyEnumeratorCallback(
if (ctx->context_.IsEmpty())
return;

args.GetReturnValue().Set(ctx->sandbox()->GetPropertyNames());
Local<Array> properties;
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V8 recommend using the MaybeLocal returning version.
(IMHO it's also cleaner then the out-param version)

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This is the MaybeLocal version.

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@refack refack Oct 14, 2018 •
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So why not:

MaybeLocal<Array> maybe = ctx->sandbox()->GetPropertyNames(ctx->context())
if (maybe.IsEmpty())
    return;
args.GetReturnValue().Set(maybe.ToLocalChecked());

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IMO, this way is easier to read and reason about (don't really have to think about Maybes).

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IMHO having an uninitialized variable is code smell, and if there's a way to avoid it, why not.
Also if (!ctx->sandbox()->GetPropertyNames(ctx->context()).ToLocal(&properties)) is a bit busy it does like 5 things...

But I leave it to the author.

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@refack Maybe a more objective advantage of the current variation here: It’s easier to handle errors from sequences of operations this way:

node/src/node.cc

Lines 1237 to 1241 in 8ce99fa

if (!args[0]->ToObject(context).ToLocal(&module) ||
!module->Get(context, env->exports_string()).ToLocal(&exports_v) ||
!exports_v->ToObject(context).ToLocal(&exports)) {
return; // Exception pending.
}

At first, I also was a fan of what you’re suggesting, but for cases like these it can become quite annoying…

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@refack refack Oct 14, 2018 •
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That's IMHO an extreme example of this pattern. What's wrong with:

  Maybe<Object> module = args[0]->ToObject(context);
  Maybe<Object> exports = module->Get(context, env->exports_string());
  Maybe<Value> exports_v = exports_v->ToObject(context);
  if (module.isEmpty() || exports.isEmpty() || exports_v.isEmpty()) {
    return;  // Exception pending.
  }

The original code also has short-circuit logic, so it is tricky to reason about.
P.S. see the // Exception pending comment, can we tell what when wrong?

We have actually have several guidelines that say to avoid this:

Google's:

  1. Local Variables - Place a function's variables in the narrowest scope possible, and initialize variables in the declaration.
  2. Output Parameters -
    Prefer using return values rather than output parameters. If output-only parameters are used they should appear after input parameters.

C++CG:

  1. ES.20: Always initialize an object
  2. F.20: For “out” output values, prefer return values to output parameters

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What's wrong with:

It doesn’t compile … MaybeLocals sadly don’t form a full monad; you can’t actually call ->Get() or ->ToObject() on one. The idea, as I understand V8, is to force API users to return to JS as soon as possible, and not attempt to do more operations.

P.S. see the // Exception pending comment, can we tell what when wrong?

No, but ideally we don’t have to care about that anyway.

It’s also a bit odd to quote Google’s style guide here… I totally see what you mean and why this pattern can be counterintuitive, but after all, this pattern is of Google’s making and V8 uses it very extensively in its own code. 😄

ES.20: Always initialize an object

I’d say that rule explicitly lists this pattern as an exception:

If you are declaring an object that is just about to be initialized from input, initializing it would cause a double initialization.

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It doesn’t compile … MaybeLocals sadly don’t form a full monad;

I tried to read this to figure out if the values are dependant. I looked several times, but missed it. That's a red light for me.

Anyway like you say, we write code so it reads well, not writes easy. The following is linear and reads much better, with only one action per line.

Maybe<Object> module = args[0]->ToObject(context);
if (module.isEmpty()) {
  return env->isolate()->ThrowException("module is empty");
}
Maybe<Object> exports = module->ToCheckedLocal()->Get(context, env->exports_string());
if (exports.isEmpty()) {
  return env->isolate()->ThrowException("exports is empty");
}
Maybe<Value> exports_v = exports_v->ToCheckedLocal()->ToObject(context);
if (exports_v.isEmpty()) {
   return env->isolate()->ThrowException("exports_v is empty");
}

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The ThrowException() bit is not quite how MaybeLocals work – an empty one typically means that there is already an exception pending – but that’s a fair point, yes.

To be clear, I don’t personally like the repetition there, but I wouldn’t object to anybody using individual checks for each call.


if (!ctx->sandbox()->GetPropertyNames(ctx->context()).ToLocal(&properties))
return;

args.GetReturnValue().Set(properties);
}

// static
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