Clarification on AllowJavascriptExecution

[madana gopal]

Hi Team,

We are using using node v6.9.0. We faced a crash, when there is a javascript call being made from our app c++ code. Crash is happening because, call is made within the DisallowJavascriptExecutionScope. This scope is set in OptimizedCompileJob::GenerateCode() in compiler.cc. Scenario is garbage collection is initiated from below code, which caused some cleanup calls to be called from our app to javascript.

OptimizedCompileJob::Status OptimizedCompileJob::GenerateCode() {

 ............................

  DisallowJavascriptExecution no_js(isolate());

  {  

   .....................

   RegisterWeakObjectsInOptimizedCode() -> it is invoking garbage collection, which landed up to the code in our app

   ..............

  }

} 

Please clarify, during this time of operation, is it good to call our javascript call with AllowJavaScriptExecutionScope. Will it have any side effects?. Please clarify.

Thanks.

Regards,

Madan

[Jakob Kummerow]

A DisallowJavascriptExecution scope indicates that the following code relies on no JavaScript code being executed. If you execute JavaScript code anyway, you will probably cause corruption and subsequent crashes or misbehavior. The CHECK-failure exists to point out the problem early and in an easy-to-understand fashion; overriding it with an AllowJavascriptExecutionScope is like removing the "Danger! Minefield!" warning sign before walking into a minefield.

You should probably keep a list of cleanup calls you want to make, and execute them when the event loop gets back to you, rather than immediately.

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[madana gopal]

Thanks Jakob.

Below is the backtrace. Here how can I identify, when I came out of event loop. Because, we want cleanup calls to get called quickly.

Backtrace:

V8_Fatal

0x5ac3ec3

v8::Function::NewInstance(v8::Local<v8::Context>, int, v8::Local<v8::Value>*) const // calling javascript function here

// app code

v8::internal::GlobalHandles::PendingPhantomCallback::Invoke(v8::internal::Isolate*)

v8::internal::GlobalHandles::DispatchPendingPhantomCallbacks(bool)

v8::internal::GlobalHandles::PostGarbageCollectionProcessing(v8::internal::GarbageCollector, v8::GCCallbackFlags)

v8::internal::Heap::PerformGarbageCollection(v8::internal::GarbageCollector, v8::GCCallbackFlags)

v8::internal::Heap::CollectGarbage(v8::internal::GarbageCollector, char const*, char const*, v8::GCCallbackFlags)

v8::internal::Factory::CopyFixedArrayAndGrow(v8::internal::Handle<v8::internal::FixedArray>, int, v8::internal::PretenureFlag)

v8::internal::ArrayList::EnsureSpace(v8::internal::Handle<v8::internal::ArrayList>, int)

v8::internal::ArrayList::Add(v8::internal::Handle<v8::internal::ArrayList>, v8::internal::Handle<v8::internal::Object>, v8::internal::Handle<v8::internal::Object>, v8::internal::ArrayList::AddMode)

v8::internal::Heap::AddRetainedMap(v8::internal::Handle<v8::internal::Map>)

0x59d0eda

v8::internal::OptimizedCompileJob::GenerateCode() //DisAllowJavascriptExecutionScope set here

v8::internal::Compiler::FinalizeOptimizedCompileJob(v8::internal::OptimizedCompileJob*)

v8::internal::OptimizingCompileDispatcher::InstallOptimizedFunctions()

v8::internal::Runtime_TryInstallOptimizedCode(int, v8::internal::Object**, v8::internal::Isolate*)

........................

We have many number of cleanup objects, for which we have to call JS code on it being garbage collected. So, we will have cleanup calls for every object being garbage collected. Could you please point the instant, at which it will be good to identify this operation is completed (i.e the point where JS enging completed its garbage collection operation and we are good to do cleanup).

Thanks.

Regards,

Madan

[Jakob Kummerow]

It's not about the garbage collection operation being completed.

Presumably you have a call to v8::Script::Run() somewhere? When that call returns, you can safely execute other scripts.

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[madana gopal]

Thanks, ok. let me go through and update. We have both Script.Run() calls and vm.runScript() calls in our project. 

So, we have to make sure, we are not making any JS calls, when either Run() or vm.runScript()  going on (as they will involve in code compilation) and need to wait for its completion. Is my understanding right Jakob?

Thanks.

Regards,

Madan

[Jakob Kummerow]

Yes.

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