Re: [v8-users] Check failed: Handle not reset in first callback. How to Reset my Persistent

[J Decker]

On Tue, Oct 9, 2018 at 4:40 PM Mike Moening  wrote:

> The example was helpful.   Encapsulating the Persistent in the callacks
> private data object seems to be the way to roll.
>
>
>> // kParameter will pass a void* parameter back to the callback,
>> kInternalFields
>> // will pass the first two internal fields back to the callback,
>> kFinalizer
>> // will pass a void* parameter back, but is invoked before the object is
>> // actually collected, so it can be resurrected. In the last case, it is
>> not
>> // possible to request a second pass callback.
>> enum class WeakCallbackType { kParameter, kInternalFields, kFinalizer };
>
>
> Does that help?
>
> Sorry, but no not really.
> Resurrection?  Sounds like a zombie movie (I probably don't need this)
> *kInternalFields - will pass the first two internal fields back to the
> callback*
>

This would be useful in a Node world, there is a utiility class ObjectWrap
which uses 1 internal field to store the user's class reference (the C++
object to be wrapped in a V8/JS reference).  It could use kInternalFields
type instead of duplicating the value to be passed as a parameter.  (Just a
possible application of that)


>
> How? Why?
>
> There has to be something better that explains the callbacks better.
>

I did this originally to track array buffer content, so I can release the
backing buffer when the typed array evaporates (is garbage collected).
I generalized to object mostly later, but then re-added 'strings' for short
lived strings associated with address/connection objects.

---
struct arrayBufferHolder  {
void *buffer; // resources associated with object
Persistent o;
/*
// other types that this might hold add deallocation handlers in
callback
Persistent s;
Persistent ab;
*/
};
typedef struct arrayBufferHolder ARRAY_BUFFER_HOLDER, *PARRAY_BUFFER_HOLDER;


/* GetHolder() returns a holder from a pool; or allocates a new one... it
reuses previous holders*/



/* ... buf and len are the backing
buffer and length of that buffer */
Local arrayBuffer = ArrayBuffer::New( isolate, buf, len );
PARRAY_BUFFER_HOLDER holder = GetHolder();
holder->o.Reset( isolate, arrayBuffer );
holder->o.SetWeak( holder, releaseBuffer,
WeakCallbackType::kParameter );
holder->buffer = buf;



/* The callback to release associated resources when `arrayBuffer` is
collcted */

void releaseBuffer( const WeakCallbackInfo  ) {
PARRAY_BUFFER_HOLDER holder = info.GetParameter();

if( !holder->o.IsEmpty() ) { // in case this is one of 3 types to release.
holder->o.ClearWeak();
holder->o.Reset();
}
Deallocate( void*, holder->buffer ); // your resource deallocation here
DropHolder( holder );  // returns holder to pool for later use. (delete
holder;?)
}

---




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Re: [v8-users] Check failed: Handle not reset in first callback. How to Reset my Persistent

[Mike Moening]

The example was helpful.   Encapsulating the Persistent in the callacks 
private data object seems to be the way to roll.
 

> // kParameter will pass a void* parameter back to the callback, 
> kInternalFields
> // will pass the first two internal fields back to the callback, kFinalizer
> // will pass a void* parameter back, but is invoked before the object is
> // actually collected, so it can be resurrected. In the last case, it is 
> not
> // possible to request a second pass callback.
> enum class WeakCallbackType { kParameter, kInternalFields, kFinalizer };


Does that help?

Sorry, but no not really.  
Resurrection?  Sounds like a zombie movie (I probably don't need this)
*kInternalFields - will pass the first two internal fields back to the 
callback*
How? Why?

There has to be something better that explains the callbacks better.

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Re: [v8-users] Check failed: Handle not reset in first callback. How to Reset my Persistent

[Jakob Kummerow]

On Tue, Oct 9, 2018 at 11:45 AM Mike Moening  wrote:

> With recent versions of v8 the SetWeak method has changed and the
> Persistent* parameter has been dropped from the destructor callback.
> The destructor callback function is **supposed** to Reset() the persistent
> or its a leak.
> If you don't V8 crashes and tells you this:
>
> Check failed: Handle not reset in first callback. See comments on
> |v8::WeakCallbackInfo
>
> Old signature:
> void MyClass::DestructorCallback(Isolate* pIsolate, Persistent*
> value, MyClass* pObj)
>
> New signature
> void MyClass::DestructorCallback(const WeakCallbackInfo& oValue)
>
> If I call it like this:
> Persistent obj(isolate, objLocal);
> obj.SetWeak(pMyObject, MyClass::DestructorCallback,
> WeakCallbackType::kParameter);
>
> How can I pass the Persistent into my DestructorCallback function so I can
> Reset it??
> Persistent cannot be copied.
> So how can I Reset() it in the callback?
> Any help would be great.
>

See this example:
https://cs.chromium.org/chromium/src/v8/src/d8.cc?l=2312=SetWeak
You can often find such hints in the V8 API Changes doc

(just
ask your favorite search engine for "V8 API Changes" to find it).


> Also the WeakCallbackType::kParameter  vs WeakCallbackType::kFinalizer
> thing has me confuzeled too.
> Which is the right type to use?
>

See the documentation:

// kParameter will pass a void* parameter back to the callback,
> kInternalFields
> // will pass the first two internal fields back to the callback, kFinalizer
> // will pass a void* parameter back, but is invoked before the object is
> // actually collected, so it can be resurrected. In the last case, it is
> not
> // possible to request a second pass callback.
> enum class WeakCallbackType { kParameter, kInternalFields, kFinalizer };


Does that help?


>
> Thanks!
>
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[v8-users] Check failed: Handle not reset in first callback. How to Reset my Persistent

[Mike Moening]

With recent versions of v8 the SetWeak method has changed and the 
Persistent* parameter has been dropped from the destructor callback.
The destructor callback function is **supposed** to Reset() the persistent 
or its a leak.
If you don't V8 crashes and tells you this:

Check failed: Handle not reset in first callback. See comments on 
|v8::WeakCallbackInfo

Old signature:
void MyClass::DestructorCallback(Isolate* pIsolate, Persistent* 
value, MyClass* pObj)

New signature
void MyClass::DestructorCallback(const WeakCallbackInfo& oValue)

If I call it like this:
Persistent obj(isolate, objLocal);
obj.SetWeak(pMyObject, MyClass::DestructorCallback, 
WeakCallbackType::kParameter);
   
How can I pass the Persistent into my DestructorCallback function so I can 
Reset it??
Persistent cannot be copied.
So how can I Reset() it in the callback?
Any help would be great.

Also the WeakCallbackType::kParameter  vs WeakCallbackType::kFinalizer 
thing has me confuzeled too.
Which is the right type to use?

Thanks!

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