Mark,

yes, I needed that persistence for the HTTP and VFS classes, but I also needed to be able to couple a dynamic C++ instance with a JS object and have a mechanism to prevent garbage collection while the C++ side is still in use. If the C++ side is no longer needed, the JS finalizer also needs to imply the C++ instance can be deleted.

That is all implemented by DuktapeObject. DuktapeObject also provides JS method invocation on the coupled JS object and a mutex for concurrency protection.

We probably need some more framework documentation than the header comments (applies to all of our framework classes…):

/***************************************************************************************************
 * DuktapeObject: coupled C++ / JS object
 *
 *  Intended for API methods to attach internal API state to a JS object and provide
 *    a standard callback invocation interface for JS objects in local scopes.
 * 
 *  - Override CallMethod() to implement specific method calls
 *  - Override Finalize() for specific destruction in JS context (garbage collection)
 *  - call Register() to prevent normal garbage collection (but not heap destruction)
 *  - call Ref() to protect against deletion (reference count)
 *  - call Lock() to protect concurrent access (recursive mutex)
 * 
 *  - GetInstance() retrieves the DuktapeObject associated with a JS object if any
 *  - Push() pushes the JS object onto the Duktape stack
 * 
 *  Note: the DuktapeObject may persist after the JS object has been finalized, e.g.
 *    if some callbacks are pending after the Duktape heap has been destroyed.
 *    Use IsCoupled() to check if the JS object is still available.
 * 
 *  Ref/Unref:
 *    Normal life cycle is from construction to finalization. Pending callbacks extend
 *    the life until the last callback has been processed. A subclass may extend the life
 *    by calling Ref(), which increases the reference count. Unref() deletes the instance
 *    if no references are left.
 */

You normally just need to use Register/Deregister & Ref/Unref, and to implement the constructor and CallMethod. Coupling of the instances normally is done on construction, as a JS object is normally already needed for the parameters and can simply be attached to.

Have a look at DuktapeHTTPRequest, DuktapeVFSLoad and DuktapeVFSSave, these are the current subclasses using this.

For the command registration I would probably couple the OvmsCommand instance with a JS command object providing an execution method.

Tell me if you need more info.

Regards,
Michael


Am 15.07.20 um 08:12 schrieb Mark Webb-Johnson:

@Michael this is probably for you.

I am trying to implement javascript command registration. The idea is that a javascript module can call something like:

OvmsCommand.Register(basecommand, name, title, callbackfn, usage, min, max)

Then we reflect that into MyCommandApp.RegisterCommand, and keep a track of which command is for which javascript callbackfn. When the command is executed, we pass it into duktape.

I also have tracking for javascript module loading and unloading, so I can DeregisterCommand() if duktape is reloaded (and also protected against commands being registered in short-lived scripts run from the command line).

To implement this, I need to store the callbackfn as a persistent reference to a duktape javascript function.

The issue with callback function references in duktape is summarised here:

https://wiki.duktape.org/howtonativepersistentreferences

When a Duktape/C function is called, Duktape places the call arguments on the value stack. While the arguments are on the value stack, they're guaranteed to be reachable and the Duktape/C function can safely work with the arguments.

However, when the Duktape/C function returns, the value stack is unwound and references in the function's value stack frame are lost. If the last reference to a particular value was in the function's value stack frame, the value will be garbage collected when the function return is processed.

The standard approach is to store the reference back in the duktape duk_push_global_stash so it won’t get garbage-collected. But, that seems messy.

I see that Michael has already implemented something that seems similar in ovms_script.{h, cpp}, for the async http callbacks. Presumably to avoid this issue. But, the approach seems very different, and I am not sure if it is stopping _all_ garbage collection for the duration of the async query, or just that particular object being garbage collected. The work seems extensive (quite a few objects involved).

So @Michael, any suggestions for this? I don’t want to reinvent the wheel...

Regards, Mark.

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