[Ovmsdev] Duktape Persistent Function References

Michael Balzer dexter at expeedo.de
Tue Sep 8 16:13:30 HKT 2020


for the synchronous callback execution, that's simply a variant of
OvmsDuktape::DuktapeRequestCallback() using the already existing
DuktapeDispatchWait() method along with passing the OvmsWriter* in
dmsg.writer. Could be named "…ExecuteCallback" to reflect the
synchronous nature.

There is no need to extend OvmsCommand. The DuktapeObject is meant to be
used as a general binding of arbitrary system objects to Duktape, so the
system objects don't need to be extended.

From my first check of your implementation, I would probably:

  * Redefine the register API to expect an object with an "execute" (and
    later possibly also a "validate") callback, and bind the
    DuktapeCommandObject to that object. A "fail" callback could also
    make sense.
  * Delegate the full OvmsCommand life cycle and execution handling to
    the DuktapeCommandObject.
  * Remove the forced unregistering of commands on script file unloading.
  * Inhibit implicit sub command deletions by parent deletions.

My API design would be like this:

// Command registration:
var cmd = OvmsCommand.Register({
  parent: "test",
  name: "dukcommand",
  title: "Test/demonstrate Duktape command registration",
  usage: "Pass 1 - 3 arguments",
  min: 1,
  max: 3,
  execute: function(verbosity, argv) {},
  fail: function(error) {}

// Command deregistration:
OvmsCommand.Unregister({ parent: "test", name: "dukcommand" });
// …or in case we still have the cmd object, simply:

The OvmsCommand execution callback would be a bound method of the
DuktapeCommandObject instance. The method passes the verbosity and
arguments vector to ExecuteMethod() via the data pointer using a custom
struct. CallMethod() then pushes these on the Duktape stack and calls
the "execute" property.

The forced unregistering limits registering commands to library plugins
(which don't have an unload operation yet). I think this is an
unnecessary limitation, I would like to be able to register a command by
adding a registration call to "ovmsmain.js" or by executing a script. Of
course that means commands registered that way need to be unregistered
explicitly, but that's up to the script developers & users then. I don't
think we should limit them here without good reason.

On a Duktape unload/reload operation, all JS objects get finalized
anyway because of heap destruction, so deletion would happen
automatically by the DuktapeConsoleCommand finalizer. If we add some
module unload operation later on, modules may e.g. define a cleanup
callback, or we can add the module filename to the DuktapeObject
registry -- the latter option would also automatically apply to all
DuktapeObject instances, eliminating the need for separate registries
for other system bindings potentially to be added in the future.

Implicit OvmsCommand deletions can happen as the order of deletion is
undefined. Inhibiting this should be a straight forward use of the
reference count though: on registration of a sub command, simply Ref()
all parent commands registered by Duktape as well. Unref() them after
the sub command has been removed, the last Unref() on a parent then
automatically deletes it (the first Ref() is done by the coupling).


Am 08.09.20 um 09:06 schrieb Mark Webb-Johnson:
> Michael,
> Very clear, and very helpful. Only thing I would suggest would be to
> have a minimum example in the documentation. The bare minimum required
> for an implementation of an object.
> Reading through what you write, it seems the correct approach is:
>   * Extend OvmsCommand to have a virtual ExecuteCommand method (same
>     parameters as ‘m_execute’ callback).
>   * Extend OvmsCommand::execute to check if m_execute is null, then
>     call ‘ExecuteCommand’ instead.
>   * Perhaps do the same for m_validate in OvmsCommand.
>   * Then, the duktape implementation can be object (rather than
>     callback function) based, as your DuktapeObject expects. The
>     javascript would call a function to register a command, with a
>     command object to be used for the callback.
>   * Need to extend the DuktapeObject system to support a synchronous
>     command (presumably implemented with a passed mutex like we do in
>     several other parts of the system).
> Is that correct, and what you were expecting? Or any other suggestions?
> Regards, Mark.
>> On 7 Sep 2020, at 2:55 AM, Michael Balzer <dexter at expeedo.de
>> <mailto:dexter at expeedo.de>> wrote:
>> Mark,
>>> Still struggling with this. It seems like your DuktapeObject will do
>>> this, but I can’t work out how it works.
>> I admit my documentation has some shortcomings. I'll try to fill that
>> gap first:
>> *_Concept #1: Reference counting_*
>> A DuktapeObject is meant to be a C++/OS extension of a Javascript
>> object, for example to hold some system binding associated with the
>> JS object.
>> The primary goal is to provide asynchronous operations on the system
>> side, initiating JS callbacks when finishing/failing. For example a
>> HTTP operation can be started by a script, passing some "done"
>> callback. The system then starts the network operation
>> asynchronously, the JS task can continue processing other scripts.
>> Once the network operation is done, the DuktapeObject sends a request
>> to the Duktape task to execute the "done" callback on itself.
>> So the DuktapeObject is normally shared by multiple contexts and
>> parallel operations. Asynchronous operation also means the JS object
>> or context may already be gone when the operation finishes. So the
>> DuktapeObject needs to stay locked in memory independent of the JS
>> context.
>> That's implemented by counting the active references to the
>> DuktapeObject (methods Ref() / Unref()). The last Unref()
>> automatically deletes the DuktapeObject instance.
>> For example: JS requests a network operation. The initial JS binding
>> (see coupling) sets the reference count to 1. The DuktapeObject
>> starts the network request, increasing the ref count to 2. If the JS
>> context now dies (decreasing the reference count), the DuktapeObject
>> will still remain valid until the network operation returns.
>> As the Ref/Unref operations need a (recursive) mutex, that's also
>> part of the DuktapeObject and exposed by the API for other uses:
>> Lock() / Unlock().
>> *_Concept #2: Coupling_*
>> Javascript does not have a destructor concept. JS objects get deleted
>> by heap destruction or by the garbage collector when no reference to
>> the JS object is left. In both cases, the actual object deletion is
>> called "finalization", and a special finalizer method/callback can be
>> installed on a JS object to be called just before the object gets
>> deallocated. That is done by the DuktapeObject::Couple() method
>> (implicitly called when constructed directly with a JS object reference).
>> There is no way to force finalization on a JS object. So a
>> DuktapeObject cannot tell Duktape to delete it's coupled object, that
>> means a DuktapeObject should normally not be deleted from outside the
>> Duktape context, at least not if still coupled to the JS object.
>> Coupling and decoupling can only be done in the Duktape context.
>> The standard finalizer DuktapeObject::Finalizer() simply decouples,
>> automatically deleting itself if the coupling was the last reference.
>> This is a virtual method, so can be overridden as necessary.
>> The coupling operation additionally adds a hidden pointer to the
>> DuktapeObject instance in the JS object. That allows to check for and
>> retreive associated DuktapeObject instances from any JS object, which
>> is provided by the GetInstance() call.
>> *_Concept #3: Registration_*
>> For asynchronous operations, it's normally very convenient to have a
>> "fire & forget" API. Example from the documentation:
>> VFS.Save({
>>   path: "/sd/mydata/telemetry.json",
>>   data: Duktape.enc('jx', telemetry),
>>   fail: function(error) {
>>     print("Error saving telemetry: " + error);
>>   }
>> });
>> I.e. you simply pass the operation arguments including the done/fail
>> callbacks to an API method and don't need to care about storing a
>> reference to some handle. In JS that normally means the object used
>> won't have any reference left after the call, so would be deleted by
>> the garbage collector on the next run.
>> To avoid garbage collection and lock the JS object in memory, we need
>> to store a reference to it in a "public" place. Duktape provides a
>> special public place for this, hidden from scripts, called the global
>> stash. DuktapeObject maintains a dedicated global object registry in
>> that stash.
>> Adding and removing the coupled object reference to/from that
>> registry is done by the Register() and Deregister() methods.
>> So for asynchronous system operations, or system integrations that
>> shall be persistent, you normally do a Register() call together with
>> the coupling, unless some ressource isn't available. Deregistration
>> is then normally done when all pending JS callbacks have been
>> executed, or when the persistent system integration has been unbound.
>> Other API designs are possible here: if you'd rather like the script
>> needing to store a reference to your operation handle, you don't need
>> to do a registration. The object will then be deleted (finalized) by
>> the garbage collector automatically after the script deletes the
>> reference.
>> *_Concept #4: Callback invocation_*
>> Triggers on the system side, for example a finished or failed network
>> operation, shall normally trigger a JS method execution.
>> JS callback methods are simply passed as part of the arguments object
>> in modern JS APIs. This allows to pass simple function definitions
>> inline, as well as to reference a separately defined general handler
>> function. JS allows functions to be excuted in the context of any
>> object, and callbacks normally are executed in the context of the API
>> object. This adds even more convenience, as the callbacks can easily
>> access the other API arguments still stored in the object, as well as
>> additional data added by the call.
>> JS callbacks cannot be executed directly from any system context,
>> they need to run in the Duktape context. So the DuktapeObject
>> callback invocation mechanism includes a general method to request a
>> callback execution by Duktape: RequestCallback()
>> Note: RequestCallback() is an asynchronous operation. A synchronous
>> variant can be added if necessary (and probably will be for command
>> execution from a console).
>> A pending callback automatically increments the reference count, so
>> the object is locked in memory until the callback has been executed
>> (or aborted) by the Duktape task.
>> The callback invocation API provides a void* for simple data (e.g. a
>> fixed string) to be passed to the callback method, but for more
>> complex data, you will normally fill some DuktapeObject member
>> variables before invoking the callback.
>> In Duktape context, the callback invocation translates the data
>> returned or provided by the system side into the Duktape callback
>> arguments and then runs the callback (if the object actually has the
>> requested callback set). The default implementation for this is
>> DuktapeObject::CallMethod(), which can be used directly for simple
>> callbacks without arguments. For more complex handling, override this
>> with your custom implementation.
>> The callbacks are by default executed on the coupled JS object, so
>> data can also be transported by setting properties on that object.
>> The callback can then simply access them via "this".
>> To simplify callback invocation from code parts that may run outside
>> or inside Duktape, it's convenient to allow calling CallMethod()
>> without a Duktape context, and let CallMethod() translate that into a
>> RequestCallback() call as necessary. Pattern:
>> duk_ret_t DuktapeHTTPRequest::CallMethod(duk_context *ctx, const
>> char* method, void* data /*=NULL*/)
>>   {
>>   if (!ctx)
>>     {
>>     RequestCallback(method, data);
>>     return 0;
>>     }
>>   …
>> A CallMethod() implementation isn't limited to executing a single
>> callback. A common example is an API defining "done" & "fail"
>> callbacks, as well as a general final "always" callback.
>> DuktapeHTTPRequest::CallMethod() also serves as an example
>> implementation for this.
>> Wow… that's become more to write & read than I expected. Please
>> provide some feedback: is that explanation sufficient & clear? I'll
>> refine it for the developer docs then.
>> Regards,
>> Michael
>> Am 01.09.20 um 19:52 schrieb Michael Balzer:
>>> Mark,
>>> I'll have a look.
>>> Regards,
>>> Michael
>>> Am 01.09.20 um 07:30 schrieb Mark Webb-Johnson:
>>>> Michael,
>>>> Still struggling with this. It seems like your DuktapeObject will
>>>> do this, but I can’t work out how it works.
>>>> Here are some notes one what I have done so far:
>>>>  1. Created a stub DuktapeConsoleCommand (derived from
>>>>     DuktapeObject) in ovms_duktape.{h,cpp}. This should hold enough
>>>>     to be able to call the javascript callback method for that
>>>>     object. It also stores the module filename (so the registration
>>>>     can be removed when the module is unloaded).
>>>>  2. Provide a DuktapeCommandMap m_cmdmap in
>>>>     ovms_duktape.{h,cpp} OvmsDuktape class that stores a mapping
>>>>     from OvmsCommand to DuktapeConsoleCommand.
>>>>  3. Created a OvmsDuktape::RegisterDuktapeConsoleCommand in
>>>>     ovms_duktape.{h,cpp) that (a) creates the OvmsCommand() object,
>>>>     (b) registers it, (c) creates the DuktapeConsoleCommand()
>>>>     object, and (d) updates a map from
>>>>     OvmsCommand->DuktapeConsoleCommand. There Is also a single
>>>>     callback DukOvmsCommandRegisterRun designed to be run by all.
>>>>  4. Created
>>>>     hooks NotifyDuktapeModuleLoad, NotifyDuktapeModuleUnload,
>>>>     and NotifyDuktapeModuleUnloadAll in OvmsDuktape. The javascript
>>>>     module is identified by filename (path to module or script on
>>>>     vfs, usually, but may also be an internal module). The Unload
>>>>     functions look through the m_cmdmap and unregister commands for
>>>>     javascript modules being unloaded.
>>>>  5. Provide an implementation for
>>>>     ovms_command DukOvmsCommandRegister to support registering
>>>>     commands from Javascript modules. This should extract the
>>>>     details, and then call
>>>>     OvmsDuktape::RegisterDuktapeConsoleCommand to do the actual
>>>>     registration. This has been implemented, except for the
>>>>     callback method (and somehow passing that method from
>>>>     Javascript in the OvmsCommand.Register javascript call).
>>>>  6. Provide a stub implementation for DukOvmsCommandRegisterRun.
>>>>     This uses m_cmdmap to lookup the DuktapeConsoleCommand object
>>>>     for the command to be run. It should execute the callback
>>>>     method (but that part is not yet implemented).
>>>> I still need help with #5 and #6. What needs to be implemented
>>>> in DuktapeConsoleCommand, and how is the parameter
>>>> in OvmsCommand.Register used to store the callback (#5)? Then how
>>>> to callback the command method from DukOvmsCommandRegisterRun (#6)?
>>>> If you have time, it is probably much quicker for you to simply
>>>> make those changes.
>>>> An alternative implementation would be to do something like
>>>> the pubsub framework, where the mapping command->callback is done
>>>> from within a javascript module. That I could do, but it seems
>>>> your DuktapeObject can do it better. 
>>>> Thanks, Mark.
>>>>> On 15 Jul 2020, at 3:34 PM, Michael Balzer <dexter at expeedo.de
>>>>> <mailto:dexter at expeedo.de>> wrote:
>>>>> 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.
>>>>>> _______________________________________________
>>>>>> OvmsDev mailing list
>>>>>> OvmsDev at lists.openvehicles.com
>>>>>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
>>>>> -- 
>>>>> Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
>>>>> Fon 02333 / 833 5735 * Handy 0176 / 206 989 26
>>>>> _______________________________________________
>>>>> OvmsDev mailing list
>>>>> OvmsDev at lists.openvehicles.com <mailto:OvmsDev at lists.openvehicles.com>
>>>>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
>>>> _______________________________________________
>>>> OvmsDev mailing list
>>>> OvmsDev at lists.openvehicles.com
>>>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
>>> -- 
>>> Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
>>> Fon 02333 / 833 5735 * Handy 0176 / 206 989 26
>>> _______________________________________________
>>> OvmsDev mailing list
>>> OvmsDev at lists.openvehicles.com
>>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
>> -- 
>> Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
>> Fon 02333 / 833 5735 * Handy 0176 / 206 989 26
>> _______________________________________________
>> OvmsDev mailing list
>> OvmsDev at lists.openvehicles.com <mailto:OvmsDev at lists.openvehicles.com>
>> http://lists.openvehicles.com/mailman/listinfo/ovmsdev
> _______________________________________________
> OvmsDev mailing list
> OvmsDev at lists.openvehicles.com
> http://lists.openvehicles.com/mailman/listinfo/ovmsdev

Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openvehicles.com/pipermail/ovmsdev/attachments/20200908/dfacc69b/attachment.htm>

More information about the OvmsDev mailing list