[Gammaray-interest] New plugin idea: Event inspector

[Christian Gagneraud]

On 15 September 2017 at 22:37, Christian Gagneraud <chgans at gmail.com> wrote:
> On 15 September 2017 at 16:06, Christian Gagneraud <chgans at gmail.com> wrote:
>> Hi there,
>>
>> I'm starting to think seriously about jumping into the gammara world.
>>
>> What I would like to get/do/contribute is a first class citizen event inspector.
>>
>> I won't waste your time with lot of words, so basically i would like
>> to hook on CustomApplication::notify, CustomObject::event and
>> CustomnObject::eventFilter, and store occurrences when some condition
>> matches.
>
> context: Linux + x86(_64) + gcc
>
> I just had a look at the Probe, the Injector and the
> FunctionOverwriter classes and skimmed quickly through some plugins ,
> and there's enough interesting stuff to start playing with this idea.
> I've already learned lot of things! ;)
> I didn't know that since Qt-5.4, installing hooks was that easy - if
> you don't mind depending on qtbase-private.
> I was surprise by the "simple" approach taken by GammaRay, I was
> expecting heavy use of SO preload instead i found a "simple is
> beautiful" approach (simple is always a relative word).
>
> I think it is doable to dynamically wrap CustomApplication::notify, as
> you can dynamically build the mangled symbol of the
> QApplication::notify overloads, same goes with QObject::Event and
> QObject::eventFilter ones: their signatures are well known and
> QMetaObject provide the rest, did i miss something?

Obviously i did miss something! ;)
First FunctionOverwriter is not a misnomer, it does overwrite the
function implementation not the symbol. So once overwritten there is
no way to call the original implementation.
Then, the function overwriter overwrites special Qt function, that are
designed for this purpose, i cannot use that to overwrite arbitray C++
symbol. Or at least not in it's current form.

I've noticed that Qt export as well QEventLoop::exec() in it's
QtCore.dynlist, not sure what QtCore.dynlist is about, but this file
contains the 3 "special" method mentioned above along this one.

I've seen as well QInternal::registerCallback and co... again not sure
how it could be used.

I don't think that installing eventFilter on objects is the best way
to monitor event dispatching and filtering.

It would be really nice to have runtime event analysis, in the mean
time i'll switch back to compile/link time hacks...

Chris

>
> The only thing that i think would need work is the C++ object
> wrapping, as FunctionOverwriter seems to be geared toward C, not C++.
> Basically i need access to the C++ "this" [1] and the
> FunctionOverwriter's trampoline code in a preloaded SO that doesn't
> depend on Qt...
>
> Well, that's the theory. I will then have to find a way for this
> preloaded SO to exec custom hooks/handlers. I don't think it should be
> that hard but memory allocators, multi-threadings and
> auto/self-induced events will certainly make the job harder thought...
>
> Now, given the high-profile of GammaRay developpers, I have the
> feeling that you never went this path by choice (ie: not for technical
> reasons obviously! ;)), would you accept this sort of hackish stuff in
> GammaRay? A pre-loaded SO dedicated to Qt event profiling?
>
> PS: If I ever go further, i will eventually want to get that working
> on arm32-linux too.
>
> Thanks,
> Chris
>
> [1] https://blog.kummerlaender.eu/article/notes_on_function_interposition_in_cpp/
>
>>
>> Meta-data associated with such occurences:
>> - entry/exit timestamps
>> - nesting level (per thread)
>> - sender object (if one and possible to deduce)
>> - sender thread
>> - receiver object
>> - receiver thread
>> - event payload
>> - filter result
>> - ...
>>
>> The end user should be able to add/remove monitoring conditions,
>> maybe simply based on:
>> - sender thread
>> - receiver object
>> - event type
>>
>> The visualisation would be a tree model (for a given thread), as I am
>> particularly interested in nesting and ordering analysis.
>>
>> I know i'm quite vague, but what do you guys think? Does such a tool
>> already exists? Open source or not? Does that sound something that
>> could be done with gammaray?
>>
>> Chris