Re: wxPython 4/wx.Yield: C++ assert failure

Hi.

I'm the NVDA developer Joseph was discussing this with. First, my thanks to 
all who have helped with this so far.

Even though I totally understand that wx.Yield can cause re-entrance (this 
is clearly documented), this wasn't previously the case when starting the 
same timer within that timer's callback. That is, *another* timer or a wx 
event might get fired within wx.Yield, but the *same* timer never fired. 
Instead, the inner call to the timer was dropped. That is what this test 
case demonstrates. In wxPython 3, you never see the re-entrance message. In 
wxPython 4, you do.

The question is whether this change is intentional/expected and whether the 
previous behaviour will be restored. It's reasonable to argue that we were 
relying on undefined behaviour (or maybe even a bug), and if that is the 
case and this won't be reverted, we'll find another way. However, I wanted 
to confirm this before putting a huge amount of work into "fixing" this.

The reason we use wx.Yield here is that we need to trigger a Windows 
message pump when sending key presses and checking for pending 
accessibility events, since keyboard events and accessibility events depend 
on the Windows message queue. I guess we could just do our own message 
pump, but I'm concerned this would end up processing wx events anyway.

Thanks!

Jamie

On Friday, April 28, 2017 at 4:30:44 AM UTC+10, [email protected] 
wrote:
>
> Hi Joseph,
>
>  
>
> The NVDA developer is likely correct, but your example shows expected 
> behavior, not a bug. You're adding an infinite number of timer events, then 
> calling wx.Yield in them. This will cause infinite recursion, as wx.Yield 
> will process the next timer event, which will call wx.Yield, which will 
> process the next timer event, which will call wx.Yield, and so on. I don't 
> think a change in wx.Yield is at fault here, it's just alerting you to the 
> recursion instead of causing it. To get to the bottom of this, you'll need 
> to find out what is triggering the recursion in NVDA.
>
>  
>
> Taking a quick look at the NVDA code's use of wx.Yield, I think the first 
> step would probably be to find out if api.py's processPendingEvents is 
> somehow calling itself multiple times. If you can identify the specific set 
> of steps that trigger recursion, the NVDA developers will probably be able 
> to come up with some ideas for workarounds or fixes.
>
>  
>
> Regards,
>
>  
>
> Kevin
>
>  
>
> *From: *<[email protected] <javascript:>> on behalf of Joseph 
> Lee <[email protected] <javascript:>>
> *Reply-To: *<[email protected] <javascript:>>
> *Date: *Thursday, April 27, 2017 at 9:36 AM
> *To: *<[email protected] <javascript:>>
> *Subject: *RE: [wxPython-dev] wxPython 4/wx.Yield: C++ assert failure
>
>  
>
> Hi Robin,
>
> At least wx.YieldIfNeeded suppresses the assertions. However, according to 
> a lead NVDA developer, this does not really resolve this problem, as NVDA 
> needs to handle other events external to wxWidgets such as accessibility 
> events. The lead developer (not me) believes that the root problem might 
> have to do with timers and certain things that are internally different 
> between wxPython classic and phoenix.
>
> Can you try out this example and see if we can trace this:
>
> https://www.dropbox.com/s/p8uuf52hzlmyd3x/wxReentrance.py?dl=1
>
>  
>
> Note that the example requires Python 2.7 with wxPython 4.0.0A1. Thanks.
>
> Cheers,
>
> Joseph
>
>  
>
> *From:* [email protected] <javascript:> [mailto:
> [email protected] <javascript:>] *On Behalf Of *Robin Dunn
> *Sent:* Thursday, April 27, 2017 12:17 AM
> *To:* [email protected] <javascript:>
> *Subject:* Re: [wxPython-dev] wxPython 4/wx.Yield: C++ assert failure
>
>  
>
> Joseph Lee wrote:
>
>
> Hi Robin and others,
>
>  
>
> In one of the projects I’m involved in (NonVisual Desktop Access), 
> numerous calls to wx.Yield function are made from various modules. After 
> porting NVDA to wxPython 4 alpha, when wx.Yield is called from some 
> modules, I get a traceback that ends with the following:
>
> wxAssertionError: C++ assertion "Assert failure" failed at 
> ..\..\src\common\evtloopcmn.cpp(110) in wxEventLoopBase::Yield(): wxYield 
> called recursively
>
>  
>
> Relevant GitHub issue for NVDA can be found at:
>
> https://github.com/nvaccess/nvda/issues/7077
>
>  
>
> The source code for a version of NVDA that uses wxPython 4 can be found at:
>
> https://github.com/josephsl/nvda
>
>  
>
> Branch is “wxPy4”. I and other NVDA developers involved in porting NVDA to 
> wxPython 4 consider this a showstopper, with a lead developer commenting 
> that this might be due to reentrancy issue.
>
>
> Yes. Basically some event(s) happens while within a wx.Yield call which 
> then results in some other code being called that uses wx.Yield.
>
> The best fix would be to find ways to avoid needing to use yield. 
> Otherwise you could switch to wx.YieldIfNeeded() (which is just a 
> convenience wrapper for wx.GetApp().Yield(True)...) That will just return 
> when the recursion is detected instead of asserting about it.  The 
> wx.EventLoopBase.Yield documentation is applicable to this as all the 
> incarnations of Yield eventually get to that method.
>
>
> -- 
> Robin Dunn
> Software Craftsman 
> http://wxPython.org
>
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