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

Hi Jamie,

 

On Mac, the sample code does trigger the re-entrance warning on wxPython 3. I suspect the same test on Linux would provide similar results. The reason it doesn't on Windows is that the time.sleep(1) call appears to be causing the timer event not to even be started, or at least fired. When I remove that time.sleep call, I get the re-entrance warning on Windows too.

 

That time.sleep(1) would stop a wx.PyTimer from being started or fired is definitely not expected or defined behavior, and is almost certainly some sort of bug, possibly with Classic's SWIG bindings on Windows.

 

So is the need to use wx.Yield because you have to process those keyboard and accessibility events right at that moment in the code? i.e. you can't wait until the normal event processing happens?

 

Regards,

 

Kevin

 

From: <[email protected]> on behalf of James Teh <[email protected]>
Reply-To: <[email protected]>
Date: Thursday, April 27, 2017 at 11:28 PM
To: wxPython-dev <[email protected]>
Subject: Re: [wxPython-dev] 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]> on behalf of Joseph Lee <[email protected]>
Reply-To: <[email protected]>
Date: Thursday, April 27, 2017 at 9:36 AM
To: <[email protected]>
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] [mailto:[email protected]] On Behalf Of Robin Dunn
Sent: Thursday, April 27, 2017 12:17 AM
To: [email protected]
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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