[Audacity-devel] Linux OverlayPanel bug

[Darrell Walisser]

I have been having this for while and assumed it was the oddity of my
system setup. I use LXDE with compositor disabled, on Ubuntu 16.04, which
is a bit unusual.

The assert (see trace below) happens frequently when starting audacity or
closing a project, but not always. It happens just before the project
window is shown, or in the process of being shown for the first time.

The first time time OverlayPanel::DrawOverlays is called, pDC is null, so
wxClientDC is created. However, dc.IsOk() will return true and yet
dc.GetHandle() will return null, leading to the assert fail shortly after. The
rest of the time, pDC is not null and there is no error.

Assuming the client dc is in fact not ready to do anything, I arrived at
the following patch. I'm not familiar enough with wxWidgets to know if this
is the best fix.

diff --git a/src/widgets/OverlayPanel.cpp b/src/widgets/OverlayPanel.cpp
index bbd5e2d..cbb7053 100644
--- a/src/widgets/OverlayPanel.cpp
+++ b/src/widgets/OverlayPanel.cpp
@@ -84,6 +84,10 @@ void OverlayPanel::DrawOverlays(bool repaint_all, wxDC
*pDC)
Maybe<wxClientDC> myDC;
auto &dc = pDC ? *pDC : (myDC.create(this), *myDC);

+ // Somehow this can be null, maybe just before project window is
realized
+ if (!dc.GetHandle())
+ return;
+
// Erase
bool done = true;
auto it2 = pairs.begin();





---
ASSERT INFO:
../src/gtk/dcclient.cpp(2043): assert "m_window" failed in DoGetSize():
GetSize() doesn't work without window

BACKTRACE:
[1] wxClientDCImpl::DoGetSize(int*, int*) const
[2] wxRect::wxRect(wxSize const&) /usr/include/wx-3.0/wx/gdicmn.h:710
[3] OverlayPanel::DrawOverlays(bool, wxDC*)
/home/ghaxt/sw/audacity/src/widgets/OverlayPanel.cpp:100
[4] TrackPanel::OnTimer(wxTimerEvent&)
/home/ghaxt/sw/audacity/src/TrackPanel.cpp:972
[5] wxAppConsoleBase::CallEventHandler(wxEvtHandler*, wxEventFunctor&,
wxEvent&) const
[6] wxEvtHandler::ProcessEventIfMatchesId(wxEventTableEntryBase const&,
wxEvtHandler*, wxEvent&)
[7] wxEventHashTable::HandleEvent(wxEvent&, wxEvtHandler*)
[8] wxEvtHandler::TryHereOnly(wxEvent&)
[9] wxEvtHandler::ProcessEventLocally(wxEvent&)
[10] wxEvtHandler::ProcessEvent(wxEvent&)
[11] wxEvtHandler::ProcessPendingEvents()
[12] wxAppConsoleBase::ProcessPendingEvents()
[13] wxApp::DoIdle()
[14] g_main_context_dispatch
[15] g_main_loop_run
[16] gtk_main
[17] wxGUIEventLoop::DoRun()
[18] wxEventLoopBase::Run()
[19] wxAppConsoleBase::MainLoop()
[20] wxEntry(int&, wchar_t**)
[21] main /home/ghaxt/sw/audacity/src/AudacityApp.cpp:726
[22] __libc_start_main
[23] _start

[Darrell Walisser]

​I found an alternate solution by deferring the timer start in TrackPanel
until the window is shown.​ This Binds() the wxEVT_SHOW on TrackPanel to
trigger the timer, then it Unbinds() as its no longer needed. So at least
the nature of the bug is a bit better defined. wxShowEvent() is not a great
idea since the docs say its only available on MSW and GTK versions.

As its not a priority right now, I'll submit a bug and we can go from there.

[Darrell Walisser]

​Not a new bug after all, I didn't think to search until now. I'll be
commenting there.

http://bugzilla.audacityteam.org/show_bug.cgi?id=1401​

[James Crook]

​I found an alternate solution by deferring the timer start in TrackPanel
until the window is shown.​ This Binds() the wxEVT_SHOW on TrackPanel to
trigger the timer, then it Unbinds() as its no longer needed. So at least
the nature of the bug is a bit better defined. wxShowEvent() is not a great
idea since the docs say its only available on MSW and GTK versions.
As its not a priority right now, I'll submit a bug and we can go from there.

Belay that. It IS a priority for 2.1.3 RC3 now. With you having it
reproducible (none of us did), it is both more serious - and more fixable.
Anything wrong with using IsShownOnScreen()?

Whatever, very glad to have your 'overlay panel bug' related to 1401.

​Not a new bug after all, I didn't think to search until now. I'll be
commenting there.

--James.