Fixed race condition cauzing application crash on Android.

[gordeevbr]

Adynathos,
Hello.

I am currently using this plugin to develop an Android application and I came across this bug.

The bug can be reproduced by launching an application with AURCameraActor set as default actor and with camera feed opening as default video source. That being said, launching with no video seems to work fine. Right after splash screen, the application "freezes" with some ripples on the screen and shortly crashes.

Using logcat I was able to retrieve the following backtrace from the crashed app:

--------- beginning of crash
F/libc ( 6285): Fatal signal 11 (SIGSEGV), code 1, fault addr 0x50 in tid 6489 (RenderThread 1)
D/clmlib ( 408): Got activities:0x0000000E
I/DEBUG ( 408): *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
I/DEBUG ( 408): UUID: e28a0bfe-226d-4d45-a1c2-2f178a4bad65
I/DEBUG ( 408): Build fingerprint: 'Sony/SGP321/SGP321:5.1.1/10.7.A.0.222/2921970873:user/release-keys'
I/DEBUG ( 408): Revision: '0'
I/DEBUG ( 408): ABI: 'arm'
I/DEBUG ( 408): pid: 6285, tid: 6489, name: RenderThread 1 >>> com.gordeevbr.demo <<<
I/DEBUG ( 408): signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x50
I/DEBUG ( 408): r0 00000000 r1 96003c80 r2 19594001 r3 00001959
I/DEBUG ( 408): r4 8ce6c000 r5 00000280 r6 988fe734 r7 00000000
I/DEBUG ( 408): r8 00008e76 r9 00270400 sl 988fe738 fp 9189baa0
I/DEBUG ( 408): ip a2fe7bd4 sp 9189ba08 lr b6e12287 pc a0f58530 cpsr 60070010
I/DEBUG ( 408):
I/DEBUG ( 408): backtrace:
I/DEBUG ( 408): #00 pc 05543530 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (UAURDriver::WriteFrameToTexture())
I/DEBUG ( 408): #1 pc 0555b3cc /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FAURVideoConfiguration::FAURVideoConfiguration(FAURVideoConfiguration const&))
I/DEBUG ( 408): #2 pc 01e989e0 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FNamedTaskThread::ProcessTasksNamedThread(int, bool)+8492)
I/DEBUG ( 408): #3 pc 01e96114 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FNamedTaskThread::ProcessTasksUntilQuit(int)+176)
I/DEBUG ( 408): #4 pc 043a838c /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (RenderingThreadMain(FEvent*)+252)
I/DEBUG ( 408): #5 pc 043b5914 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FRenderingThread::Run()+20)
I/DEBUG ( 408): #6 pc 01f02730 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FRunnableThreadPThread::Run()+284)
I/DEBUG ( 408): #7 pc 01e913a0 /data/app/com.gordeevbr.demo-1/lib/arm/libUE4.so (FRunnableThreadPThread::_ThreadProc(void*)+176)
I/DEBUG ( 408): #8 pc 000135e7 /system/lib/libc.so (__pthread_start(void*)+30)
I/DEBUG ( 408): #9 pc 00011513 /system/lib/libc.so (__start_thread+6)

Lines 00 and 01 of backtrace I had to retrieve with addr2line.

After inspecting every variable in the 'ENQUEUE_UNIQUE_RENDER_COMMAND_ONEPARAMETER' block of 'UAURDriver::WriteFrameToTexture()' for null pointers, I noticed that indeed 'UpdateParameters->Texture2DResource->GetTexture2DRHI()' sometimes returned null pointer just right before the crash. My guess is that this is a race condition.

After applying changes from this commit, I was not able to reproduce the error anymore. Please review it and accept if necessary.

[adynathos]

Thank you very much for diagnosing and fixing the problem.
There is never enough checks for null 😄

I also see than in UE4.17 there may be some better way of drawing the video on the texture, they mention some Image Plate Actor

[gordeevbr]

@adynathos no problem. I am currently using this plugin for a project, so I might be contributing some of the stuff I develop. Hopefully, you don't mind contributions.
Yes, I also saw the news about Image Plate Actor, but didn't research into it yet. I guess I will try to do so.
Android ARCore support for UE4 might make things easier for AR development, but it's only supported on Androids 7.0+, so it's not the best option at the moment.