Bug 4425 - Background Execution sample throws UIKitThreadAccessException when multitasking on simulator
Summary: Background Execution sample throws UIKitThreadAccessException when multitaski...
Status: RESOLVED FIXED
Alias: None
Product: iOS
Classification: Xamarin
Component: General ()
Version: 5.3.x
Hardware: PC Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
URL:
Depends on:
Blocks:
 
Reported: 2012-04-13 12:32 UTC by PJ
Modified: 2012-04-16 10:35 UTC (History)
2 users (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED FIXED

Description PJ 2012-04-13 12:32:58 UTC
Steps to reproduce:
1. Run the BackgroundExecution Sample on simulator
2. Press [Start Long Running Task]
3. Press the home button
4. Open another app (eg Safari)

Result:
Stacktrace: https://gist.github.com/2377539

I'm only seeing this behavior on simulator, but 360logica got the same result on Release / iPad2.


Found on:
MD 2.8.8.4
Mono 2.10.9
MT 5.3.3 5ee2421f4f7722353285c61220c5e97090b17fe1
Comment 1 Sebastien Pouliot 2012-04-13 14:45:10 UTC
Release builds do not contain the UI thread checks (unless some extra command line options were included). Sure it's not a Debug build (or at debug is enabled) ?
Comment 2 PJ 2012-04-13 14:48:00 UTC
Yeah I couldn't reproduce that result either.

I'll check again to see if they had a false negative when they're back on Sunday night.
Comment 3 Sebastien Pouliot 2012-04-13 16:33:55 UTC
the UI check comes from BackgroundTimeRemaining
http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIApplication_Class/Reference/Reference.html#//apple_ref/occ/instp/UIApplication/backgroundTimeRemaining

It's not marked as thread safe but, sadly, Apple docs are rarely clear about thread-safety.

In this case the API is meant to be called (at least useful) on a background thread (since main thread is suspended). Googling shows code that use it inside background thread (including sample the "IOS 5 Programming Cookbook" book).

Fixed in 662319ab2cffed8d12c3b48ca034ce1af2d4f981

but please try this again, without the fix, since it should not happen on release builds.
Comment 4 PJ 2012-04-16 09:58:49 UTC
Heyo,

I verified that this error occurs on the same build I found the initial issue on using Release | iPhoneSimulator


http://screencast.com/t/EQHhlhRERsA


Have not yet tested the fix.
Comment 5 Sebastien Pouliot 2012-04-16 10:07:07 UTC
Please check (to be 100% sure) but "Release | iPhoneSimulator" does not use the linker (by default) so the UI checks won't be removed. That's perfectly normal.

OTOH the default, for devices, is to use the linker and *if* not on debug the UI checks should not be enabled (unless some other command lines options are added). If this can be duplicated on Release|iPhone (like the original description said) then there's an issue.
Comment 6 PJ 2012-04-16 10:35:08 UTC
I have not yet been able to reproduce on device, and I can't again now. Think we're good!

That Release | iPad result was from 360 and it was just a false negative upon re-test.