Bug 51313 - NavigationPage.PopAsync throwing IndexOutOfBounds
Summary: NavigationPage.PopAsync throwing IndexOutOfBounds
Status: RESOLVED INVALID
Alias: None
Product: Forms
Classification: Xamarin
Component: Forms ()
Version: 2.3.3
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: ---
Assignee: Bugzilla
URL:
Depends on:
Blocks:
 
Reported: 2017-01-09 21:24 UTC by Brandon Risell
Modified: 2017-02-15 14:09 UTC (History)
4 users (show)

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

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Status:
RESOLVED INVALID

Description Brandon Risell 2017-01-09 21:24:25 UTC
We ran into a bug where NavigationPage.PopAsync() was called from a couple threads very quickly. Both threads are able to get past a safety check, because neither has started yet. They then both try to remove the calling page from the backing collection. One succeeds and the other throws an index out of bounds exception.

You can see where it's checking if a current navigation is happening:
https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Core/NavigationPage.cs#L117

and then both threads continue onto:
https://github.com/xamarin/Xamarin.Forms/blob/master/Xamarin.Forms.Core/NavigationPage.cs#L129


Here is a console app that reproduces the issue:


class MainClass
{
    private static ManualResetEvent _mre = new ManualResetEvent(false);
    private static NavigationPage _nav = new NavigationPage();

    public static void Main(string[] args)
    {
        var iterationCount = 0;
        while (true)
        {
            iterationCount++;
            System.Console.WriteLine($"Attempt #{iterationCount} to trigger the thread issue");

            var page1 = new ContentPage { Content = new Label() };
            var page2 = new ContentPage { Content = new Label() };

            _nav.Navigation.PushAsync(page1).Wait();
            _nav.Navigation.PushAsync(page2).Wait();

            var thread1 = new Thread(ThreadToPopNav);
            var thread2 = new Thread(ThreadToPopNav);

            thread1.Start();
            thread2.Start();

            Thread.Sleep(500);

            _mre.Set();

            thread1.Join();
            thread2.Join();

            _mre.Reset();
        }
    }

    private static void ThreadToPopNav()
    {
        // Wait for signal
        _mre.WaitOne();

        // Pop it like its hot
        _nav.Navigation.PopAsync().Wait();

        System.Console.WriteLine($"{Thread.CurrentThread.ManagedThreadId}: {DateTime.Now.Ticks}");
    }
}
Comment 1 adrianknight89 2017-01-10 01:39:36 UTC
CurrentNavigationTask needs to be dealt with properly. I think it leads to a memory leak. Not sure if it's related to your issue though.
Comment 2 Stephane Delcroix 2017-02-15 14:09:45 UTC
you're not supposed to Push() or Pop() from anything but the main thread. If you do you can get unexpected result, like this one, but also there is a high chance that the platform rendering won't work correctly.