How to prevent iOS crash reporters from crashing MonoTouch apps?

Question

There are plenty iOS crash reporting libraries in iOS, including TestFlight and HockeyApp. If you don't want to depend on services, you can still use libraries like PLCrashReporter. Binding these libraries is fairly trivial because their public API usually consists of a couple of classes with several initialization methods.

However, when trying to use TestFlight, and later HockeyApp in our application, our app began to randomly crash. Turns out, this is a known issue reported several times, but Xamarin doesn't warn about it, it is relatively obscure and we found it the hard way.

We have learned that all iOS crash reporters prevent Mono from catching null reference exceptions:

try {
    object o = null;
    o.GetHashCode ();
} catch {
    // Catch block isn't called with crash reporting enabled.
    // Instead, the app will crash.
}

Why does this happen? Quoting Rolf, a Xamarin developer,

A null reference exception is actually a SIGSEGV signal at first. Usually the mono runtime handles this and translates it into a nullreference exception, allowing the execution to continue. The problem is that SIGSEGV signals are a very bad thing in ObjC apps (and when it occurs outside of managed code), so any crash reporting solution will report it as a crash (and kill the app) - this happens before MonoTouch gets a chance to handle the SIGSEGV, so there is nothing MonoTouch can do about this.

I'm sure many use TestFlight in MonoTouch apps without knowing it causes crashes.
Isn't it ironic?

How do you make crash reporting libraries not crash MonoTouch apps?

Answer 1

Put this in AppDelegate.cs:

[DllImport ("libc")]
private static extern int sigaction (Signal sig, IntPtr act, IntPtr oact);

enum Signal {
    SIGBUS = 10,
    SIGSEGV = 11
}

static void EnableCrashReporting ()
{
    IntPtr sigbus = Marshal.AllocHGlobal (512);
    IntPtr sigsegv = Marshal.AllocHGlobal (512);

    // Store Mono SIGSEGV and SIGBUS handlers
    sigaction (Signal.SIGBUS, IntPtr.Zero, sigbus);
    sigaction (Signal.SIGSEGV, IntPtr.Zero, sigsegv);

    // Enable crash reporting libraries
    EnableCrashReportingUnsafe ();

    // Restore Mono SIGSEGV and SIGBUS handlers            
    sigaction (Signal.SIGBUS, sigbus, IntPtr.Zero);
    sigaction (Signal.SIGSEGV, sigsegv, IntPtr.Zero);

    Marshal.FreeHGlobal (sigbus);
    Marshal.FreeHGlobal (sigsegv);
}

static void EnableCrashReportingUnsafe ()
{
    // Run your crash reporting library initialization code here--
    // this example uses HockeyApp but it should work well
    // with TestFlight or other libraries.

    // Verify in documentation that your library of choice
    // installs its sigaction hooks before leaving this method.

    var manager = BITHockeyManager.SharedHockeyManager;
    manager.Configure (HockeyAppId, null);
    manager.StartManager ();
}

Call EnableCrashReporting () in the beginning of FinishedLaunching method.
Wrap this call in #if !DEBUG directive if you want.

---

How does it work?

I followed Rolf's suggestion:

One possible solution is to allow mono to handle all SIGSEGV signals (technically speaking the crash reporting lib should either not handle the SIGSEGV signal, or it should chain to mono's handler and not do any processing by itself). If mono determines that the SIGSEGV signal is not from managed code (i.e. something very bad happened), it will raise a SIGABORT signal (which the crash reporting lib should already handle and treat as a crash). As you can understand this is something that has to be done in the crash reporting library.

And Landon Fuller's Objective C implementation:

#import <signal.h>

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    /* Save Mono's signal handler actions */
    struct sigaction sigbus_action, sigsegv_action;
    sigaction(SIGBUS, NULL, &sigbus_action);
    sigaction(SIGSEGV, NULL, &sigsegv_action);

    // Enable the crash reporter here. Ie, [[PLCrashReporter sharedReporter] enableCrashReporterAndReturnError:],
    // or whatever is the correct initialization mechanism for the crash reporting service you're using

    /* Restore Mono's signal handlers */
    sigaction(SIGBUS, &sigbus_action, NULL);
    sigaction(SIGSEGV, &sigsegv_action, NULL);

    return YES;
}

I used Banshee source code as a reference point for how to call sigaction from MonoTouch.

Hope it helps!