Calling performSelector for a @objc_method created by rubicon from Objective C crashes in Python 3.9

[yilei]

Describe the bug
In Python 3.9, calling [obj performSelector:NSSelectorFromString(@"name")] crashes with EXC_BAD_ACCESS (code=2, address=0x1056f7f48) if the obj is an instance of a class created by rubicon with @objc_method def name(self): ....

To Reproduce
Steps to reproduce the behavior:

1. Create a briefcase iOS project:

   $ briefcase new
   # Give it a name `py39crashdemo`
   $ cd py39crashdemo
   $ briefcase create iOS
   

2. Add the objc class definition using rubicon in app.py:

   $ cat iOS/Xcode/py39crashdemo/py39crashdemo/app/py39crashdemo/app.py
   """
   My first application
   """
   from rubicon import objc
   class Runner(objc.NSObject):
       @objc.objc_method
       def execScript(self):
           pass
   def main():
       # This should start and launch your app!
       Interop = objc.ObjCClass("Interop")  # Interop is a class defined in objc.
       Interop.sharedRunner = Runner.alloc().init()
   

3. Add the objective code that invokes the execScript method:

    $ cat iOS/Xcode/py39crashdemo/py39crashdemo/main.m
    //
    //  main.m
    //  A main module for starting Python projects under iOS.
    //
    
    #import <Foundation/Foundation.h>
    #import <UIKit/UIKit.h>
    #include <Python.h>
    #include <dlfcn.h>
    
    NSObject *_sharedRunner;
    
    @interface Interop: NSObject
    @property(class) NSObject *sharedRunner;
    @end
    @implementation Interop
    + (void)setSharedRunner:(NSObject *)sharedRunner {
        _sharedRunner = sharedRunner;
    }
    + (NSObject *)sharedRunner {
        return _sharedRunner;
    }
    @end
    
    @interface PythonAppDelegate : NSObject<UIApplicationDelegate>
    @end
    @implementation PythonAppDelegate
    - (void)applicationDidFinishLaunching:(UIApplication *)application {
        [Interop.sharedRunner performSelector:NSSelectorFromString(@"execScript")];  // <---- This is the crash
    }
    @end
    
    int main(int argc, char *argv[]) {
    // ... below are untouched
   

4. Run the iOS application, and it crashes with EXC_BAD_ACCESS (code=2, address=0x1056f7f48) at [Interop.sharedRunner performSelector:NSSelectorFromString(@"execScript")];

Expected behavior
It should not crash.

Environment:

• Operating System: macOS 11.3
• Python version: 3.9.4
• Software versions:
  • Briefcase: 0.3.5
  • Xcode: 12.5
  • Toga: n/a
  • ...

Additional context
I had an earlier conversion on BeeWare Discoard with @freakboy3742, and I'm instructed to file this bug.

[dgelessus]

Thank you for the bug report! (For reference, here is a link to the Discord discussion.)

To check if the crash has anything to do with performSelector:, can you try calling the Python-defined method directly and see if that crashes as well? If you do [Interop.sharedRunner execScript] in the Objective-C code, the compiler will give you a warning (because NSObject doesn't declare an execScript method), but it should still compile and run.

If you want to avoid the warning, you can define a protocol on the Objective-C side for the Python-defined Runner class to implement:

@protocol RunnerProtocol
-(id)execScript;
@end
NSObject<RunnerProtocol> *_sharedRunner;
// and all other declarations rewritten to use NSObject<RunnerProtocol>

And on the Python side:

RunnerProtocol = objc.ObjCProtocol("RunnerProtocol")
class Runner(objc.NSObject, protocols=[RunnerProtocol]):
    # ...

Then you should be able to call [Interop.sharedRunner execScript] without any compiler warnings.

[yilei]

Thanks. I need to use the protocol approach (otherwise it fails to compile, probably there is a flag to turn it to warning/error?), but calling [Interop.sharedRunner execScript] crashes in the same way.

[freakboy3742]

Doing some housekeeping; I've transferred this issue to the Rubicon-ObjC repository, since it doesn't appear to be specifically related to the Apple support build.