objective C exceptions not handled properly

[timotheecour]

objective C exceptions not handled properly

D20191113T130903

Example

clang++ -shared -framework Foundation -o $build_D/temp/libt0782b.dylib $timn_D/tests/nim/all/t0782b.mm
nim cpp -r $timn_D/tests/nim/all/t0782.nim

# t0782.nim:
{.passL: "-L$build_D/temp/ -lt0782b".}
proc fun_cpp() {.importc.}
proc fun_objc() {.importc.}

proc main()=
  when defined(case_fun_cpp):
    fun_cpp()
  else:
    fun_objc()

main()

// t0782b.mm:
#include <stdio.h>
#include <exception>
#include <stdexcept>

// #import <Foundation/NSExceptions.h>
#import <Foundation/Foundation.h>

extern "C"{
void fun_objc(){
  printf("in fun1\n");
  NSException* myException = [NSException
          exceptionWithName:@"MyException"
          reason:@"blah blah"
          userInfo:nil];
  @throw myException;
}

void fun_cpp(){
  printf("in fun2\n");
  throw std::runtime_error("bad");
}

}

Current Output

in fun1
Error: unhandled unknown cpp exception

Expected Output

should show stacktrace, exception name, reason, userData (see https://developer.apple.com/documentation/foundation/nsexception)

Additional Information

• this is because of current implementation of setTerminate in excpt.nim which doesn't handle the objc case.

• this is especially problematic when such exceptions are thrown in an unsuspecting way, eg if calling a (nim-wrapped) cv::imshow from a non-main-thread, on OSX, it'll throw a objc exception, but nim will hide all useful info and just write Error: unhandled unknown cpp exception instead of smthg like:

2019-11-13 13:07:43.073 smartcam_main[48737:17265707] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'NSWindow drag regions should only be invalidated on the Main Thread!'
*** First throw call stack:
(
        0   CoreFoundation                      0x00007fff2dc9bf53 __exceptionPreprocess + 250
        1   libobjc.A.dylib                     0x00007fff63d61835 objc_exception_throw + 48
        2   CoreFoundation                      0x00007fff2dcb652c -[NSException raise] + 9
        3   AppKit                              0x00007fff2ae6a6b2 -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] + 310
        4   AppKit                              0x00007fff2ae67bc1 -[NSWindow _initContent:styleMask:backing:defer:contentView:] + 1416
        5   AppKit                              0x00007fff2ae67633 -[NSWindow initWithContentRect:styleMask:backing:defer:] + 42
        6   AppKit                              0x00007fff2b0fa0d8 -[NSWindow initWithContentRect:styleMask:backing:defer:screen:] + 52
        7   libopencv_highgui.4.1.dylib         0x000000010e055496 cvNamedWindow + 368
        8   libopencv_highgui.4.1.dylib         0x000000010e054fa4 cvShowImage + 95

when nim's setTerminate isn't called

[timotheecour]

I tried adapting https://github.com/wix/Detox/blob/master/detox/ios/Detox/DetoxCrashHandler.mm and it works.

except.nim:

  const timn_objc_handler = defined(timn_objc_handler) # D20191113T130903
  when timn_objc_handler:
    {.passL: "-L$build_D/temp/ -lnimextras_objc".}
  setTerminate proc() {.noconv.} =
    proc terminate_objc_handler(msg: cstring) {.importc: "terminate_objc_handler".}
    # Remove ourself as a handler, reinstalling the default handler.
    setTerminate(nil)
    var msg = "Unknown error in unexpected exception handler"
    try:
      raise
    except Exception:
      msg = currException.getStackTrace() & "Error: unhandled exception: " &
        currException.msg & " [" & $currException.name & "]"
    except StdException as e:
      when timn_objc_handler:
        terminate_objc_handler($e.what())
      msg = "Error: unhandled cpp exception: " & $e.what()
    except:
      when timn_objc_handler:
        terminate_objc_handler("")
      msg = "Error: unhandled unknown cpp exception"

    when defined(genode):
      # stderr not available by default, use the LOG session
      echo msg
    else:
      writeToStdErr msg & "\n"

    quit 1

nimextras_objc.mm:

extern "C"
void terminate_objc_handler(const char* msg){
    // prelude
    @try {
      __cxxabiv1::__cxa_rethrow();
    }
    @catch (id e) { // handle objc exception to print stacktrace, name, userInfo, reason
    } 
    @catch (...) {  // handle C++ exception ; use __cxa_current_exception_type, __cxa_demangle etc
    }
}

clang++ -shared -framework Foundation -o $build_D/temp/libnimextras_objc.dylib $timn_D/src/timn/nimextras_objc.mm

it works but I'm wondering whether there's a simpler way