[PLAT-6163] Fix recrash deadlocks

[nickdowell]

Goal

Fix deadlocks that could occur if the crash handling code (or the app's onCrashHandler callback) crashes.

Changeset

• Fixes a deadlock caused by the Mach exception handling threads being suspended due to context->reservedThreads being clobbered in bsg_kscrashsentry_clearContext().
• Fixes a deadlock due to thread_terminate being called on original crashing thread when handling a nested Mach exception.
• Ensures that bsg_kscrashsentry_beginHandlingCrash() is called when an unhandled NSException is detected, so that crashes during crash handling can be identified.

There remains a deadlock on macOS 10.14 and earlier if a signal handler crashes. This appears to be a bug in the OS and not something we can work around.

Sample code

#include <signal.h>
#include <stdlib.h>

static void signal_handler(int signum) {
  // Crashing the signal handler in this way will deadlock on macOS <= 10.14
  // on more recent versions of macOS the process will terminate as expected.
  static volatile int *ptr;
  *ptr = 42;
}

int main(int argc, const char * argv[]) {
  signal(SIGABRT, signal_handler);
  abort();
  return 0;
}

Testing

E2E tests have been set up on a separate branch (will be included in a future PR) and passed on CI: https://buildkite.com/bugsnag/bugsnag-cocoa/builds/4195

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size did not change - 1,253,256 bytes

Generated by 🚫 Danger