[runtime] Detect recursion when handling unhandled Objective-C except…

…ions. Fixes #14796. (#20276)

Detect recursion when handling unhandled Objective-C exceptions, which can happen if:

* We install a handler for unhandled Objective-C exceptions.
* An unhandled Objective-C exception is caught.
* We convert the unhandled Objective-C exception to a managed exception, and throw that.
* Nobody handles the managed exception either, so we convert it to an Objective-C exception and throw that.
* We re-enter the unhandled Objective-C exception handler, and the cycle continues.
* Eventually the process crashes due to a stack overflow.

Fixes #14796.

runtime/runtime.m

@@ -1133,18 +1133,24 @@ -(void) xamarinSetFlags: (enum XamarinGCHandleFlags) flags;
 	abort ();
 }
 
+extern "C" {
+	static thread_local int xamarin_handling_unhandled_exceptions = 0;
+}
+
 static void
 exception_handler (NSException *exc)
 {
 	// COOP: We won't get here in coop-mode, because we don't set the uncaught objc exception handler in that case.
 	LOG (PRODUCT ": Received unhandled ObjectiveC exception: %@ %@", [exc name], [exc reason]);
 
-	if (xamarin_is_gc_coop) {
-		PRINT ("Uncaught Objective-C exception: %@", exc);
-		assert (false); // Re-throwing the Objective-C exception will probably just end up with infinite recursion
+	if (xamarin_handling_unhandled_exceptions == 1) {
+		PRINT ("Detected recursion when handling uncaught Objective-C exception: %@", exc);
+		abort ();
 	}
 
+	xamarin_handling_unhandled_exceptions = 1;
 	xamarin_throw_ns_exception (exc);
+	xamarin_handling_unhandled_exceptions = 0;
 }
 
 #if defined (DEBUG)
@@ -2369,6 +2375,14 @@ -(void) xamarinSetFlags: (enum XamarinGCHandleFlags) flags;
 			xamarin_gchandle_free (exception_gchandle);
 		}
 
+		// If we end up here as part of an unhandled Objective-C exception, we're also trying to detect an infinite loop by
+		// setting the xamarin_handling_unhandled_exceptions variable to 1 before processing the unhandled Objective-C exception,
+		// and clearing it afterwards. However, the clearing of the variable will never happen if Mono unwinds native
+		// stack frames, and then on the next unhandled Objective-C exception we'll think we're recursing when we're really not.
+		// (FWIW this is yet another reason why letting Mono unhandled native frames is a really bad idea).
+		// So here we work around that by clearing the variable before letting Mono unwind native frames.
+		xamarin_handling_unhandled_exceptions = 0;
+
 		mono_raise_exception ((MonoException *) exception);
 #endif
 		break;