[debugger] Work around a debugger issue when using the interpreter. (#15509)

There's a bug in the Mono runtime where the interpreter does not disable optimizations when the debugger is attached, which leads to the interpreter optimizing code and the debugger ending up rather confused.

The bug is fixed in the Mono runtime (https://github.com/dotnet/runtime/pull/71436), but there's no immediate way for the runtime to release this fix, so here we're implementing a workaround that disables interpreter optimizations if the debugging is enabled. It's somewhat clunky because the Mono external API wasn't designed for this, so we have to abuse the API a bit to accomplish the effect we want.

This is somewhat risky (since we're changing the startup path in a pretty big way), but there's an escape hatch via an environment variable, and also the workaround will not be in effect for release builds.

While the runtime issue exists in legacy mono/mono as well, we'll fix the Mono runtime for legacy, because we don't have to wait to consume legacy mono (https://github.com/xamarin/xamarin-macios/pull/15507). This means that the workaround is for .NET scenarios only.

This is a backport of ##15451.

Co-authored-by: Thays Grazia <thaystg@gmail.com>

runtime/monovm-bridge.m

@@ -60,6 +60,12 @@ xamarin_bridge_setup ()
 void
 xamarin_bridge_initialize ()
 {
+#if DOTNET
+	bool use_mono_workaround = xamarin_init_mono_debug && getenv ("XAMARIN_SKIP_INTERPRETER_DEBUGGING_WORKAROUND") == 0;
+#else
+	bool use_mono_workaround = false;
+#endif
+
 	if (xamarin_register_modules != NULL)
 		xamarin_register_modules ();
 	DEBUG_LAUNCH_TIME_PRINT ("\tAOT register time");
@@ -69,9 +75,9 @@ xamarin_bridge_initialize ()
 	DEBUG_LAUNCH_TIME_PRINT ("\tDebug init time");
 #endif
 	
-	if (xamarin_init_mono_debug)
+	if (xamarin_init_mono_debug && !use_mono_workaround)
 		mono_debug_init (MONO_DEBUG_FORMAT_MONO);
-	
+
 	mono_install_assembly_preload_hook (xamarin_assembly_preload_hook, NULL);
 	mono_install_load_aot_data_hook (xamarin_load_aot_data, xamarin_free_aot_data, NULL);
 
@@ -85,7 +91,32 @@ xamarin_bridge_initialize ()
 	mono_install_unhandled_exception_hook (xamarin_unhandled_exception_handler, NULL);
 	mono_install_ftnptr_eh_callback (xamarin_ftnptr_exception_handler);
 
-	mono_jit_init_version ("MonoTouch", "mobile");
+	const char *argc[] = {
+		"",
+		"--interp=-all",
+		"",
+		NULL,
+	};
+
+	if (use_mono_workaround) {
+		// This is a workaround for a runtime bug that prevents debugging from working properly.
+		// We call mono_main to disable interpreter optimizations when debugging, because
+		// mono's own interpreter initialization doesn't take into account that the debugger
+		// might be attached, and the subsequent optimizations can break the debugger.
+		// The stdout dance is to hide mono's "helpful" output telling us the command line
+		// arguments are incorrect (which they would be normally, but we're abusing mono_main
+		// to get the side effects).
+		fflush (stdout);
+		int originalStdout = dup (STDOUT_FILENO);
+		int devnull = open ("/dev/null", O_RDWR);
+		dup2 (devnull, STDOUT_FILENO);
+		mono_main (2, (char**)argc);
+		fflush (stdout);
+		dup2 (originalStdout, STDOUT_FILENO);
+	} else {
+		mono_jit_init_version ("MonoTouch", "mobile");
+	}
+	
 	/*
 	  As part of mono initialization a preload hook is added that overrides ours, so we need to re-instate it here.
 	  This is wasteful, but there's no way to manipulate the preload hook list except by adding to it.