`CrashHandler` breaks custom crash handlers

[0x0ACB]

Tested versions

4.7

System information

Windows 11, Ubuntu 26.04

Issue description

This is mainly an issue with libgodot but the CrashHandlers for Windows and Linux will not restore previous signal handlers when disabled. Since they can not be disabled before initializing godot and will always set their own signal handlers it is currently impossible to setup custom crash handling beforehand with something like breakpad.

godot/platform/windows/crash_handler_windows_signal.cpp

Lines 307 to 327 in 8c7e6c5

	void CrashHandler::disable() {
	if (disabled) {
	return;
	}
	#if defined(CRASH_HANDLER_EXCEPTION)
	signal(SIGSEGV, nullptr);
	signal(SIGFPE, nullptr);
	signal(SIGILL, nullptr);
	#endif
	disabled = true;
	}
	void CrashHandler::initialize() {
	#if defined(CRASH_HANDLER_EXCEPTION)
	signal(SIGSEGV, CrashHandlerException);
	signal(SIGFPE, CrashHandlerException);
	signal(SIGILL, CrashHandlerException);
	#endif
	}

godot/platform/linuxbsd/crash_handler_linuxbsd.cpp

Lines 259 to 279 in 8c7e6c5

	void CrashHandler::disable() {
	if (disabled) {
	return;
	}
	#ifdef CRASH_HANDLER_ENABLED
	signal(SIGSEGV, SIG_DFL);
	signal(SIGFPE, SIG_DFL);
	signal(SIGILL, SIG_DFL);
	#endif
	disabled = true;
	}
	void CrashHandler::initialize() {
	#ifdef CRASH_HANDLER_ENABLED
	signal(SIGSEGV, handle_crash);
	signal(SIGFPE, handle_crash);
	signal(SIGILL, handle_crash);
	#endif
	}

Steps to reproduce

init_breakpad();
const auto instance = static_cast<GodotInstance*>(libgodot_create_godot_instance(argc, argv, gdextension_init));
OS::get_singleton()->disable_crash_handler();
CRASH_NOW();

Minimal reproduction project (MRP)

[NoctemCat]

Some languages also have signal handlers installed by their runtime, which makes LibGodot currently awkward to use.

Similar issue: #119079

[bruvzg]

Relevant for macOS as well (but not sure if libgodot is usable on macOS). Saving/restoring signal handlers should be trivial to do, and should not make any difference for the standalone engine, so I do not see any reason not to do it. I will check it tomorrow.