[jvm] haxe.EventLoop.main fails if first accessed from non-main thread

[kevinresol]

9127a0b

	static function main() {
		// trace(sys.thread.Thread.main()); // uncomment to fix
		final t = new java.lang.Thread(() -> {
			trace(sys.thread.Thread.current() == sys.thread.Thread.main()); // prints true?! huh?
			trace(haxe.EventLoop.main.run(() -> trace("run in main loop")));
		});

		t.start();
		t.join();
	}

Run this on jvm and get Exception in thread "main" You can't run this loop from a different thread

The problem is that the static var that stores the main thread reference is run on the thread that first accessed it, so it might reference the wrong thread. In other words sys.thread.Thread.main() cannot reliably return the main thread unless it is first accessed from the main thread.

[kevinresol]

I think the simplest fix is to somehow force init of the sys.thread.Thread class in Jvm.init()
for example I added a trace(sys.thread.Thread.main()); there and problem is fixed

[kevinresol]

I think the root problem here is that the current code is written with an assumption that static function __init__ is always being run in the main thread, unfortunately that is not the case. Because __init__ is generated as <cinit> in jvm and that is run on the thread that first accesses the class.

In order to fix that, we either have to change how __init__ is generated on JVM, or drop the assumption that __init__ is always running in main thread when writing haxe code.

See #8933