/*
 * Copyright © 2024 Nick Bowler
 *
 * Helpers for hosts using native Windows threading API.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * The init_once implementation is adapted from the Pthreads-win32 library
 * implementation based on MCS (Mellor-Crummy Scott) locks, originally
 * distributed with the following copyright and permission notice:
 *
 *      Pthreads-win32 - POSIX Threads Library for Win32
 *      Copyright(C) 1998 John E. Bossom
 *      Copyright(C) 1999,2005 Pthreads-win32 contributors
 *
 *      This library is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU Lesser General Public
 *      License as published by the Free Software Foundation; either
 *      version 2 of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

static DWORD tls_key = TLS_OUT_OF_INDEXES;
#define tls_key_valid (tls_key != TLS_OUT_OF_INDEXES)

static void init_once_cb(void);

static void *tls_get(void)
{
	return TlsGetValue(tls_key);
}

static BOOL tls_set(void *p)
{
	return TlsSetValue(tls_key, p);
}

/*
 * Synchronize with another thread's call to init_once_wait on the same
 * HANDLE object, which must be initialized to null.  If init_once_wait
 * got there first, we receive a handle to the event object which can
 * then be signaled.
 */
static void init_once_signal(HANDLE *ep)
{
	/*
	 * Note that INVALID_HANDLE_VALUE is distinct from a null pointer
	 * and also distinct from any handle returned by CreateEvent.
	 */
	HANDLE e = INVALID_HANDLE_VALUE;

	if ((e = InterlockedCompareExchangePointer(ep, e, NULL)))
		SetEvent(e);
}

/*
 * Synchronize with another thread's call to init_once_signal on the same
 * HANDLE object.
 */
static void init_once_wait(HANDLE *ep)
{
#if _WIN64
	if (!InterlockedAdd64((LONG64 *)ep, 0)) // load with memory barrier
#else
	if (!InterlockedAdd((LONG *)ep, 0))
#endif
	{
		HANDLE e;

		e = CreateEvent(NULL, FALSE, FALSE, NULL);
		if (!InterlockedCompareExchangePointer(ep, e, NULL))
			WaitForSingleObject(e, INFINITE);
		CloseHandle(e);
	}
}

/*
 * This implementation using an MCS lock variation requires only a single
 * pointer of shared global state initialized to null, and in the uncontended
 * case does not require allocation of any Windows resources whatsoever.
 *
 * These locks are described in the paper:
 *
 *  "Algorithms for Scalable Synchronization on Shared-Memory Multiprocessors"
 *   by John M. Mellor-Crummey and Michael L. Scott.
 *   ACM Transactions on Computer Systems Volume 9, Issue 1 (Feb. 1991).
 *
 * The basic idea is that each thread has a local state, which includes a
 * pointer to the next waiting thread.  The global state is a tail pointer to
 * the last waiting thread.  The running thread holds the lock and also the
 * pointer to the first waiter.
 *
 * On acquire, atomically swap our fresh local state with the global tail
 * pointer, becoming the new last waiter.  We receive a pointer to the previous
 * last waiter (or nothing, in the unlocked case).  At this point it is safe
 * for a new thread to come along and update the tail pointer again.  If
 * needed, we then update the last waiter to point to our thread, signal
 * that this is completed, and then wait to be signaled.
 *
 * On release, if the tail pointer points to us there are no waiters, and this
 * can be confirmed with an atomic compare and exchange to the done state,
 * which is equivalent to the original state except that a subsequent acquirer
 * will know that the initialization has been previously completed.
 *
 * If that didn't unlock the lock, we need to wait for the signal from the next
 * thread (which may not have updated our next pointer yet), then signal the
 * next thread to wake up.  Eventually the queue will empty and the lock is
 * left in the done state, at which point a simple atomic load can determine
 * that nothing else needs to happen.
 */
static int init_once(void)
{
	struct once_state * const init_done = (void *)-1;

	struct once_state {
		struct once_state *next;
		HANDLE ready_event, next_event;
	};

	static void * volatile tail;
	struct once_state local = {0};
	struct once_state *p;

	/* fast path for the normal (init completed) case. */
	if (tail == init_done)
		return 1;

	if (!(p = InterlockedExchangePointer(&tail, &local))) {
		/* we're number one! */
		tls_key = TlsAlloc();
		init_once_cb();
	} else if (p != init_done) {
		/* contended, wait for predecessor */
		p->next = &local;
		init_once_signal(&p->next_event);
		init_once_wait(&local.ready_event);
	}

	p = InterlockedCompareExchangePointer(&tail, init_done, &local);
	if (p != &local) {
		/* contended, wait for successor */
		init_once_wait(&local.next_event);
		init_once_signal(&local.next->ready_event);
	}

	return 1;
}

#if !TEST_W32_NO_DLLMAIN

/*
 * On Windows, DLLs are notified of thread exit via the DllMain entry point.
 * This works in all versions.
 */
#if !DLL_EXPORT
static
#endif
BOOL WINAPI DllMain(HINSTANCE hinst, DWORD reason, LPVOID p)
{
	if (reason == DLL_THREAD_DETACH && tls_key != TLS_OUT_OF_INDEXES)
		free(TlsGetValue(tls_key));

	return TRUE;
}

/*
 * We can achieve similar behaviour with static linking executables by
 * putting a pointer to the entry point in a special section.
 *
 * I believe this is supported beginning around Windows XP.
 */
#if !DLL_EXPORT
#pragma data_seg(".CRT$XLF")
#if __GNUC__
__attribute__((section(".CRT$XLF")))
#endif
PIMAGE_TLS_CALLBACK cdecl__tls_hook = (PIMAGE_TLS_CALLBACK)DllMain;
#pragma data_seg()
#endif

#endif