// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements runtime support for signal handling.
//
// Most synchronization primitives are not available from
// the signal handler (it cannot block and cannot use locks)
// so the handler communicates with a processing goroutine
// via struct sig, below.
//
// Ownership for sig.Note passes back and forth between
// the signal handler and the signal goroutine in rounds.
// The initial state is that sig.note is cleared (setup by signal_enable).
// At the beginning of each round, mask == 0.
// The round goes through three stages:
//
// (In parallel)
// 1a) One or more signals arrive and are handled
// by sigsend using cas to set bits in sig.mask.
// The handler that changes sig.mask from zero to non-zero
// calls notewakeup(&sig).
// 1b) Sigrecv calls notesleep(&sig) to wait for the wakeup.
//
// 2) Having received the wakeup, sigrecv knows that sigsend
// will not send another wakeup, so it can noteclear(&sig)
// to prepare for the next round. (Sigsend may still be adding
// signals to sig.mask at this point, which is fine.)
//
// 3) Sigrecv uses cas to grab the current sig.mask and zero it,
// triggering the next round.
//
// The signal handler takes ownership of the note by atomically
// changing mask from a zero to non-zero value. It gives up
// ownership by calling notewakeup. The signal goroutine takes
// ownership by returning from notesleep (caused by the notewakeup)
// and gives up ownership by clearing mask.

package signal
#include "config.h"
#include "runtime.h"
#include "arch.h"
#include "malloc.h"
#include "defs.h"

static struct {
	Note;
	uint32 mask[(NSIG+31)/32];
	uint32 wanted[(NSIG+31)/32];
	uint32 kick;
	bool inuse;
} sig;

// Called from sighandler to send a signal back out of the signal handling thread.
bool
__go_sigsend(int32 s)
{
	uint32 bit, mask;

	if(!sig.inuse || s < 0 || (size_t)s >= 32*nelem(sig.wanted) || !(sig.wanted[s/32]&(1U<<(s&31))))
		return false;
	bit = 1 << (s&31);
	for(;;) {
		mask = sig.mask[s/32];
		if(mask & bit)
			break;		// signal already in queue
		if(runtime_cas(&sig.mask[s/32], mask, mask|bit)) {
			// Added to queue.
			// Only send a wakeup if the receiver needs a kick.
			if(runtime_cas(&sig.kick, 1, 0))
				runtime_notewakeup(&sig);
			break;
		}
	}
	return true;
}

// Called to receive the next queued signal.
// Must only be called from a single goroutine at a time.
func signal_recv() (m uint32) {
	static uint32 recv[nelem(sig.mask)];
	int32 i, more;
	
	for(;;) {
		// Serve from local copy if there are bits left.
		for(i=0; i<NSIG; i++) {
			if(recv[i/32]&(1U<<(i&31))) {
				recv[i/32] ^= 1U<<(i&31);
				m = i;
				goto done;
			}
		}

		// Get a new local copy.
		// Ask for a kick if more signals come in
		// during or after our check (before the sleep).
		if(sig.kick == 0) {
			runtime_noteclear(&sig);
			runtime_cas(&sig.kick, 0, 1);
		}

		more = 0;
		for(i=0; (size_t)i<nelem(sig.mask); i++) {
			for(;;) {
				m = sig.mask[i];
				if(runtime_cas(&sig.mask[i], m, 0))
					break;
			}
			recv[i] = m;
			if(m != 0)
				more = 1;
		}
		if(more)
			continue;

		// Sleep waiting for more.
		runtime_entersyscall();
		runtime_notesleep(&sig);
		runtime_exitsyscall();
	}

done:;
	// goc requires that we fall off the end of functions
	// that return values instead of using our own return
	// statements.
}

// Must only be called from a single goroutine at a time.
func signal_enable(s uint32) {
	int32 i;

	if(!sig.inuse) {
		// The first call to signal_enable is for us
		// to use for initialization.  It does not pass
		// signal information in m.
		sig.inuse = true;	// enable reception of signals; cannot disable
		runtime_noteclear(&sig);
		return;
	}
	
	if(~s == 0) {
		// Special case: want everything.
		for(i=0; (size_t)i<nelem(sig.wanted); i++)
			sig.wanted[i] = ~(uint32)0;
		runtime_sigenable(s);
		return;
	}

	if(s >= nelem(sig.wanted)*32)
		return;
	sig.wanted[s/32] |= 1U<<(s&31);
	runtime_sigenable(s);
}