[9fans] alt and chanfree

["Mathieu Lonjaret" <mathieu.lonjaret@gmail.com>]

Hi all,

I'm doing something like the following to synchronize a bunch of
coroutines.
Everything goes fine while the different threads are working (and are
regularly sending over their respective channel). However, as soon as
one terminates and calls chanfree(), the next call to alt() fails (as
in I get a "pagefault" message). I can tell because if I never call
chanfree() I never hit that error.
Now in this simplified version I haven't shown that I in fact actually
dynamically reorganize the Alt * (adding/removing an Alt everytime a
caller starts/terminates) so it's totally possible I messed up there.

Before I dig deeper to find out what I'm doing wrong in my Alt *,
I wanted to make sure that I'm globally doing the right thing, i.e. if
it's ok and even recommended to free the Channel as soon as the thread
that was sending on it does not need it anymore.
Can anyone please confirm that?

Thanks,
Mathieu

static void
caller(void *arg)
{
	struct Params{ Torrent *tor; Peer *peer; Channel *c;} *params;
	uchar chanmsg[1];

	params = arg;
	// in callerworks() we send() on params->c at various points to synchronize, all goes well.
	callerworks(params->tor, params->peer, params->c);
	chanmsg[0] = 0;
	send(params->c, chanmsg);
	// peer is done working now
	chanfree(params->c);
}

void
callers(void *arg)
{
	Torrent *tor = arg;
	Peer *peer;
	uchar m[1];
	int i;
	int n;
	Alt *a;
	struct Params{Torrent *tor; Peer *peer; Channel *c;} *params;

	a = emalloc((MAXPEERS+1)*sizeof(Alt));
	for (i = 0; i<MAXPEERS; i++){
		a[i].v = m;
		a[i].c = chancreate(sizeof(m), 0);
		a[i].op = CHANRCV;

		// prepare params for the thread
		peer = emalloc(sizeof(Peer));
		params = emalloc(sizeof(struct Params));
		params->tor = tor;
		params->peer = peer;
		params->c = a[i].c;
		threadcreate(caller, params, STACK);
	}
	a[MAXPEERS].v = nil;
	a[MAXPEERS].c = nil;
	a[MAXPEERS].op = CHANEND;

	for(;;){
		n = alt(a);
		if(n < 0)
			error("with alt");

		if (m[0] == 0){
			// a caller has terminated
			dosomestuff()
		}
	}
}