/*
a->op is Recv: 从对应得Channel的发送队列随机取出一个Alt，
并读取数据，Alt对应的Task可以进入就绪队列了
a->op is Send: 将数据拷贝到Buffer中
*/
static void
altexec(Alt *a)
{
int i;
Altarray *ar;
Alt *other;
Channel *c;
c = a->c;
ar = chanarray(c, otherop(a->op));
if(ar && ar->n){
// 这里的逻辑目前看都是a->op == RECV才跑得到
i = rand()%ar->n;
other = ar->a[i];
// a->v = other->v
altcopy(a, other);
// 将other从Channel的队列（接收队列）中删除
altalldequeue(other->xalt);
other->xalt[0].xalt = other;
// Task重新进入就绪队列
taskready(other->task);
}else {
// 将a->v的数据放到buffer里面
altcopy(a, nil);
}
}
static void
altexec(Alt *a)
{
int i;
Altarray *ar;
Alt *other;
Channel *c;
c = a->c;
ar = chanarray(c, otherop(a->op));
if(ar && ar->n){
i = rand()%ar->n;
other = ar->a[i];
altcopy(a, other);
altalldequeue(other->xalt);
other->xalt[0].xalt = other;
taskready(other->task);
}else
altcopy(a, nil);
}
static int
altexec(Alt *a, int spl)
{
volatile Alt *b;
int i, n, otherop;
Channel *c;
void *me, *waiter, *buf;
c = a->c;
/* rendezvous with others */
otherop = (CHANSND+CHANRCV) - a->op;
n = 0;
b = nil;
me = a->v;
for(i=0; i<c->nentry; i++)
if(c->qentry[i] && c->qentry[i]->op==otherop && *c->qentry[i]->tag==nil)
if(nrand(++n) == 0)
b = c->qentry[i];
if(b != nil){
_threaddebug(DBGCHAN, "rendez %s alt %p chan %p alt %p", a->op==CHANRCV?"recv":"send", a, c, b);
waiter = b->v;
if(c->s && c->n){
/*
* if buffer is full and there are waiters
* and we're meeting a waiter,
* we must be receiving.
*
* we use the value in the channel buffer,
* copy the waiter's value into the channel buffer
* on behalf of the waiter, and then wake the waiter.
*/
if(a->op!=CHANRCV)
abort();
buf = altexecbuffered(a, 1);
altcopy(me, buf, c->e);
altcopy(buf, waiter, c->e);
}else{
if(a->op==CHANRCV)
altcopy(me, waiter, c->e);
else
altcopy(waiter, me, c->e);
}
*b->tag = c; /* commits us to rendezvous */
_threaddebug(DBGCHAN, "unlocking the chanlock");
unlock(&chanlock);
_procsplx(spl);
_threaddebug(DBGCHAN, "chanlock is %lud", *(ulong*)&chanlock);
while(_threadrendezvous(b->tag, 0) == Intred)
;
return 1;
}
buf = altexecbuffered(a, 0);
if(a->op==CHANRCV)
altcopy(me, buf, c->e);
else
altcopy(buf, me, c->e);
unlock(&chanlock);
_procsplx(spl);
return 1;
}