/*
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);
}
