从源码看 select 看 case 同时满足时的选择

简介

select的所有case对应一个数组,每次select时候,都会对这个数组进行打乱操作
所以case同时满足时候选择是不确定的

源码

runtime/select.go 中

func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
    ...
	for i := 1; i < ncases; i++ {
		j := fastrandn(uint32(i + 1))
		pollorder[i] = pollorder[j]
		pollorder[j] = uint16(i)
	}
    ...
	for i := 0; i < ncases; i++ {
		casi = int(pollorder[i])
		cas = &scases[casi]
		c = cas.c

		switch cas.kind {
		case caseNil:
			continue

		case caseRecv:
			sg = c.sendq.dequeue()
			if sg != nil {
				goto recv
			}
			if c.qcount > 0 {
				goto bufrecv
			}
			if c.closed != 0 {
				goto rclose
			}

		case caseSend:
			if raceenabled {
				racereadpc(c.raceaddr(), cas.pc, chansendpc)
			}
			if c.closed != 0 {
				goto sclose
			}
			sg = c.recvq.dequeue()
			if sg != nil {
				goto send
			}
			if c.qcount < c.dataqsiz {
				goto bufsend
			}

		case caseDefault:
			dfli = casi
			dfl = cas
		}
	}
}