// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package runtime
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// This file contains the implementation of Go select statements.
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import (
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"unsafe"
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)
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const debugSelect = false
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// scase.kind values.
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// Known to compiler.
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// Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselect.
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const (
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caseNil = iota
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caseRecv
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caseSend
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caseDefault
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)
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// Select case descriptor.
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// Known to compiler.
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// Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
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type scase struct {
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c *hchan // chan
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elem unsafe.Pointer // data element
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kind uint16
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pc uintptr // race pc (for race detector / msan)
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releasetime int64
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}
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var (
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chansendpc = funcPC(chansend)
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chanrecvpc = funcPC(chanrecv)
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)
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func selectsetpc(cas *scase) {
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cas.pc = getcallerpc()
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}
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func sellock(scases []scase, lockorder []uint16) {
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var c *hchan
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for _, o := range lockorder {
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c0 := scases[o].c
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if c0 != nil && c0 != c {
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c = c0
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lock(&c.lock)
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}
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}
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}
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func selunlock(scases []scase, lockorder []uint16) {
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// We must be very careful here to not touch sel after we have unlocked
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// the last lock, because sel can be freed right after the last unlock.
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// Consider the following situation.
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// First M calls runtime·park() in runtime·selectgo() passing the sel.
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// Once runtime·park() has unlocked the last lock, another M makes
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// the G that calls select runnable again and schedules it for execution.
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// When the G runs on another M, it locks all the locks and frees sel.
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// Now if the first M touches sel, it will access freed memory.
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for i := len(scases) - 1; i >= 0; i-- {
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c := scases[lockorder[i]].c
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if c == nil {
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break
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}
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if i > 0 && c == scases[lockorder[i-1]].c {
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continue // will unlock it on the next iteration
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}
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unlock(&c.lock)
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}
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}
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func selparkcommit(gp *g, _ unsafe.Pointer) bool {
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// This must not access gp's stack (see gopark). In
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// particular, it must not access the *hselect. That's okay,
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// because by the time this is called, gp.waiting has all
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// channels in lock order.
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var lastc *hchan
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for sg := gp.waiting; sg != nil; sg = sg.waitlink {
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if sg.c != lastc && lastc != nil {
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// As soon as we unlock the channel, fields in
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// any sudog with that channel may change,
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// including c and waitlink. Since multiple
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// sudogs may have the same channel, we unlock
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// only after we've passed the last instance
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// of a channel.
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unlock(&lastc.lock)
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}
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lastc = sg.c
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}
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if lastc != nil {
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unlock(&lastc.lock)
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}
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return true
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}
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func block() {
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gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
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}
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// selectgo implements the select statement.
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//
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// cas0 points to an array of type [ncases]scase, and order0 points to
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// an array of type [2*ncases]uint16. Both reside on the goroutine's
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// stack (regardless of any escaping in selectgo).
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//
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// selectgo returns the index of the chosen scase, which matches the
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// ordinal position of its respective select{recv,send,default} call.
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// Also, if the chosen scase was a receive operation, it reports whether
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// a value was received.
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func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
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if debugSelect {
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print("select: cas0=", cas0, "\n")
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}
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cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
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order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
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scases := cas1[:ncases:ncases]
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pollorder := order1[:ncases:ncases]
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lockorder := order1[ncases:][:ncases:ncases]
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// Replace send/receive cases involving nil channels with
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// caseNil so logic below can assume non-nil channel.
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for i := range scases {
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cas := &scases[i]
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if cas.c == nil && cas.kind != caseDefault {
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*cas = scase{}
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}
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}
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var t0 int64
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if blockprofilerate > 0 {
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t0 = cputicks()
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for i := 0; i < ncases; i++ {
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scases[i].releasetime = -1
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}
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}
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// The compiler rewrites selects that statically have
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// only 0 or 1 cases plus default into simpler constructs.
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// The only way we can end up with such small sel.ncase
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// values here is for a larger select in which most channels
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// have been nilled out. The general code handles those
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// cases correctly, and they are rare enough not to bother
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// optimizing (and needing to test).
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// generate permuted order
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for i := 1; i < ncases; i++ {
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j := fastrandn(uint32(i + 1))
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pollorder[i] = pollorder[j]
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pollorder[j] = uint16(i)
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}
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// sort the cases by Hchan address to get the locking order.
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// simple heap sort, to guarantee n log n time and constant stack footprint.
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for i := 0; i < ncases; i++ {
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j := i
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// Start with the pollorder to permute cases on the same channel.
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c := scases[pollorder[i]].c
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for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
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k := (j - 1) / 2
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lockorder[j] = lockorder[k]
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j = k
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}
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lockorder[j] = pollorder[i]
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}
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for i := ncases - 1; i >= 0; i-- {
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o := lockorder[i]
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c := scases[o].c
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lockorder[i] = lockorder[0]
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j := 0
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for {
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k := j*2 + 1
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if k >= i {
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break
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}
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if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
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k++
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}
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if c.sortkey() < scases[lockorder[k]].c.sortkey() {
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lockorder[j] = lockorder[k]
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j = k
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continue
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}
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break
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}
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lockorder[j] = o
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}
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if debugSelect {
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for i := 0; i+1 < ncases; i++ {
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if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
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print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
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throw("select: broken sort")
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}
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}
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}
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// lock all the channels involved in the select
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sellock(scases, lockorder)
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var (
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gp *g
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sg *sudog
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c *hchan
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k *scase
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sglist *sudog
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sgnext *sudog
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qp unsafe.Pointer
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nextp **sudog
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)
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loop:
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// pass 1 - look for something already waiting
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var dfli int
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var dfl *scase
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var casi int
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var cas *scase
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var recvOK bool
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for i := 0; i < ncases; i++ {
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casi = int(pollorder[i])
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cas = &scases[casi]
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c = cas.c
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switch cas.kind {
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case caseNil:
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continue
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case caseRecv:
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sg = c.sendq.dequeue()
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if sg != nil {
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goto recv
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}
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if c.qcount > 0 {
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goto bufrecv
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}
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if c.closed != 0 {
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goto rclose
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}
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case caseSend:
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if raceenabled {
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racereadpc(c.raceaddr(), cas.pc, chansendpc)
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}
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if c.closed != 0 {
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goto sclose
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}
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sg = c.recvq.dequeue()
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if sg != nil {
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goto send
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}
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if c.qcount < c.dataqsiz {
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goto bufsend
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}
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case caseDefault:
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dfli = casi
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dfl = cas
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}
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}
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if dfl != nil {
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selunlock(scases, lockorder)
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casi = dfli
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cas = dfl
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goto retc
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}
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// pass 2 - enqueue on all chans
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gp = getg()
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if gp.waiting != nil {
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throw("gp.waiting != nil")
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}
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nextp = &gp.waiting
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for _, casei := range lockorder {
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casi = int(casei)
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cas = &scases[casi]
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if cas.kind == caseNil {
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continue
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}
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c = cas.c
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sg := acquireSudog()
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sg.g = gp
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sg.isSelect = true
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// No stack splits between assigning elem and enqueuing
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// sg on gp.waiting where copystack can find it.
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sg.elem = cas.elem
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sg.releasetime = 0
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if t0 != 0 {
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sg.releasetime = -1
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}
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sg.c = c
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// Construct waiting list in lock order.
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*nextp = sg
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nextp = &sg.waitlink
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switch cas.kind {
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case caseRecv:
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c.recvq.enqueue(sg)
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case caseSend:
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c.sendq.enqueue(sg)
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}
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}
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// wait for someone to wake us up
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gp.param = nil
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gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
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sellock(scases, lockorder)
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gp.selectDone = 0
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sg = (*sudog)(gp.param)
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gp.param = nil
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// pass 3 - dequeue from unsuccessful chans
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// otherwise they stack up on quiet channels
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// record the successful case, if any.
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// We singly-linked up the SudoGs in lock order.
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casi = -1
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cas = nil
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sglist = gp.waiting
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// Clear all elem before unlinking from gp.waiting.
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for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
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sg1.isSelect = false
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sg1.elem = nil
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sg1.c = nil
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}
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gp.waiting = nil
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for _, casei := range lockorder {
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k = &scases[casei]
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if k.kind == caseNil {
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continue
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}
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if sglist.releasetime > 0 {
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k.releasetime = sglist.releasetime
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}
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if sg == sglist {
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// sg has already been dequeued by the G that woke us up.
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casi = int(casei)
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cas = k
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} else {
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c = k.c
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if k.kind == caseSend {
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c.sendq.dequeueSudoG(sglist)
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} else {
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c.recvq.dequeueSudoG(sglist)
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}
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}
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sgnext = sglist.waitlink
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sglist.waitlink = nil
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releaseSudog(sglist)
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sglist = sgnext
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}
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if cas == nil {
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// We can wake up with gp.param == nil (so cas == nil)
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// when a channel involved in the select has been closed.
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// It is easiest to loop and re-run the operation;
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// we'll see that it's now closed.
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// Maybe some day we can signal the close explicitly,
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// but we'd have to distinguish close-on-reader from close-on-writer.
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// It's easiest not to duplicate the code and just recheck above.
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// We know that something closed, and things never un-close,
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// so we won't block again.
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goto loop
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}
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c = cas.c
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if debugSelect {
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print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
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}
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if cas.kind == caseRecv {
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recvOK = true
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}
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if raceenabled {
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if cas.kind == caseRecv && cas.elem != nil {
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raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
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} else if cas.kind == caseSend {
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raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
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}
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}
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if msanenabled {
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if cas.kind == caseRecv && cas.elem != nil {
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msanwrite(cas.elem, c.elemtype.size)
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} else if cas.kind == caseSend {
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msanread(cas.elem, c.elemtype.size)
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}
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}
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selunlock(scases, lockorder)
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goto retc
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bufrecv:
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// can receive from buffer
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if raceenabled {
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if cas.elem != nil {
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raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
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}
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raceacquire(chanbuf(c, c.recvx))
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racerelease(chanbuf(c, c.recvx))
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}
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if msanenabled && cas.elem != nil {
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msanwrite(cas.elem, c.elemtype.size)
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}
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recvOK = true
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qp = chanbuf(c, c.recvx)
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if cas.elem != nil {
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typedmemmove(c.elemtype, cas.elem, qp)
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}
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typedmemclr(c.elemtype, qp)
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c.recvx++
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if c.recvx == c.dataqsiz {
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c.recvx = 0
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}
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c.qcount--
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selunlock(scases, lockorder)
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goto retc
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bufsend:
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// can send to buffer
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if raceenabled {
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raceacquire(chanbuf(c, c.sendx))
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racerelease(chanbuf(c, c.sendx))
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raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
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}
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if msanenabled {
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msanread(cas.elem, c.elemtype.size)
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}
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typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
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c.sendx++
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if c.sendx == c.dataqsiz {
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c.sendx = 0
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}
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c.qcount++
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selunlock(scases, lockorder)
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goto retc
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recv:
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// can receive from sleeping sender (sg)
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recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
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if debugSelect {
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print("syncrecv: cas0=", cas0, " c=", c, "\n")
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}
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recvOK = true
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goto retc
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rclose:
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// read at end of closed channel
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selunlock(scases, lockorder)
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recvOK = false
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if cas.elem != nil {
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typedmemclr(c.elemtype, cas.elem)
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}
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if raceenabled {
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raceacquire(c.raceaddr())
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}
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goto retc
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send:
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// can send to a sleeping receiver (sg)
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if raceenabled {
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raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
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}
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if msanenabled {
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msanread(cas.elem, c.elemtype.size)
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}
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send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
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if debugSelect {
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print("syncsend: cas0=", cas0, " c=", c, "\n")
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}
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goto retc
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retc:
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if cas.releasetime > 0 {
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blockevent(cas.releasetime-t0, 1)
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}
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return casi, recvOK
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sclose:
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// send on closed channel
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selunlock(scases, lockorder)
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panic(plainError("send on closed channel"))
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}
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func (c *hchan) sortkey() uintptr {
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// TODO(khr): if we have a moving garbage collector, we'll need to
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// change this function.
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return uintptr(unsafe.Pointer(c))
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}
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// A runtimeSelect is a single case passed to rselect.
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// This must match ../reflect/value.go:/runtimeSelect
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type runtimeSelect struct {
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dir selectDir
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typ unsafe.Pointer // channel type (not used here)
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ch *hchan // channel
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val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
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}
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// These values must match ../reflect/value.go:/SelectDir.
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type selectDir int
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const (
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_ selectDir = iota
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selectSend // case Chan <- Send
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selectRecv // case <-Chan:
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selectDefault // default
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)
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//go:linkname reflect_rselect reflect.rselect
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func reflect_rselect(cases []runtimeSelect) (int, bool) {
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if len(cases) == 0 {
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block()
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}
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sel := make([]scase, len(cases))
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order := make([]uint16, 2*len(cases))
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for i := range cases {
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rc := &cases[i]
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switch rc.dir {
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case selectDefault:
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sel[i] = scase{kind: caseDefault}
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case selectSend:
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sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
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case selectRecv:
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sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
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}
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if raceenabled || msanenabled {
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selectsetpc(&sel[i])
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}
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}
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return selectgo(&sel[0], &order[0], len(cases))
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}
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func (q *waitq) dequeueSudoG(sgp *sudog) {
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x := sgp.prev
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y := sgp.next
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if x != nil {
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if y != nil {
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// middle of queue
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x.next = y
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y.prev = x
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sgp.next = nil
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sgp.prev = nil
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return
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}
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// end of queue
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x.next = nil
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q.last = x
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sgp.prev = nil
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return
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}
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if y != nil {
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// start of queue
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y.prev = nil
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q.first = y
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sgp.next = nil
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return
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}
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// x==y==nil. Either sgp is the only element in the queue,
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// or it has already been removed. Use q.first to disambiguate.
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if q.first == sgp {
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q.first = nil
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q.last = nil
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}
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}
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