runtime: record channel in sudog

Given a G, there's currently no way to find the channel it's blocking
on. We'll need this information to fix a (probably theoretical) bug in
select and to implement concurrent stack shrinking, so record the
channel in the sudog.

For #12967.

Change-Id: If8fb63a140f1d07175818824d08c0ebeec2bdf66
Reviewed-on: https://go-review.googlesource.com/20035
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/select.go

@@ -331,6 +331,7 @@ func selecttype(size int32) *Type {
 	sudog.List.Append(Nod(ODCLFIELD, newname(Lookup("releasetime")), typenod(Types[TUINT64])))
 	sudog.List.Append(Nod(ODCLFIELD, newname(Lookup("ticket")), typenod(Types[TUINT32])))
 	sudog.List.Append(Nod(ODCLFIELD, newname(Lookup("waitlink")), typenod(Ptrto(Types[TUINT8]))))
+	sudog.List.Append(Nod(ODCLFIELD, newname(Lookup("c")), typenod(Ptrto(Types[TUINT8]))))
 	typecheck(&sudog, Etype)
 	sudog.Type.Noalg = true
 	sudog.Type.Local = true

src/runtime/chan.go

@@ -209,6 +209,7 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 	mysg.waitlink = nil
 	mysg.g = gp
 	mysg.selectdone = nil
+	mysg.c = c
 	gp.waiting = mysg
 	gp.param = nil
 	c.sendq.enqueue(mysg)
@@ -229,6 +230,7 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 	if mysg.releasetime > 0 {
 		blockevent(mysg.releasetime-t0, 2)
 	}
+	mysg.c = nil
 	releaseSudog(mysg)
 	return true
 }
@@ -469,6 +471,7 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r
 	gp.waiting = mysg
 	mysg.g = gp
 	mysg.selectdone = nil
+	mysg.c = c
 	gp.param = nil
 	c.recvq.enqueue(mysg)
 	goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)
@@ -483,6 +486,7 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r
 	}
 	closed := gp.param == nil
 	gp.param = nil
+	mysg.c = nil
 	releaseSudog(mysg)
 	return true, !closed
 }

src/runtime/proc.go

@@ -329,6 +329,9 @@ func releaseSudog(s *sudog) {
 	if s.waitlink != nil {
 		throw("runtime: sudog with non-nil waitlink")
 	}
+	if s.c != nil {
+		throw("runtime: sudog with non-nil c")
+	}
 	gp := getg()
 	if gp.param != nil {
 		throw("runtime: releaseSudog with non-nil gp.param")

src/runtime/runtime2.go

@@ -215,13 +215,14 @@ type gobuf struct {
 // selecttype.
 type sudog struct {
 	g           *g
-	selectdone  *uint32
+	selectdone  *uint32 // CAS to 1 to win select race (may point to stack)
 	next        *sudog
 	prev        *sudog
-	elem        unsafe.Pointer // data element
+	elem        unsafe.Pointer // data element (may point to stack)
 	releasetime int64
 	ticket      uint32
 	waitlink    *sudog // g.waiting list
+	c           *hchan // channel
 }
 
 type gcstats struct {

src/runtime/select.go

@@ -385,6 +385,7 @@ loop:
 			sg.releasetime = -1
 		}
 		sg.waitlink = gp.waiting
+		sg.c = c
 		gp.waiting = sg
 
 		switch cas.kind {
@@ -416,6 +417,7 @@ loop:
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
 		sg1.selectdone = nil
 		sg1.elem = nil
+		sg1.c = nil
 	}
 	gp.waiting = nil
 	for i := int(sel.ncase) - 1; i >= 0; i-- {