[dev.garbage] runtime: make sure G.param and SudoG.elem do not hold stale pointers

In old conservative Go, this could cause memory leaks.
A new pickier collector might reasonably crash when it saw one of these.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://golang.org/cl/147480043

src/runtime/chan.go

diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index 48925b2..10503f4 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -140,10 +140,11 @@
 			unlock(&c.lock)
 
 			recvg := sg.g
-			recvg.param = unsafe.Pointer(sg)
 			if sg.elem != nil {
 				memmove(unsafe.Pointer(sg.elem), ep, uintptr(c.elemsize))
+				sg.elem = nil
 			}
+			recvg.param = unsafe.Pointer(sg)
 			if sg.releasetime != 0 {
 				sg.releasetime = cputicks()
 			}
@@ -179,6 +180,7 @@
 			}
 			panic("send on closed channel")
 		}
+		gp.param = nil
 		if mysg.releasetime > 0 {
 			blockevent(int64(mysg.releasetime)-t0, 2)
 		}
@@ -278,6 +280,7 @@
 			break
 		}
 		gp := sg.g
+		sg.elem = nil
 		gp.param = nil
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
@@ -292,6 +295,7 @@
 			break
 		}
 		gp := sg.g
+		sg.elem = nil
 		gp.param = nil
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
@@ -372,6 +376,7 @@
 			if ep != nil {
 				memmove(ep, sg.elem, uintptr(c.elemsize))
 			}
+			sg.elem = nil
 			gp := sg.g
 			gp.param = unsafe.Pointer(sg)
 			if sg.releasetime != 0 {
@@ -409,9 +414,11 @@
 		if mysg.releasetime > 0 {
 			blockevent(mysg.releasetime-t0, 2)
 		}
+		haveData := gp.param != nil
+		gp.param = nil
 		releaseSudog(mysg)
 
-		if gp.param != nil {
+		if haveData {
 			// a sender sent us some data. It already wrote to ep.
 			selected = true
 			received = true

src/runtime/proc.go

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 9b95868..eefe823 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -148,6 +148,9 @@
 	c := gomcache()
 	s := c.sudogcache
 	if s != nil {
+		if s.elem != nil {
+			gothrow("acquireSudog: found s.elem != nil in cache")
+		}
 		c.sudogcache = s.next
 		return s
 	}
@@ -162,12 +165,22 @@
 	// which keeps the garbage collector from being invoked.
 	mp := acquirem()
 	p := new(sudog)
+	if p.elem != nil {
+		gothrow("acquireSudog: found p.elem != nil after new")
+	}
 	releasem(mp)
 	return p
 }
 
 //go:nosplit
 func releaseSudog(s *sudog) {
+	if s.elem != nil {
+		gothrow("runtime: sudog with non-nil elem")
+	}
+	gp := getg()
+	if gp.param != nil {
+		gothrow("runtime: releaseSudog with non-nil gp.param")
+	}
 	c := gomcache()
 	s.next = c.sudogcache
 	c.sudogcache = s

src/runtime/select.go

diff --git a/src/runtime/select.go b/src/runtime/select.go
index 7716d2d..1bcea8c 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -368,6 +368,7 @@
 	// someone woke us up
 	sellock(sel)
 	sg = (*sudog)(gp.param)
+	gp.param = nil
 
 	// pass 3 - dequeue from unsuccessful chans
 	// otherwise they stack up on quiet channels
@@ -376,6 +377,10 @@
 	// iterating through the linked list they are in reverse order.
 	cas = nil
 	sglist = gp.waiting
+	// Clear all elem before unlinking from gp.waiting.
+	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
+		sg1.elem = nil
+	}
 	gp.waiting = nil
 	for i := int(sel.ncase) - 1; i >= 0; i-- {
 		k = &scases[pollorder[i]]
@@ -506,6 +511,7 @@
 	if cas.elem != nil {
 		memmove(cas.elem, sg.elem, uintptr(c.elemsize))
 	}
+	sg.elem = nil
 	gp = sg.g
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {
@@ -541,6 +547,7 @@
 	if sg.elem != nil {
 		memmove(sg.elem, cas.elem, uintptr(c.elemsize))
 	}
+	sg.elem = nil
 	gp = sg.g
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {

src/runtime/sema.go

diff --git a/src/runtime/sema.go b/src/runtime/sema.go
index beacd67..142d308 100644
--- a/src/runtime/sema.go
+++ b/src/runtime/sema.go
@@ -168,6 +168,7 @@
 	} else {
 		root.head = s.next
 	}
+	s.elem = nil
 	s.next = nil
 	s.prev = nil
 }