cmd/compile: clear hidden value at end of channel range body

While we’re here, clean up a few comments.

Fixes #15281

Change-Id: Ia6173e9941133db08f57bc80bdd3c5722122bfdb
Reviewed-on: https://go-review.googlesource.com/22082
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -154,7 +154,7 @@ func walkrange(n *Node) {
 		v2 = n.List.Second()
 	}
 
-	// n->list has no meaning anymore, clear it
+	// n.List has no meaning anymore, clear it
 	// to avoid erroneous processing by racewalk.
 	n.List.Set(nil)
 
@@ -217,9 +217,9 @@ func walkrange(n *Node) {
 			n.Right.Ninit.Set1(a)
 		}
 
-		// orderstmt allocated the iterator for us.
-	// we only use a once, so no copy needed.
 	case TMAP:
+		// orderstmt allocated the iterator for us.
+		// we only use a once, so no copy needed.
 		ha := a
 
 		th := hiter(t)
@@ -254,8 +254,8 @@ func walkrange(n *Node) {
 			body = []*Node{a}
 		}
 
-		// orderstmt arranged for a copy of the channel variable.
 	case TCHAN:
+		// orderstmt arranged for a copy of the channel variable.
 		ha := a
 
 		n.Left = nil
@@ -278,9 +278,13 @@ func walkrange(n *Node) {
 		} else {
 			body = []*Node{Nod(OAS, v1, hv1)}
 		}
+		// Zero hv1. This prevents hv1 from being the sole, inaccessible
+		// reference to an otherwise GC-able value during the next channel receive.
+		// See issue 15281.
+		body = append(body, Nod(OAS, hv1, nil))
 
-		// orderstmt arranged for a copy of the string variable.
 	case TSTRING:
+		// orderstmt arranged for a copy of the string variable.
 		ha := a
 
 		ohv1 := temp(Types[TINT])

test/fixedbugs/issue15281.go

+// run
+
+// Copyright 2016 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package main
+
+import "runtime"
+
+func main() {
+	{
+		x := inuse()
+		c := make(chan []byte, 10)
+		c <- make([]byte, 10<<20)
+		close(c)
+		f1(c, x)
+	}
+	{
+		x := inuse()
+		c := make(chan []byte, 10)
+		c <- make([]byte, 10<<20)
+		close(c)
+		f2(c, x)
+	}
+}
+
+func f1(c chan []byte, start int64) {
+	for x := range c {
+		if delta := inuse() - start; delta < 9<<20 {
+			println("BUG: f1: after alloc: expected delta at least 9MB, got: ", delta)
+			println(x)
+		}
+		x = nil
+		if delta := inuse() - start; delta > 1<<20 {
+			println("BUG: f1: after alloc: expected delta below 1MB, got: ", delta)
+			println(x)
+		}
+	}
+}
+
+func f2(c chan []byte, start int64) {
+	for {
+		x, ok := <-c
+		if !ok {
+			break
+		}
+		if delta := inuse() - start; delta < 9<<20 {
+			println("BUG: f2: after alloc: expected delta at least 9MB, got: ", delta)
+			println(x)
+		}
+		x = nil
+		if delta := inuse() - start; delta > 1<<20 {
+			println("BUG: f2: after alloc: expected delta below 1MB, got: ", delta)
+			println(x)
+		}
+	}
+}
+
+func inuse() int64 {
+	runtime.GC()
+	var st runtime.MemStats
+	runtime.ReadMemStats(&st)
+	return int64(st.Alloc)
+}