runtime: prevent the GC from seeing the content of a frame in runfinq()

Fixes #5348.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/8954044

src/pkg/runtime/mgc0.c

@@ -2191,7 +2191,7 @@ runfinq(void)
 				framesz = sizeof(uintptr) + f->nret;
 				if(framecap < framesz) {
 					runtime·free(frame);
-					frame = runtime·mal(framesz);
+					frame = runtime·mallocgc(framesz, FlagNoPointers, 0, 1);
 					framecap = framesz;
 				}
 				*(void**)frame = f->arg;

test/fixedbugs/issue5348.go

+// run
+
+// Copyright 2013 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.
+
+// Issue 5348: finalizers keep data live for a surprising amount of time
+
+package main
+
+import (
+	"runtime"
+)
+
+type T struct {
+	S *string
+}
+
+func newString(s string) *string {
+	return &s
+}
+
+var c = make(chan int)
+
+func foo() {
+	t := &T{S: newString("foo")}
+	runtime.SetFinalizer(t, func(p *T) { c <- 0 })
+	runtime.SetFinalizer(t.S, func(p *string) { c <- 0 })
+}
+
+func main() {
+	foo()
+	runtime.GC()
+	<-c
+	runtime.GC()
+	<-c
+}