test: demo for issue 7695

Cgo writes C function declarations pretending every arg is a pointer.
If the C function is deferred, it does not inhibit stack copying on split.
The stack copying code believes the C declaration, possibly misinterpreting
integers as pointers.

Probably the right fix for Go 1.3 is to make deferred C functions inhibit
stack copying.

For Go 1.4 and beyond we probably need to make cgo generate Go code
for 6g here, not C code for 6c.

Update #7695

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/83820043

misc/cgo/test/backdoor/backdoor.go

@@ -5,3 +5,4 @@
 package backdoor
 
 func LockedOSThread() bool // in runtime.c
+func Issue7695(x1, x2, x3, x4, x5, x6, x7, x8 uintptr)

misc/cgo/test/backdoor/runtime.c

@@ -23,3 +23,10 @@ void
 	b = runtime·lockedOSThread();
 	FLUSH(&b);
 }
+
+// This is what a cgo-compiled stub declaration looks like.
+void
+·Issue7695(struct{void *y[8*sizeof(void*)];}p)
+{
+	USED(p);
+}

misc/cgo/test/issue7695_test.go

+// Copyright 2014 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.
+
+// Demo of deferred C function with untrue prototype
+// breaking stack copying. See golang.org/issue/7695.
+
+package cgotest
+
+import (
+	"testing"
+
+	"./backdoor"
+)
+
+func TestIssue7695(t *testing.T) {
+	defer backdoor.Issue7695(1, 0, 2, 0, 0, 3, 0, 4)
+	recurse(100)
+}
+
+func recurse(n int) {
+	var x [128]int
+	n += x[0]
+	if n > 0 {
+		recurse(n - 1)
+	}
+}