test: add test that variables captured by deferred funcs are current on fault

This came up in private mail.
It works today and I want to make sure it stays working.

Change-Id: I13ebdc2dfadb3c72d7f179be89883137320c05d0
Reviewed-on: https://go-review.googlesource.com/7390Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rob Pike <r@golang.org>

test/recover4.go

+// +build linux darwin
+// run
+
+// Copyright 2015 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.
+
+// Test that if a slice access causes a fault, a deferred func
+// sees the most recent value of the variables it accesses.
+// This is true today; the role of the test is to ensure it stays true.
+//
+// In the test, memcopy is the function that will fault, during dst[i] = src[i].
+// The deferred func recovers from the error and returns, making memcopy
+// return the current value of n. If n is not being flushed to memory
+// after each modification, the result will be a stale value of n.
+//
+// The test is set up by mmapping a 64 kB block of memory and then
+// unmapping a 16 kB hole in the middle of it. Running memcopy
+// on the resulting slice will fault when it reaches the hole.
+
+package main
+
+import (
+	"log"
+	"runtime/debug"
+	"syscall"
+	"unsafe"
+)
+
+func memcopy(dst, src []byte) (n int, err error) {
+	defer func() {
+		err = recover().(error)
+	}()
+
+	for i := 0; i < len(dst) && i < len(src); i++ {
+		dst[i] = src[i]
+		n++
+	}
+	return
+}
+
+func main() {
+	// Turn the eventual fault into a panic, not a program crash,
+	// so that memcopy can recover.
+	debug.SetPanicOnFault(true)
+
+	// Map 64 kB block of data with 16 kB hole in middle.
+	data, err := syscall.Mmap(-1, 0, 64*1024, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
+	if err != nil {
+		log.Fatalf("mmap: %v", err)
+	}
+
+	// Note: Cannot call syscall.Munmap, because Munmap checks
+	// that you are unmapping a whole region returned by Mmap.
+	// We are trying to unmap just a hole in the middle.
+	if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[32*1024])), 16*1024, 0); err != 0 {
+		log.Fatalf("munmap: %v", err)
+	}
+
+	other := make([]byte, 64*1024)
+
+	// Check that memcopy returns the actual amount copied
+	// before the fault (32kB - 5, the offset we skip in the argument).
+	n, err := memcopy(data[5:], other)
+	if err == nil {
+		log.Fatal("no error from memcopy across memory hole")
+	}
+	if n != 32*1024-5 {
+		log.Fatal("memcopy returned %d, want %d", n, 32*1024-5)
+	}
+}