Commit 0c31992e authored by Russ Cox's avatar Russ Cox

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: 's avatarAustin Clements <austin@google.com>
Reviewed-by: 's avatarRob Pike <r@golang.org>
parent 5764befa
// +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)
}
}
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