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Source file test/recover4.go

Documentation: test

  // +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)
  
  	size := syscall.Getpagesize()
  
  	// Map 16 pages of data with a 4-page hole in the middle.
  	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
  	if err != nil {
  		log.Fatalf("mmap: %v", err)
  	}
  
  	other := make([]byte, 16*size)
  
  	// 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[8*size])), uintptr(4*size), 0); err != 0 {
  		log.Fatalf("munmap: %v", err)
  	}
  
  	// Check that memcopy returns the actual amount copied
  	// before the fault (8*size - 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 != 8*size-5 {
  		log.Fatal("memcopy returned %d, want %d", n, 8*size-5)
  	}
  }
  

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