github.com/karrick/go@v0.0.0-20170817181416-d5b0ec858b37/test/recover4.go (about)

     1  // +build linux darwin
     2  // run
     3  
     4  // Copyright 2015 The Go Authors. All rights reserved.
     5  // Use of this source code is governed by a BSD-style
     6  // license that can be found in the LICENSE file.
     7  
     8  // Test that if a slice access causes a fault, a deferred func
     9  // sees the most recent value of the variables it accesses.
    10  // This is true today; the role of the test is to ensure it stays true.
    11  //
    12  // In the test, memcopy is the function that will fault, during dst[i] = src[i].
    13  // The deferred func recovers from the error and returns, making memcopy
    14  // return the current value of n. If n is not being flushed to memory
    15  // after each modification, the result will be a stale value of n.
    16  //
    17  // The test is set up by mmapping a 64 kB block of memory and then
    18  // unmapping a 16 kB hole in the middle of it. Running memcopy
    19  // on the resulting slice will fault when it reaches the hole.
    20  
    21  package main
    22  
    23  import (
    24  	"log"
    25  	"runtime/debug"
    26  	"syscall"
    27  	"unsafe"
    28  )
    29  
    30  func memcopy(dst, src []byte) (n int, err error) {
    31  	defer func() {
    32  		err = recover().(error)
    33  	}()
    34  
    35  	for i := 0; i < len(dst) && i < len(src); i++ {
    36  		dst[i] = src[i]
    37  		n++
    38  	}
    39  	return
    40  }
    41  
    42  func main() {
    43  	// Turn the eventual fault into a panic, not a program crash,
    44  	// so that memcopy can recover.
    45  	debug.SetPanicOnFault(true)
    46  
    47  	size := syscall.Getpagesize()
    48  
    49  	// Map 16 pages of data with a 4-page hole in the middle.
    50  	data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
    51  	if err != nil {
    52  		log.Fatalf("mmap: %v", err)
    53  	}
    54  
    55  	other := make([]byte, 16*size)
    56  
    57  	// Note: Cannot call syscall.Munmap, because Munmap checks
    58  	// that you are unmapping a whole region returned by Mmap.
    59  	// We are trying to unmap just a hole in the middle.
    60  	if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 {
    61  		log.Fatalf("munmap: %v", err)
    62  	}
    63  
    64  	// Check that memcopy returns the actual amount copied
    65  	// before the fault (8*size - 5, the offset we skip in the argument).
    66  	n, err := memcopy(data[5:], other)
    67  	if err == nil {
    68  		log.Fatal("no error from memcopy across memory hole")
    69  	}
    70  	if n != 8*size-5 {
    71  		log.Fatal("memcopy returned %d, want %d", n, 8*size-5)
    72  	}
    73  }