file_descriptor_shuffle.h revision c407dc5cd9bdc5668497f21b26b09d988ab439de 
1// Copyright (c) 2009 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef BASE_FILE_DESCRIPTOR_SHUFFLE_H_
6#define BASE_FILE_DESCRIPTOR_SHUFFLE_H_
7
8// This code exists to perform the shuffling of file descriptors which is
9// commonly needed when forking subprocesses. The naive approve is very simple,
10// just call dup2 to setup the desired descriptors, but wrong. It's tough to
11// handle the edge cases (like mapping 0 -> 1, 1 -> 0) correctly.
12//
13// In order to unittest this code, it's broken into the abstract action (an
14// injective multimap) and the concrete code for dealing with file descriptors.
15// Users should use the code like this:
16//   base::InjectiveMultimap file_descriptor_map;
17//   file_descriptor_map.push_back(base::InjectionArc(devnull, 0, true));
18//   file_descriptor_map.push_back(base::InjectionArc(devnull, 2, true));
19//   file_descriptor_map.push_back(base::InjectionArc(pipe[1], 1, true));
20//   base::ShuffleFileDescriptors(file_descriptor_map);
21//
22// and trust the the Right Thing will get done.
23
24#include <vector>
25
26namespace base {
27
28// A Delegate which performs the actions required to perform an injective
29// multimapping in place.
30class InjectionDelegate {
31 public:
32  // Duplicate |fd|, an element of the domain, and write a fresh element of the
33  // domain into |result|. Returns true iff successful.
34  virtual bool Duplicate(int* result, int fd) = 0;
35  // Destructively move |src| to |dest|, overwriting |dest|. Returns true iff
36  // successful.
37  virtual bool Move(int src, int dest) = 0;
38  // Delete an element of the domain.
39  virtual void Close(int fd) = 0;
40
41 protected:
42  virtual ~InjectionDelegate() {}
43};
44
45// An implementation of the InjectionDelegate interface using the file
46// descriptor table of the current process as the domain.
47class FileDescriptorTableInjection : public InjectionDelegate {
48  bool Duplicate(int* result, int fd);
49  bool Move(int src, int dest);
50  void Close(int fd);
51};
52
53// A single arc of the directed graph which describes an injective multimapping.
54struct InjectionArc {
55  InjectionArc(int in_source, int in_dest, bool in_close)
56      : source(in_source),
57        dest(in_dest),
58        close(in_close) {
59  }
60
61  int source;
62  int dest;
63  bool close;  // if true, delete the source element after performing the
64               // mapping.
65};
66
67typedef std::vector<InjectionArc> InjectiveMultimap;
68
69bool PerformInjectiveMultimap(const InjectiveMultimap& map,
70                              InjectionDelegate* delegate);
71
72bool PerformInjectiveMultimapDestructive(InjectiveMultimap* map,
73                                         InjectionDelegate* delegate);
74
75// This function will not call malloc but will mutate |map|
76static inline bool ShuffleFileDescriptors(InjectiveMultimap* map) {
77  FileDescriptorTableInjection delegate;
78  return PerformInjectiveMultimapDestructive(map, &delegate);
79}
80
81}  // namespace base
82
83#endif  // BASE_FILE_DESCRIPTOR_SHUFFLE_H_
84