file_descriptor_shuffle.h revision 3345a6884c488ff3a535c2c9acdd33d74b37e311 
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#pragma once
8
9// This code exists to perform the shuffling of file descriptors which is
10// commonly needed when forking subprocesses. The naive approve is very simple,
11// just call dup2 to setup the desired descriptors, but wrong. It's tough to
12// handle the edge cases (like mapping 0 -> 1, 1 -> 0) correctly.
13//
14// In order to unittest this code, it's broken into the abstract action (an
15// injective multimap) and the concrete code for dealing with file descriptors.
16// Users should use the code like this:
17//   base::InjectiveMultimap file_descriptor_map;
18//   file_descriptor_map.push_back(base::InjectionArc(devnull, 0, true));
19//   file_descriptor_map.push_back(base::InjectionArc(devnull, 2, true));
20//   file_descriptor_map.push_back(base::InjectionArc(pipe[1], 1, true));
21//   base::ShuffleFileDescriptors(file_descriptor_map);
22//
23// and trust the the Right Thing will get done.
24
25#include <vector>
26
27namespace base {
28
29// A Delegate which performs the actions required to perform an injective
30// multimapping in place.
31class InjectionDelegate {
32 public:
33  // Duplicate |fd|, an element of the domain, and write a fresh element of the
34  // domain into |result|. Returns true iff successful.
35  virtual bool Duplicate(int* result, int fd) = 0;
36  // Destructively move |src| to |dest|, overwriting |dest|. Returns true iff
37  // successful.
38  virtual bool Move(int src, int dest) = 0;
39  // Delete an element of the domain.
40  virtual void Close(int fd) = 0;
41
42 protected:
43  virtual ~InjectionDelegate() {}
44};
45
46// An implementation of the InjectionDelegate interface using the file
47// descriptor table of the current process as the domain.
48class FileDescriptorTableInjection : public InjectionDelegate {
49  bool Duplicate(int* result, int fd);
50  bool Move(int src, int dest);
51  void Close(int fd);
52};
53
54// A single arc of the directed graph which describes an injective multimapping.
55struct InjectionArc {
56  InjectionArc(int in_source, int in_dest, bool in_close)
57      : source(in_source),
58        dest(in_dest),
59        close(in_close) {
60  }
61
62  int source;
63  int dest;
64  bool close;  // if true, delete the source element after performing the
65               // mapping.
66};
67
68typedef std::vector<InjectionArc> InjectiveMultimap;
69
70bool PerformInjectiveMultimap(const InjectiveMultimap& map,
71                              InjectionDelegate* delegate);
72
73bool PerformInjectiveMultimapDestructive(InjectiveMultimap* map,
74                                         InjectionDelegate* delegate);
75
76// This function will not call malloc but will mutate |map|
77static inline bool ShuffleFileDescriptors(InjectiveMultimap* map) {
78  FileDescriptorTableInjection delegate;
79  return PerformInjectiveMultimapDestructive(map, &delegate);
80}
81
82}  // namespace base
83
84#endif  // BASE_FILE_DESCRIPTOR_SHUFFLE_H_
85