1// Copyright (c) 2011 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_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
6#define BASE_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
7
8// This code exists to shuffle file descriptors, which is commonly needed when
9// forking subprocesses. The naive approach (just call dup2 to set up the
10// desired descriptors) is very simple, but wrong: it won't handle edge cases
11// (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
26#include "base/base_export.h"
27#include "base/compiler_specific.h"
28
29namespace base {
30
31// A Delegate which performs the actions required to perform an injective
32// multimapping in place.
33class InjectionDelegate {
34 public:
35  // Duplicate |fd|, an element of the domain, and write a fresh element of the
36  // domain into |result|. Returns true iff successful.
37  virtual bool Duplicate(int* result, int fd) = 0;
38  // Destructively move |src| to |dest|, overwriting |dest|. Returns true iff
39  // successful.
40  virtual bool Move(int src, int dest) = 0;
41  // Delete an element of the domain.
42  virtual void Close(int fd) = 0;
43
44 protected:
45  virtual ~InjectionDelegate() {}
46};
47
48// An implementation of the InjectionDelegate interface using the file
49// descriptor table of the current process as the domain.
50class BASE_EXPORT FileDescriptorTableInjection : public InjectionDelegate {
51  virtual bool Duplicate(int* result, int fd) OVERRIDE;
52  virtual bool Move(int src, int dest) OVERRIDE;
53  virtual void Close(int fd) OVERRIDE;
54};
55
56// A single arc of the directed graph which describes an injective multimapping.
57struct InjectionArc {
58  InjectionArc(int in_source, int in_dest, bool in_close)
59      : source(in_source),
60        dest(in_dest),
61        close(in_close) {
62  }
63
64  int source;
65  int dest;
66  bool close;  // if true, delete the source element after performing the
67               // mapping.
68};
69
70typedef std::vector<InjectionArc> InjectiveMultimap;
71
72BASE_EXPORT bool PerformInjectiveMultimap(const InjectiveMultimap& map,
73                                          InjectionDelegate* delegate);
74
75BASE_EXPORT bool PerformInjectiveMultimapDestructive(
76    InjectiveMultimap* map,
77    InjectionDelegate* delegate);
78
79// This function will not call malloc but will mutate |map|
80static inline bool ShuffleFileDescriptors(InjectiveMultimap* map) {
81  FileDescriptorTableInjection delegate;
82  return PerformInjectiveMultimapDestructive(map, &delegate);
83}
84
85}  // namespace base
86
87#endif  // BASE_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
88