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
27 namespace base {
28
29 // A Delegate which performs the actions required to perform an injective
30 // multimapping in place.
31 class 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:
~InjectionDelegate()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.
48 class FileDescriptorTableInjection : public InjectionDelegate {
49 virtual bool Duplicate(int* result, int fd);
50 virtual bool Move(int src, int dest);
51 virtual void Close(int fd);
52 };
53
54 // A single arc of the directed graph which describes an injective multimapping.
55 struct InjectionArc {
InjectionArcInjectionArc56 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
68 typedef std::vector<InjectionArc> InjectiveMultimap;
69
70 bool PerformInjectiveMultimap(const InjectiveMultimap& map,
71 InjectionDelegate* delegate);
72
73 bool PerformInjectiveMultimapDestructive(InjectiveMultimap* map,
74 InjectionDelegate* delegate);
75
76 // This function will not call malloc but will mutate |map|
ShuffleFileDescriptors(InjectiveMultimap * map)77 static 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