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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 
29 namespace base {
30 
31 // A Delegate which performs the actions required to perform an injective
32 // multimapping in place.
33 class 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() = default;
46 };
47 
48 // An implementation of the InjectionDelegate interface using the file
49 // descriptor table of the current process as the domain.
50 class BASE_EXPORT FileDescriptorTableInjection : public InjectionDelegate {
51   bool Duplicate(int* result, int fd) override;
52   bool Move(int src, int dest) override;
53   void Close(int fd) override;
54 };
55 
56 // A single arc of the directed graph which describes an injective multimapping.
57 struct InjectionArc {
InjectionArcInjectionArc58   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 
70 typedef std::vector<InjectionArc> InjectiveMultimap;
71 
72 BASE_EXPORT bool PerformInjectiveMultimap(const InjectiveMultimap& map,
73                                           InjectionDelegate* delegate);
74 
75 BASE_EXPORT bool PerformInjectiveMultimapDestructive(
76     InjectiveMultimap* map,
77     InjectionDelegate* delegate);
78 
79 // This function will not call malloc but will mutate |map|
ShuffleFileDescriptors(InjectiveMultimap * map)80 static 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