base/file_descriptor_shuffle.h

   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:
  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   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.
  55 struct 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
  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|
  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_