base/file_descriptor_shuffle.h

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