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
   8 // This code exists to perform the shuffling of file descriptors which is
   9 // commonly needed when forking subprocesses. The naive approve is very simple,
  10 // just call dup2 to setup the desired descriptors, but wrong. It's tough to
  11 // handle the edge cases (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() {}
  46 };
  47
  48 // An implementation of the InjectionDelegate interface using the file
  49 // descriptor table of the current process as the domain.
  50 class 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.
  57 struct 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
  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|
  80 static inline bool ShuffleFileDescriptors(InjectiveMultimap* map) {
  81   FileDescriptorTableInjection delegate;
  82   return PerformInjectiveMultimapDestructive(map, &delegate);
  83 }
  84
  85 }  // namespace base
  86
  87 #endif  // BASE_FILE_DESCRIPTOR_SHUFFLE_H_