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      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   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 {
     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_
     85