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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() {}
     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 {
     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_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
     88