ipc/glue/FileDescriptorShuffle.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_ipc_FileDescriptorShuffle_h
   8 #define mozilla_ipc_FileDescriptorShuffle_h
   9
  10 #include "mozilla/Span.h"
  11 #include "nsTArray.h"
  12
  13 #include <functional>
  14 #include <utility>
  15
  16 // This class converts a set of file descriptor mapping, which may
  17 // contain conflicts (like {a->b, b->c} or {a->b, b->a}) into a
  18 // sequence of dup2() operations that can be performed between fork
  19 // and exec, or with posix_spawn_file_actions_adddup2.  It may create
  20 // temporary duplicates of fds to use as the source of a dup2; they
  21 // are closed on destruction.
  22 //
  23 // The dup2 sequence is guaranteed to not contain dup2(x, x) for any
  24 // x; if such an element is present in the input, it will be dup2()ed
  25 // from a temporary fd to ensure that the close-on-exec bit is cleared.
  26 //
  27 // In general, this is *not* guaranteed to minimize the use of
  28 // temporary fds.
  29
  30 namespace mozilla {
  31 namespace ipc {
  32
  33 class FileDescriptorShuffle {
  34  public:
  35   FileDescriptorShuffle() = default;
  36   ~FileDescriptorShuffle();
  37
  38   using MappingRef = mozilla::Span<const std::pair<int, int>>;
  39
  40   // Translate the given mapping, creating temporary fds as needed.
  41   // Can fail (return false) on failure to duplicate fds.
  42   bool Init(MappingRef aMapping);
  43
  44   // Accessor for the dup2() sequence.  Do not use the returned value
  45   // or the fds contained in it after this object is destroyed.
  46   MappingRef Dup2Sequence() const { return mMapping; }
  47
  48   // Tests whether the given fd is used as a destination in this mapping.
  49   // Can be used to close other fds after performing the dup2()s.
  50   bool MapsTo(int aFd) const;
  51
  52   // Forget the information, so that it's destructor will not try to
  53   // delete FDs duped by itself.
  54   void Forget() { mTempFds.Clear(); }
  55
  56  private:
  57   nsTArray<std::pair<int, int>> mMapping;
  58   nsTArray<int> mTempFds;
  59   int mMaxDst;
  60
  61   FileDescriptorShuffle(const FileDescriptorShuffle&) = delete;
  62   void operator=(const FileDescriptorShuffle&) = delete;
  63 };
  64
  65 }  // namespace ipc
  66 }  // namespace mozilla
  67
  68 #endif  // mozilla_ipc_FileDescriptorShuffle_h