FileDescriptorShuffle.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */

/* vim: set ts=8 sts=2 et sw=2 tw=80: */

/* This Source Code Form is subject to the terms of the Mozilla Public

 * License, v. 2.0. If a copy of the MPL was not distributed with this

 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef mozilla_ipc_FileDescriptorShuffle_h

#define mozilla_ipc_FileDescriptorShuffle_h

#include "mozilla/Span.h"

#include "nsTArray.h"

#include <functional>

#include <utility>

// This class converts a set of file descriptor mapping, which may

// contain conflicts (like {a->b, b->c} or {a->b, b->a}) into a

// sequence of dup2() operations that can be performed between fork

// and exec, or with posix_spawn_file_actions_adddup2.  It may create

// temporary duplicates of fds to use as the source of a dup2; they

// are closed on destruction.

//

// The dup2 sequence is guaranteed to not contain dup2(x, x) for any

// x; if such an element is present in the input, it will be dup2()ed

// from a temporary fd to ensure that the close-on-exec bit is cleared.

//

// In general, this is *not* guaranteed to minimize the use of

// temporary fds.

namespace mozilla {

namespace ipc {

class FileDescriptorShuffle {

 public:

  FileDescriptorShuffle() = default;

  ~FileDescriptorShuffle();

  using MappingRef = mozilla::Span<const std::pair<int, int>>;

  // Translate the given mapping, creating temporary fds as needed.

  // Can fail (return false) on failure to duplicate fds.

  bool Init(MappingRef aMapping);

  // Accessor for the dup2() sequence.  Do not use the returned value

  // or the fds contained in it after this object is destroyed.

  MappingRef Dup2Sequence() const { return mMapping; }

  // Tests whether the given fd is used as a destination in this mapping.

  // Can be used to close other fds after performing the dup2()s.

  bool MapsTo(int aFd) const;

  // Forget the information, so that it's destructor will not try to

  // delete FDs duped by itself.

  void Forget() { mTempFds.Clear(); }

 private:

  nsTArray<std::pair<int, int>> mMapping;

  nsTArray<int> mTempFds;

  int mMaxDst;

  FileDescriptorShuffle(const FileDescriptorShuffle&) = delete;

  void operator=(const FileDescriptorShuffle&) = delete;

};

}  // namespace ipc

}  // namespace mozilla

#endif  // mozilla_ipc_FileDescriptorShuffle_h