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// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// This code exists to perform the shuffling of file descriptors which is
// commonly needed when forking subprocesses. The naive approve is very simple,
// just call dup2 to setup the desired descriptors, but wrong. It's tough to
// handle the edge cases (like mapping 0 -> 1, 1 -> 0) correctly.
// In order to unittest this code, it's broken into the abstract action (an
// injective multimap) and the concrete code for dealing with file descriptors.
// Users should use the code like this:
// base::InjectiveMultimap file_descriptor_map;
// file_descriptor_map.push_back(base::InjectionArc(devnull, 0, true));
// file_descriptor_map.push_back(base::InjectionArc(devnull, 2, true));
// file_descriptor_map.push_back(base::InjectionArc(pipe[1], 1, true));
// base::ShuffleFileDescriptors(file_descriptor_map);
// and trust the the Right Thing will get done.
#include <vector>
namespace base {
// A Delegate which performs the actions required to perform an injective
// multimapping in place.
class InjectionDelegate {
// Duplicate |fd|, an element of the domain, and write a fresh element of the
// domain into |result|. Returns true iff successful.
virtual bool Duplicate(int* result, int fd) = 0;
// Destructively move |src| to |dest|, overwriting |dest|. Returns true iff
// successful.
virtual bool Move(int src, int dest) = 0;
// Delete an element of the domain.
virtual void Close(int fd) = 0;
// An implementation of the InjectionDelegate interface using the file
// descriptor table of the current process as the domain.
class FileDescriptorTableInjection : public InjectionDelegate {
bool Duplicate(int* result, int fd);
bool Move(int src, int dest);
void Close(int fd);
// A single arc of the directed graph which describes an injective multimapping.
struct InjectionArc {
InjectionArc(int in_source, int in_dest, bool in_close)
: source(in_source),
close(in_close) {
int source;
int dest;
bool close; // if true, delete the source element after performing the
// mapping.
typedef std::vector<InjectionArc> InjectiveMultimap;
bool PerformInjectiveMultimap(const InjectiveMultimap& map,
InjectionDelegate* delegate);
bool PerformInjectiveMultimapDestructive(InjectiveMultimap* map,
InjectionDelegate* delegate);
// This function will not call malloc but will mutate |map|
static inline bool ShuffleFileDescriptors(InjectiveMultimap *map) {
FileDescriptorTableInjection delegate;
return PerformInjectiveMultimapDestructive(map, &delegate);
} // namespace base
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