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+// Copyright (c) 2011 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.
+
+#ifndef BASE_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
+#define BASE_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_
+
+// 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>
+
+#include "base/base_export.h"
+#include "base/compiler_specific.h"
+
+namespace base {
+
+// A Delegate which performs the actions required to perform an injective
+// multimapping in place.
+class InjectionDelegate {
+ public:
+  // 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;
+
+ protected:
+  virtual ~InjectionDelegate() {}
+};
+
+// An implementation of the InjectionDelegate interface using the file
+// descriptor table of the current process as the domain.
+class FileDescriptorTableInjection : public InjectionDelegate {
+  virtual bool Duplicate(int* result, int fd) OVERRIDE;
+  virtual bool Move(int src, int dest) OVERRIDE;
+  virtual void Close(int fd) OVERRIDE;
+};
+
+// 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),
+        dest(in_dest),
+        close(in_close) {
+  }
+
+  int source;
+  int dest;
+  bool close;  // if true, delete the source element after performing the
+               // mapping.
+};
+
+typedef std::vector<InjectionArc> InjectiveMultimap;
+
+BASE_EXPORT bool PerformInjectiveMultimap(const InjectiveMultimap& map,
+                                          InjectionDelegate* delegate);
+
+BASE_EXPORT 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
+
+#endif  // BASE_POSIX_FILE_DESCRIPTOR_SHUFFLE_H_