Linux: refactor zygote support

base/global_descriptors_posix.h

+// 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.
+
+#ifndef BASE_GLOBAL_DESCRIPTORS_POSIX_H_
+#define BASE_GLOBAL_DESCRIPTORS_POSIX_H_
+
+#include "build/build_config.h"
+
+#include <vector>
+#include <utility>
+
+#include <stdint.h>
+
+#include "base/singleton.h"
+
+namespace base {
+
+// It's common practice to install file descriptors into well known slot
+// numbers before execing a child; stdin, stdout and stderr are ubiqutous
+// examples.
+//
+// However, when using a zygote model, this becomes troublesome. Since the
+// descriptors which need to be in these slots generally aren't known, any code
+// could open a resource and take one of the reserved descriptors. Simply
+// overwriting the slot isn't a viable solution.
+//
+// We could try to fill the reserved slots as soon as possible, but this is a
+// fragile solution since global constructors etc are able to open files.
+//
+// Instead, we retreat from the idea of installing descriptors in specific
+// slots and add a layer of indirection in the form of this singleton object.
+// It maps from an abstract key to a descriptor. If independent modules each
+// need to define keys, then values should be chosen randomly so as not to
+// collide.
+class GlobalDescriptors {
+ public:
+  typedef uint32_t Key;
+
+  // Get a descriptor given a key. It is a fatal error if the key is not known.
+  int Get(Key key) const;
+  // Get a descriptor give a key. Returns -1 on error.
+  int MaybeGet(Key key) const;
+
+  typedef std::vector<std::pair<Key, int> > Mapping;
+
+  // Set the descriptor for the given key.
+  void Set(Key key, int fd);
+
+  void Reset(const Mapping& mapping) {
+    descriptors_ = mapping;
+  }
+
+ private:
+  GlobalDescriptors() { }
+  friend struct DefaultSingletonTraits<GlobalDescriptors>;
+
+  Mapping descriptors_;
+};
+
+}  // namespace base
+
+#endif  // BASE_GLOBAL_DESCRIPTORS_POSIX_H_