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base/posix/global_descriptors.h

 // Copyright (c) 2012 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_GLOBAL_DESCRIPTORS_H_
 #define BASE_POSIX_GLOBAL_DESCRIPTORS_H_
 
 #include "build/build_config.h"
 
 #include <vector>
 #include <utility>
 
 #include <stdint.h>
 
 #include "base/files/memory_mapped_file.h"
+#include "base/files/scoped_file.h"
 #include "base/memory/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 BASE_EXPORT GlobalDescriptors {

[dcheng, 2017/02/15 08:05:27]

Note that this is deprecated and new code should use FileDescriptorStore, with a
https://crbug.com/xyz link for tracking this cleanup.

[Jay Civelli, 2017/02/15 19:53:47]

Done.

  public:
   typedef uint32_t Key;
   struct Descriptor {
     Descriptor(Key key, int fd);
     Descriptor(Key key, int fd, base::MemoryMappedFile::Region region);
 
     // Globally unique key.
     Key key;
     // Actual FD.
     int fd;
     // Optional region, defaults to kWholeFile.
     base::MemoryMappedFile::Region region;
   };
   typedef std::vector<Descriptor> Mapping;
 
   // Often we want a canonical descriptor for a given Key. In this case, we add
   // the following constant to the key value:
   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
 
   // Return the singleton instance of GlobalDescriptors.
   static GlobalDescriptors* GetInstance();
 
   // 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 given a key. Returns -1 on error.
   int MaybeGet(Key key) const;
 
+  // Returns a descriptor given a key and removes it from this class mappings.
+  // Also populates |region| if it's not null.

[dcheng, 2017/02/15 08:05:27]

I might be missing something, but it seems like nothing actually passes null for
|region|. I'd rather just require it be present (ditto for the new class)

[Jay Civelli, 2017/02/15 19:53:47]

Done.

+  // It is a fatal error if the key is not known.
+  base::ScopedFD TakeFD(Key key, base::MemoryMappedFile::Region* region);
+
   // Get a region given a key. It is a fatal error if the key is not known.
   base::MemoryMappedFile::Region GetRegion(Key key) const;
 
   // Set the descriptor for the given |key|. This sets the region associated
   // with |key| to kWholeFile.
   void Set(Key key, int fd);
 
   // Set the descriptor and |region| for the given |key|.
   void Set(Key key, int fd, base::MemoryMappedFile::Region region);
 
   void Reset(const Mapping& mapping);
 
  private:
   friend struct DefaultSingletonTraits<GlobalDescriptors>;
   GlobalDescriptors();
   ~GlobalDescriptors();
 
   Mapping descriptors_;
 };
 
 }  // namespace base
 
 #endif  // BASE_POSIX_GLOBAL_DESCRIPTORS_H_