Make changing a record's type an atomic operation.

base/metrics/persistent_memory_allocator.h

 // Copyright (c) 2015 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_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 #define BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <atomic>
@@ skipping 223 matching lines @@
   }
 
   // Get the number of bytes allocated to a block. This is useful when storing
   // arrays in order to validate the ending boundary. The returned value will
   // include any padding added to achieve the required alignment and so could
   // be larger than given in the original Allocate() request.
   size_t GetAllocSize(Reference ref) const;
 
   // Access the internal "type" of an object. This generally isn't necessary
   // but can be used to "clear" the type and so effectively mark it as deleted
-  // even though the memory stays valid and allocated.
+  // even though the memory stays valid and allocated. Changing the type is
+  // an atomic compare/exchange and so requires knowing the existing value.
+  // It will return false if the existing type is not what is expected.
   uint32_t GetType(Reference ref) const;
-  void SetType(Reference ref, uint32_t type_id);
+  bool ChangeType(Reference ref, uint32_t to_type_id, uint32_t from_type_id);
 
   // Reserve space in the memory segment of the desired |size| and |type_id|.
   // A return value of zero indicates the allocation failed, otherwise the
   // returned reference can be used by any process to get a real pointer via
   // the GetAsObject() call.
   Reference Allocate(size_t size, uint32_t type_id);
 
   // Allocated objects can be added to an internal list that can then be
   // iterated over by other processes. If an allocated object can be found
   // another way, such as by having its reference within a different object
@@ skipping 151 matching lines @@
 
  private:
   std::unique_ptr<MemoryMappedFile> mapped_file_;
 
   DISALLOW_COPY_AND_ASSIGN(FilePersistentMemoryAllocator);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_