Pass the GUID for the SharedMemoryHandle used by base::FieldTrialList.

base/metrics/field_trial.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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 54 matching lines @@
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/files/file.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
+#include "base/memory/shared_memory_handle.h"
 #include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/pickle.h"
 #include "base/process/launch.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
 namespace base {
 
@@ skipping 481 matching lines @@
 #if defined(OS_WIN)
   // On Windows, we need to explicitly pass down any handles to be inherited.
   // This function adds the shared memory handle to field trial state to the
   // list of handles to be inherited.
   static void AppendFieldTrialHandleIfNeeded(
       base::HandlesToInheritVector* handles);
 #endif
 
 #if defined(OS_POSIX) && !defined(OS_NACL)
   // On POSIX, we also need to explicitly pass down this file descriptor that
-  // should be shared with the child process. Returns kInvalidPlatformFile if no
-  // handle exists or was not initialized properly.
-  static PlatformFile GetFieldTrialHandle();
+  // should be shared with the child process. Returns an invalid handle if it
+  // was not initialized properly.
+  static base::SharedMemoryHandle GetFieldTrialHandle();
 #endif
 
   // Adds a switch to the command line containing the field trial state as a
   // string (if not using shared memory to share field trial state), or the
   // shared memory handle + length.
   // Needs the |field_trial_handle_switch| argument to be passed in since base/
   // can't depend on content/.
   static void CopyFieldTrialStateToFlags(const char* field_trial_handle_switch,
                                          const char* enable_features_switch,
                                          const char* disable_features_switch,
@@ skipping 49 matching lines @@
       PersistentMemoryAllocator const& allocator);
 
  private:
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest,
                            DoNotAddSimulatedFieldTrialsToAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AssociateFieldTrialParams);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, ClearParamsFromSharedMemory);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest,
+                           SerializeSharedMemoryHandleMetadata);
+
+  // Serialization and deserialization doesn't actually transport the underlying

[Alexei Svitkine (slow), 2017/05/05 18:02:16]

Please start this comment explaining what serialization/deserialization is for
(i.e. used to tell child processes about the segment). The current comment you
have makes sense as a second sentence.

[erikchen, 2017/05/05 19:41:51]

Done.

+  // OS resource - that must be done by the Process launcher.
+  static std::string SerializeSharedMemoryHandleMetadata(
+      const SharedMemoryHandle& shm);
+#if defined(OS_WIN)
+  static SharedMemoryHandle DeserializeSharedMemoryHandleMetadata(
+      const std::string& switch_value);
+#elif defined(OS_POSIX) && !defined(OS_NACL)
+  static SharedMemoryHandle DeserializeSharedMemoryHandleMetadata(
+      int fd,
+      const std::string& switch_value);
+#endif
 
 #if defined(OS_WIN)
   // Takes in |handle_switch| from the command line which represents the shared
   // memory handle for field trials, parses it, and creates the field trials.
   // Returns true on success, false on failure.
-  static bool CreateTrialsFromHandleSwitch(const std::string& handle_switch);
+  // |switch_value| also contains the serialized GUID.
+  static bool CreateTrialsFromSwitchValue(const std::string& switch_value);
 #endif
 
 #if defined(OS_POSIX) && !defined(OS_NACL)
   // On POSIX systems that use the zygote, we look up the correct fd that backs
   // the shared memory segment containing the field trials by looking it up via
   // an fd key in GlobalDescriptors. Returns true on success, false on failure.
-  static bool CreateTrialsFromDescriptor(int fd_key);
+  // |switch_value| also contains the serialized GUID.
+  static bool CreateTrialsFromDescriptor(int fd_key,
+                                         const std::string& switch_value);
 #endif
 
   // Takes an unmapped SharedMemoryHandle, creates a SharedMemory object from it
   // and maps it with the correct size.
   static bool CreateTrialsFromSharedMemoryHandle(SharedMemoryHandle shm_handle);
 
   // Expects a mapped piece of shared memory |shm| that was created from the
   // browser process's field_trial_allocator and shared via the command line.
   // This function recreates the allocator, iterates through all the field
   // trials in it, and creates them via CreateFieldTrial(). Returns true if
@@ skipping 53 matching lines @@
 
   // Allocator in shared memory containing field trial data. Used in both
   // browser and child processes, but readonly in the child.
   // In the future, we may want to move this to a more generic place if we want
   // to start passing more data other than field trials.
   std::unique_ptr<FieldTrialAllocator> field_trial_allocator_ = nullptr;
 
   // Readonly copy of the handle to the allocator. Needs to be a member variable
   // because it's needed from both CopyFieldTrialStateToFlags() and
   // AppendFieldTrialHandleIfNeeded().
-  PlatformFile readonly_allocator_handle_ = kInvalidPlatformFile;
+  base::SharedMemoryHandle readonly_allocator_handle_;
 
   // Tracks whether CreateTrialsFromCommandLine() has been called.
   bool create_trials_from_command_line_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_