Share field trial allocator on zygote-using Linuxes

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 493 matching lines @@
   static void CreateTrialsFromCommandLine(
       const base::CommandLine& cmd_line,
       const char* field_trial_handle_switch);
 
 #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);
+#elif defined(OS_POSIX)
+  // On POSIX, we also need to explicitly pass down any fds to be inherited via
+  // |files_to_register| in child_process_launcher.cc. This gets the handle so
+  // we can add it to the mapping there.

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

You check the return value of this for -1 but don't document that. Please do.

[lawrencewu, 2016/11/23 18:59:44]

Done.

+  static int FieldTrialHandle();

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Functions should use a verb. GetFieldTrialHandle()?

[lawrencewu, 2016/11/23 18:59:44]

Done.

 #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,
                                          base::CommandLine* cmd_line);
 
@@ skipping 23 matching lines @@
 
   // Return the number of active field trials.
   static size_t GetFieldTrialCount();
 
  private:
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
 
 #if defined(OS_WIN)
-  // Takes in |handle| that should have been retrieved from the command line and
-  // creates a SharedMemoryHandle from it, and then calls
+  // Takes in |cli_switch| that should have been retrieved from the command line
+  // and constructs a SharedMemoryHandle from it, and then calls
   // CreateTrialsFromSharedMemory(). Returns true on success, false on failure.
-  static bool CreateTrialsFromWindowsHandle(HANDLE handle);
+  static bool CreateTrialsFromWindowsSwitch(std::string cli_switch);

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Don't pass strings by value.

const std::string&

Also, avoid abbreviations/acronyms - so command_line_switch - or handle_switch.

[lawrencewu, 2016/11/23 18:59:44]

Done.

+#elif defined(OS_POSIX)
+  // Takes in |cli_switch| that should have been retrieved from the command
+  // line, looks up the fd using GlobalDescriptor, constructs a
+  // SharedMemoryHandle from it, and then calls CreateTrialsFromSharedMemory().
+  // Returns true on success, false on failure.
+  static bool CreateTrialsFromPosixSwitch(std::string cli_switch);

[Alexei Svitkine (slow), 2016/11/23 17:49:55]

Do we need two separate functions? Why not keeping it the same one and have the
details be in the implementation?

[lawrencewu, 2016/11/23 18:59:44]

Yeah, that works better. Done.

 #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
   // successful and false otherwise.
   static bool CreateTrialsFromSharedMemory(
       std::unique_ptr<base::SharedMemory> shm);
 
   // Instantiate the field trial allocator, add all existing field trials to it,
   // and duplicates its handle to a read-only handle, which gets stored in
@@ skipping 48 matching lines @@
   // 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;
 
 #if defined(OS_WIN)
   // Readonly copy of the handle to the allocator. Needs to be a member variable
   // because it's needed from both CopyFieldTrialStateToFlags() and
   // AppendFieldTrialHandleIfNeeded().
   HANDLE readonly_allocator_handle_ = nullptr;
+#elif defined(OS_POSIX)
+  int readonly_allocator_handle_ = -1;
 #endif
 
   // 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_