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 47 matching lines @@
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/command_line.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/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/process/launch.h"
 #include "base/strings/string_piece.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
@@ skipping 411 matching lines @@
   // through a command line argument to the browser process. Created field
   // trials will be marked "used" for the purposes of active trial reporting
   // if they are prefixed with |kActivationMarker|. Trial names in
   // |ignored_trial_names| are ignored when parsing |trials_string|.
   static bool CreateTrialsFromString(
       const std::string& trials_string,
       const std::set<std::string>& ignored_trial_names);
 
   // Achieves the same thing as CreateTrialsFromString, except wraps the logic
   // by taking in the trials from the command line, either via shared memory
-  // handle or command line argument.
-  // If using shared memory to pass around the list of field trials, then
-  // expects |field_trial_handle_switch| command line argument to
-  // contain the shared memory handle.
-  // If not, then create the trials as before (using the kForceFieldTrials
-  // switch). Needs the |field_trial_handle_switch| argument to be passed in
-  // since base/ can't depend on content/.
-  static void CreateTrialsFromCommandLine(
-      const base::CommandLine& cmd_line,
-      const char* field_trial_handle_switch);
+  // handle or command line argument. A bit of a misnomer since on POSIX we
+  // get the trials from opening |field_trial_descriptor_id| if using shared
+  // memory. On Windows, we expect the |cmd_line| switch for
+  // |field_trial_handle_switch| to contain the shared memory handle that
+  // contains the field trial allocator.
+  // We need the |field_trial_handle_switch| and |field_trial_descriptor_id|
+  // arguments to be passed in since base/ can't depend on content/.
+  static void CreateTrialsFromCommandLine(const base::CommandLine& cmd_line,
+                                          const char* field_trial_handle_switch,
+                                          int field_trial_descriptor_id);
 
 #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 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();
 #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 25 matching lines @@
   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);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest,
                            DoNotAddSimulatedFieldTrialsToAllocator);
 
 #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
-  // CreateTrialsFromSharedMemory(). Returns true on success, false on failure.
-  static bool CreateTrialsFromWindowsHandle(HANDLE handle);
+  // 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);
 #endif
 
+#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_MACOSX) && \
+    !defined(OS_ANDROID)
+  // 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);

[Alexei Svitkine (slow), 2016/11/29 19:32:47]

Nit: You use fd_key here but field_trial_descriptor_id elsewhere. Make
consistent (both in header and .cc).

I think fd_key is fine as a name since it's short and it still accurately
describes what it's doing.

[lawrencewu, 2016/11/29 19:52:44]

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 43 matching lines @@
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
   // 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;
 
-#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;
-#endif
+  PlatformFile readonly_allocator_handle_ = kInvalidPlatformFile;
 
   // 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_