Share field trial allocator on zygote-using Linuxes

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 34b9782bee0fe06c51abf1383c3d98c247c4a958..c61afd93b4c15dc65e3a7bf3d80cac83892c7c6a 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -65,6 +65,7 @@
 
 #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"
@@ -76,6 +77,13 @@
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
+// On systems that use the zygote process to spawn child processes, we must
+// retrieve the correct fd using the mapping in GlobalDescriptors.
+#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_MACOSX) && \
+    !defined(OS_ANDROID)
+#define POSIX_WITH_ZYGOTE 1

[Alexei Svitkine (slow), 2016/11/28 16:56:13]

I don't think we should define this in the header file.

Otherwise, this define bleeds into all files including this header, which we
don't want. (If we wanted such a thing to exist, it should be in a more
appropriate place in base.)

Otherwise, suggest spelling things out for the one time it's used in the .h and
then keeping the define in the .cc.

[lawrencewu, 2016/11/29 14:38:07]

Done.

+#endif
+
 namespace base {
 
 class FieldTrialList;
@@ -503,9 +511,9 @@ class BASE_EXPORT FieldTrialList {
   // 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);
+  static void CreateTrialsFromCommandLine(const base::CommandLine& cmd_line,
+                                          const char* field_trial_handle_switch,
+                                          const int field_trial_handle);

[Alexei Svitkine (slow), 2016/11/28 16:56:13]

Nit: No reason to mark primitive params (int) as const.

Also, what is "field_trial_handle"? It's not the actual handle - it looks like
it's the id in the posix global descriptors map. If so, name it as such and also
document it in the comment.

[lawrencewu, 2016/11/29 14:38:07]

Done.

 
 #if defined(OS_WIN)
   // On Windows, we need to explicitly pass down any handles to be inherited.
@@ -513,6 +521,11 @@ class BASE_EXPORT FieldTrialList {
   // 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
@@ -558,12 +571,23 @@ class BASE_EXPORT FieldTrialList {
                            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(POSIX_WITH_ZYGOTE)
+  // 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 CreateTrialsFromFdKey(const int fd_key);

[Alexei Svitkine (slow), 2016/11/28 16:56:13]

Nit: No const

[lawrencewu, 2016/11/29 14:38:07]

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
@@ -627,12 +651,10 @@ class BASE_EXPORT FieldTrialList {
   // 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;