Initial implementation for sharing field trial state (win)

content/app/content_main_runner.cc

Index: content/app/content_main_runner.cc
diff --git a/content/app/content_main_runner.cc b/content/app/content_main_runner.cc
index 01bb5f00f42ce6a9499708f7a53c5766054c62fc..08ee11752797f0984044190ea2ff8b6d826cc775 100644
--- a/content/app/content_main_runner.cc
+++ b/content/app/content_main_runner.cc
@@ -26,12 +26,14 @@
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
+#include "base/memory/shared_memory.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/path_service.h"
 #include "base/process/launch.h"
 #include "base/process/memory.h"
+#include "base/process/process.h"
 #include "base/process/process_handle.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/strings/string_number_conversions.h"
@@ -146,12 +148,34 @@ void InitializeFieldTrialAndFeatureList(
 
   // Ensure any field trials in browser are reflected into the child
   // process.
+#if defined(OS_WIN)
+  if (command_line.HasSwitch(switches::kFieldTrialHandle)) {
+    std::string arg =

[Alexei Svitkine (slow), 2016/10/04 18:49:35]

Suggest moving this code to base/metrics. We can have a static FieldTrialList
static method that is CreateTrialsFromCommandLine().

Then, this logic can be inside there - to either use the handle or the force
flag.

Also, I suggest not having ifdefs around this part assuming the code compiles on
non-Windows platforms.

[lawrencewu, 2016/10/05 20:57:56]

Done. Regarding the ifdefs, I think they're necessary because HANDLE is a
windows-specific type. In the future, the ifdef's scope will be reduced (so each
platform just encodes the handle/fd their own specific way), but for now I think
we should keep them outside the if of the switch.

[lawrencewu, 2016/10/06 20:14:57]

Done. Regarding the ifdefs, I think they're necessary because HANDLE is a
windows-specific type. In the future, the ifdef's scope will be reduced (so each
platform just encodes the handle/fd their own specific way), but for now I think
we need them here.

+        command_line.GetSwitchValueASCII(switches::kFieldTrialHandle);
+    size_t token = arg.find(",");
+    int field_trial_handle = std::stoi(arg.substr(0, token));
+    int field_trial_length = std::stoi(arg.substr(token + 1, arg.length()));
+
+    HANDLE handle = reinterpret_cast<HANDLE>(field_trial_handle);
+    base::SharedMemoryHandle shm_handle =
+        base::SharedMemoryHandle(handle, base::GetCurrentProcId());
+
+    base::SharedMemory shared_memory(shm_handle, false);
+    shared_memory.Map(field_trial_length);
+
+    char* field_trial_state = static_cast<char*>(shared_memory.memory());
+    bool result = base::FieldTrialList::CreateTrialsFromString(
+        std::string(field_trial_state), std::set<std::string>());
+    DCHECK(result);
+  }
+#else
   if (command_line.HasSwitch(switches::kForceFieldTrials)) {
     bool result = base::FieldTrialList::CreateTrialsFromString(
         command_line.GetSwitchValueASCII(switches::kForceFieldTrials),
         std::set<std::string>());
     DCHECK(result);
   }
+#endif
 
   std::unique_ptr<base::FeatureList> feature_list(new base::FeatureList);
   feature_list->InitializeFromCommandLine(