Initial implementation for sharing field trial state (win)

content/browser/browser_child_process_host_impl.cc

Index: content/browser/browser_child_process_host_impl.cc
diff --git a/content/browser/browser_child_process_host_impl.cc b/content/browser/browser_child_process_host_impl.cc
index e3eacd3ce01dead3132bee37df83760295395796..8cb7c71ad83ea1c25830ba3dc3057d1a82b39bf4 100644
--- a/content/browser/browser_child_process_host_impl.cc
+++ b/content/browser/browser_child_process_host_impl.cc
@@ -208,7 +208,8 @@ void BrowserChildProcessHostImpl::TerminateAll() {
 
 // static
 void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
-    base::CommandLine* cmd_line) {
+    base::CommandLine* cmd_line,
+    base::SharedMemory* shared_memory) {
   std::string enabled_features;
   std::string disabled_features;
   base::FeatureList::GetInstance()->GetFeatureOverrides(&enabled_features,
@@ -223,14 +224,34 @@ void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
   std::string field_trial_states;
   base::FieldTrialList::AllStatesToString(&field_trial_states);
   if (!field_trial_states.empty()) {
-    cmd_line->AppendSwitchASCII(switches::kForceFieldTrials,
-                                field_trial_states);
+    // Use shared memory to pass field_trial_states if we can, otherwise
+    // fallback to the command line.
+    if (shared_memory) {

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

I also suggest putting this code in base/metrics as well. The function would be
responsible for adding the data to the command line.

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

Done.

+#if defined(OS_WIN)
+      size_t length = field_trial_states.size() + 1;
+      shared_memory->CreateAndMapAnonymous(length);
+      memcpy(shared_memory->memory(), field_trial_states.c_str(), length);
+
+      // HANDLE is just typedef'd to void *
+      auto uintptr_handle =
+          reinterpret_cast<std::uintptr_t>(shared_memory->handle().GetHandle());
+      std::string field_trial_handle =
+          std::to_string(uintptr_handle) + "," + std::to_string(length);
+
+      cmd_line->AppendSwitchASCII(switches::kFieldTrialHandle,
+                                  field_trial_handle);
+#endif
+    } else {
+      cmd_line->AppendSwitchASCII(switches::kForceFieldTrials,
+                                  field_trial_states);
+    }
   }
 }
 
 void BrowserChildProcessHostImpl::Launch(
     SandboxedProcessLauncherDelegate* delegate,
     base::CommandLine* cmd_line,
+    const base::SharedMemory* field_trial_state,
     bool terminate_on_shutdown) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
 
@@ -258,11 +279,7 @@ void BrowserChildProcessHostImpl::Launch(
 
   notify_child_disconnected_ = true;
   child_process_.reset(new ChildProcessLauncher(
-      delegate,
-      cmd_line,
-      data_.id,
-      this,
-      child_token_,
+      delegate, cmd_line, data_.id, this, field_trial_state, child_token_,
       base::Bind(&BrowserChildProcessHostImpl::OnMojoError,
                  weak_factory_.GetWeakPtr(),
                  base::ThreadTaskRunnerHandle::Get()),