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..4601cc7e04a6f5a0e34073ba534e86d1c16351ed 100644
--- a/content/browser/browser_child_process_host_impl.cc
+++ b/content/browser/browser_child_process_host_impl.cc
@@ -38,6 +38,7 @@
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/child_process_data.h"
 #include "content/public/browser/content_browser_client.h"
+#include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/mojo_channel_switches.h"
 #include "content/public/common/process_type.h"
@@ -207,7 +208,8 @@ void BrowserChildProcessHostImpl::TerminateAll() {
 }
 
 // static
-void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
+std::unique_ptr<base::SharedMemory>
+BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
     base::CommandLine* cmd_line) {
   std::string enabled_features;
   std::string disabled_features;
@@ -219,18 +221,28 @@ void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
     cmd_line->AppendSwitchASCII(switches::kDisableFeatures, disabled_features);
 
   // If we run base::FieldTrials, we want to pass to their state to the
-  // child process so that it can act in accordance with each state.
-  std::string field_trial_states;
-  base::FieldTrialList::AllStatesToString(&field_trial_states);
-  if (!field_trial_states.empty()) {
-    cmd_line->AppendSwitchASCII(switches::kForceFieldTrials,
-                                field_trial_states);
+  // child process so that it can act in accordance with each state. Use shared
+  // memory to pass the state if the feature is enabled, otherwise fallback to
+  // passing it via the command line as a string.
+  if (base::FeatureList::IsEnabled(

[Alexei Svitkine (slow), 2016/10/06 13:45:30]

Can this whole logic live in the helper function (including the old path)? Then,
the feature can move to base too and can probably just be declared inside the
field trials .cc file and not exposed elsewhere.

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

Done.

+          features::kShareFieldTrialStateViaSharedMemory)) {
+    return base::FieldTrialList::CopyFieldTrialStateToSharedMemory(
+        cmd_line, switches::kFieldTrialHandle);
+  } else {
+    std::string field_trial_states;
+    base::FieldTrialList::AllStatesToString(&field_trial_states);
+    if (!field_trial_states.empty()) {
+      cmd_line->AppendSwitchASCII(switches::kForceFieldTrials,
+                                  field_trial_states);
+    }
+    return std::unique_ptr<base::SharedMemory>(nullptr);
   }
 }
 
 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 +270,7 @@ 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()),