Initial implementation for sharing field trial state (win)

content/browser/browser_child_process_host_impl.cc

 // 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.
 
 #include "content/browser/browser_child_process_host_impl.h"
 
 #include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/dump_without_crashing.h"
@@ skipping 20 matching lines @@
 #include "content/browser/profiler_message_filter.h"
 #include "content/browser/tracing/trace_message_filter.h"
 #include "content/common/child_process_host_impl.h"
 #include "content/common/child_process_messages.h"
 #include "content/common/mojo/mojo_child_connection.h"
 #include "content/public/browser/browser_child_process_host_delegate.h"
 #include "content/public/browser/browser_child_process_observer.h"
 #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"
 #include "content/public/common/result_codes.h"
 #include "ipc/attachment_broker.h"
 #include "ipc/attachment_broker_privileged.h"
 #include "mojo/edk/embedder/embedder.h"
 
 #if defined(OS_MACOSX)
 #include "content/browser/mach_broker_mac.h"
@@ skipping 149 matching lines @@
   // Make a copy since the BrowserChildProcessHost dtor mutates the original
   // list.
   BrowserChildProcessList copy = g_child_process_list.Get();
   for (BrowserChildProcessList::iterator it = copy.begin();
        it != copy.end(); ++it) {
     delete (*it)->delegate();  // ~*HostDelegate deletes *HostImpl.
   }
 }
 
 // static
-void BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
+std::unique_ptr<base::SharedMemory>
+BrowserChildProcessHostImpl::CopyFeatureAndFieldTrialFlags(
     base::CommandLine* cmd_line) {
   std::string enabled_features;
   std::string disabled_features;
   base::FeatureList::GetInstance()->GetFeatureOverrides(&enabled_features,
                                                         &disabled_features);
   if (!enabled_features.empty())
     cmd_line->AppendSwitchASCII(switches::kEnableFeatures, enabled_features);
   if (!disabled_features.empty())
     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);
 
   GetContentClient()->browser()->AppendExtraCommandLineSwitches(
       cmd_line, data_.id);
 
   const base::CommandLine& browser_command_line =
       *base::CommandLine::ForCurrentProcess();
   static const char* const kForwardSwitches[] = {
     switches::kDisableLogging,
     switches::kEnableLogging,
     switches::kIPCConnectionTimeout,
     switches::kLoggingLevel,
     switches::kTraceToConsole,
     switches::kV,
     switches::kVModule,
   };
   cmd_line->CopySwitchesFrom(browser_command_line, kForwardSwitches,
                              arraysize(kForwardSwitches));
 
   if (child_connection_) {
     cmd_line->AppendSwitchASCII(switches::kMojoApplicationChannelToken,
                                 child_connection_->service_token());
   }
 
   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()),
       terminate_on_shutdown));
 }
 
 const ChildProcessData& BrowserChildProcessHostImpl::GetData() const {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   return data_;
 }
@@ skipping 326 matching lines @@
 
 #if defined(OS_WIN)
 
 void BrowserChildProcessHostImpl::OnObjectSignaled(HANDLE object) {
   OnChildDisconnected();
 }
 
 #endif
 
 }  // namespace content