Make a channel and time limited persistent PreRead experiment

chrome/app/client_util.cc

Index: chrome/app/client_util.cc
===================================================================
--- chrome/app/client_util.cc	(revision 98219)
+++ chrome/app/client_util.cc	(working copy)
@@ -11,6 +11,7 @@
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/rand_util.h"  // For PreRead experiment.
 #include "base/string_util.h"
 #include "base/utf_string_conversions.h"
 #include "base/version.h"
@@ -83,10 +84,51 @@
   return true;
 }
 
+#if defined(OS_WIN)
+// These constants are used by the PreRead experiment.
+const wchar_t kPreReadRegistryValue[] = L"PreReadExperimentGroup";
+const int kPreReadExpiryYear = 2012;
+const int kPreReadExpiryMonth = 1;
+const int kPreReadExpiryDay = 1;
+
+bool PreReadShouldRun() {
+  base::Time::Exploded exploded = { 0 };
+  exploded.year = kPreReadExpiryYear;
+  exploded.month = kPreReadExpiryMonth;
+  exploded.day_of_month = kPreReadExpiryDay;
+
+  base::Time expiration_time = base::Time::FromLocalExploded(exploded);
+
+  // Get the build time. This is copied from base::FieldTrial::GetBuildTime.

[Sigurður Ásgeirsson, 2011/08/25 17:44:14]

MetricsLogBase::GetBuildTime() is a public static member, can you just invoke it
from here?

[chrisha, 2011/08/25 19:39:17]

Unfortunately this is in seconds-since-unix-epoch, which base::Time can't use.
And FieldTrial::GetBuildTime is private.

+  base::Time build_time;
+  const char* kDateTime = __DATE__ " " __TIME__;
+  bool result = base::Time::FromString(kDateTime, &build_time);
+  DCHECK(result);
+
+  // If the experiment is expired, don't run it.
+  if (build_time > expiration_time)
+    return false;
+
+#if defined(GOOGLE_CHROME_BUILD)
+  // The experiment should only run on canary and dev.
+  BrowserDistribution* dist = BrowserDistribution::GetDistribution();
+  std::wstring channel;
+  if (!dist->GetChromeChannel(&channel))
+    return false;
+  return channel == L"canary" || channel == L"dev";

[Sigurður Ásgeirsson, 2011/08/25 17:44:14]

looks like the canary channel name is "canary build", but can also be referred
to by "GoogleChromeSxSDistribution::ChannelName()". See 
http://codesearch.google.com/#h0RrPvyPu-c/chrome/app/breakpad_win.cc&exact_pa...

[chrisha, 2011/08/25 19:39:17]

Looking at that file, the channel name is actually "canary", but it's definitely
smarter to refer to it directly via GoogleChromeSxSDistribution.

Done.

+#else
+  // If we're running Chromium, we have no concept of channel so we
+  // run the experiment.
+  return true;
+#endif
+}
+#endif
+
 // Expects that |dir| has a trailing backslash. |dir| is modified so it
 // contains the full path that was tried. Caller must check for the return
 // value not being null to determine if this path contains a valid dll.
-HMODULE LoadChromeWithDirectory(std::wstring* dir) {
+HMODULE LoadChromeWithDirectory(const std::wstring& reg_key,  // For PreRead.
+                                std::wstring* dir) {
   ::SetCurrentDirectoryW(dir->c_str());
   const CommandLine& cmd_line = *CommandLine::ForCurrentProcess();
 #ifdef _WIN64
@@ -138,6 +180,57 @@
       key.Close();
     }
 
+    // The PreRead experiment is unable to use the standard FieldTrial
+    // mechanism as PreReading happens in chrome.exe prior to loading

[Sigurður Ásgeirsson, 2011/08/25 17:44:14]

PreReading -> pre-reading?

Good comment.

[chrisha, 2011/08/25 19:39:17]

Done.

+    // chrome.dll. As such, we use a custom approach. If the experiment is
+    // running (not expired, and we're running a version of chrome from an
+    // appropriate channel) then we look to the registry for the experiment
+    // group. If it is not found, we toss a coin and persist the result in the
+    // registry. The coin-toss value is communicated to chrome.dll via an
+    // environment variable, which indicates to chrome.dll that the experiment
+    // is running, causing it to report sub-histogram results. If the experiment
+    // is expired or not running, the registry key is removed.
+
+    base::win::RegKey exp_key(
+        HKEY_CURRENT_USER, reg_key.c_str(),
+        KEY_CREATE_SUB_KEY | KEY_QUERY_VALUE | KEY_SET_VALUE);
+
+    if (exp_key.Valid()) {
+      if (PreReadShouldRun()) {

[Sigurður Ásgeirsson, 2011/08/25 17:44:14]

nit: the code would look a little cleaner with this in a utility function, as
then you can do early return on failure branches.

[chrisha, 2011/08/25 19:39:17]

Done.

+        bool exp_is_live = false;
+        DWORD coin_toss = 0;
+
+        // Coin already tossed?
+        if (exp_key.ReadValueDW(kPreReadRegistryValue, &coin_toss) ==
+            ERROR_SUCCESS) {
+          exp_is_live = true;
+        } else {
+          // Coin not tossed? Toss it, and save the value to the registry.
+          coin_toss = base::RandInt(0, 1);
+          DCHECK(coin_toss == 0 || coin_toss == 1);
+          if (exp_key.WriteValue(kPreReadRegistryValue, coin_toss) ==
+              ERROR_SUCCESS) {
+            exp_is_live = true;
+          }
+        }
+
+        // If the experiment is running, indicate it to chrome.dll via an
+        // environment variable.
+        if (exp_is_live) {
+          pre_read = coin_toss;
+          scoped_ptr<base::Environment> env(base::Environment::Create());
+          env->SetVar(chrome::kPreReadEnvironmentVariable,
+                      pre_read ? "1" : "0");
+        }
+      } else {
+        // The experiment is no longer running. Remove the registry key
+        // storing the coin toss.
+        exp_key.DeleteValue(kPreReadRegistryValue);
+      }
+
+      exp_key.Close();
+    }
+
     if (pre_read) {
       TRACE_EVENT_BEGIN_ETW("PreReadImage", 0, "");
       file_util::PreReadImage(dir->c_str(), pre_read_size, pre_read_step_size);
@@ -161,7 +254,7 @@
   GoogleUpdateSettings::UpdateDidRunState(false, system_level);
 }
 
-}
+}  // namespace
 //=============================================================================
 
 MainDllLoader::MainDllLoader() : dll_(NULL) {
@@ -183,7 +276,8 @@
 HMODULE MainDllLoader::Load(std::wstring* out_version, std::wstring* out_file) {
   std::wstring dir(GetExecutablePath());
   *out_file = dir;
-  HMODULE dll = LoadChromeWithDirectory(out_file);
+  // 'GetRegistryPath()' was added in support of PreRead experiment.

[Sigurður Ásgeirsson, 2011/08/25 17:44:14]

I don't think you particularly need to explain this.

[chrisha, 2011/08/25 19:39:17]

Done.

+  HMODULE dll = LoadChromeWithDirectory(GetRegistryPath(), out_file);
   if (dll)
     return dll;
 
@@ -222,7 +316,8 @@
     *out_file = dir;
     *out_version = version_string;
     out_file->append(*out_version).append(L"\\");
-    dll = LoadChromeWithDirectory(out_file);
+    // 'GetRegistryPath()' was added in support of PreRead experiment.
+    dll = LoadChromeWithDirectory(GetRegistryPath(), out_file);
     if (!dll) {
       LOG(ERROR) << "Failed to load Chrome DLL from " << out_file;
     }