Migrate users from "WebKit" flag to "Web Platform".

We changed the name of the "Experimental WebKit Features" flag, and changed
the actual flag at the same time[1]. I decided at the time to simply throw away
the old flag; I've been reliably informed that developers aren't particularly
thrilled with that decision. "Why have my experimental features stopped
working?" they ask. "Didn't you read my G+ post?" isn't a particularly helpful
answer, especially since most people (unbelievably!) don't follow me on
The Internets™.

This patch hacks a migration system into about:flags, which I hope we never
ever have to use again, and can safely throw away after a release cycle or two.

[1]: https://codereview.chromium.org/14940009/

R=jochen@chromium.org,thakis@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19520007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@212794 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 70 insertions, 2 deletions

diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 6ca3032..e239a43 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -309,6 +309,7 @@ const Experiment::Choice kTabCaptureDownscaleQualityChoices[] = {
 //   array of choices.
 // See the documentation of Experiment for details on the fields.
 //
+
 // When adding a new choice, add it to the end of the list.
 const Experiment kExperiments[] = {
   {
@@ -1636,6 +1637,15 @@ const Experiment kExperiments[] = {
 const Experiment* experiments = kExperiments;
 size_t num_experiments = arraysize(kExperiments);
 
+const Migration kMigrations[] = {
+  // TODO(mkwst): Remove this once some reasonable amount of time has passed.
+  {"enable-experimental-webkit-features",
+   "enable-experimental-web-platform-features"}
+};
+
+const Migration* migrations = kMigrations;
+size_t num_migrations = arraysize(kMigrations);
+
 // Stores and encapsulates the little state that about:flags has.
 class FlagsState {
  public:
@@ -1704,6 +1714,17 @@ bool ValidateExperiment(const Experiment& e) {
   return true;
 }
 
+// Given the set of currently active experiments, migrate deprecated experiments
+// to their shiny new counterparts.
+void MigrateExperiments(std::set<std::string>* experimentList) {
+  for (size_t i = 0; i < num_migrations; ++i) {
+    if (experimentList->count(migrations[i].from)) {
+      experimentList->erase(experimentList->find(migrations[i].from));
+      experimentList->insert(migrations[i].to);
+    }
+  }
+}
+
 // Removes all experiments from prefs::kEnabledLabsExperiments that are
 // unknown, to prevent this list to become very long as experiments are added
 // and removed.
@@ -1714,7 +1735,11 @@ void SanitizeList(FlagsStorage* flags_storage) {
     AddInternalName(experiments[i], &known_experiments);
   }
 
-  std::set<std::string> enabled_experiments = flags_storage->GetFlags();
+  // Store the original list of Experiments, then copy it for processing.
+  std::set<std::string> unsanitized_experiments = flags_storage->GetFlags();
+  std::set<std::string> enabled_experiments = unsanitized_experiments;
+
+  MigrateExperiments(&enabled_experiments);
 
   std::set<std::string> new_enabled_experiments;
   std::set_intersection(
@@ -1722,7 +1747,7 @@ void SanitizeList(FlagsStorage* flags_storage) {
       enabled_experiments.begin(), enabled_experiments.end(),
       std::inserter(new_enabled_experiments, new_enabled_experiments.begin()));
 
-  if (new_enabled_experiments != enabled_experiments)
+  if (new_enabled_experiments != unsanitized_experiments)
     flags_storage->SetFlags(new_enabled_experiments);
 }
 
@@ -2114,6 +2139,16 @@ void SetExperiments(const Experiment* e, size_t count) {
   }
 }
 
+void SetMigrations(const Migration* e, size_t count) {
+  if (!e) {
+    migrations = kMigrations;
+    num_migrations = arraysize(kMigrations);
+  } else {
+    migrations = e;
+    num_migrations = count;
+  }
+}
+
 const Experiment* GetExperiments(size_t* count) {
   *count = num_experiments;
   return experiments;
diff --git a/chrome/browser/about_flags.h b/chrome/browser/about_flags.h
index 5d67e97..c7fdc53 100644
--- a/chrome/browser/about_flags.h
+++ b/chrome/browser/about_flags.h
@@ -105,6 +105,13 @@ struct Experiment {
   string16 DescriptionForChoice(int index) const;
 };
 
+// Migration is used internally by about_flags to move users from old,
+// busted flags to the new hotness. This is exposed only for testing.
+struct Migration {
+  const char* const from;
+  const char* const to;
+};
+
 // Reads the Labs |prefs| (called "Labs" for historical reasons) and adds the
 // commandline flags belonging to the active experiments to |command_line|.
 void ConvertFlagsToSwitches(FlagsStorage* flags_storage,
@@ -163,6 +170,10 @@ void ClearState();
 // NOT take ownership of the supplied Experiments.
 void SetExperiments(const Experiment* e, size_t count);
 
+// Sets the list of migrations. Pass in NULL to use the default set. This does
+// NOT take ownership of the supplied Migrations.
+void SetMigrations(const Migration* e, size_t count);
+
 // Returns the current set of experiments.
 const Experiment* GetExperiments(size_t* count);
 
diff --git a/chrome/browser/about_flags_unittest.cc b/chrome/browser/about_flags_unittest.cc
index 34219c3..3d9fbd2 100644
--- a/chrome/browser/about_flags_unittest.cc
+++ b/chrome/browser/about_flags_unittest.cc
@@ -110,6 +110,13 @@ static Experiment kExperiments[] = {
   },
 };
 
+static Migration kMigrations[] = {
+  {
+    kFlags1,
+    kFlags2
+  }
+};
+
 class AboutFlagsTest : public ::testing::Test {
  protected:
   AboutFlagsTest() : flags_storage_(&prefs_) {
@@ -223,6 +230,21 @@ TEST_F(AboutFlagsTest, ConvertFlagsToSwitches) {
   EXPECT_TRUE(command_line.HasSwitch(kSwitch1));
 }
 
+TEST_F(AboutFlagsTest, ConvertFlagsToSwitchesMigration) {
+  testing::SetMigrations(kMigrations, arraysize(kMigrations));
+  SetExperimentEnabled(&flags_storage_,kFlags1, true);
+
+  CommandLine command_line(CommandLine::NO_PROGRAM);
+
+  EXPECT_FALSE(command_line.HasSwitch(kSwitch1));
+  EXPECT_FALSE(command_line.HasSwitch(kSwitch2));
+
+  ConvertFlagsToSwitches(&flags_storage_, &command_line);
+
+  EXPECT_FALSE(command_line.HasSwitch(kSwitch1));
+  EXPECT_TRUE(command_line.HasSwitch(kSwitch2));
+}
+
 TEST_F(AboutFlagsTest, CompareSwitchesToCurrentCommandLine) {
   SetExperimentEnabled(&flags_storage_, kFlags1, true);