Introduce a new framework for back-and-forth tracked/protected preferences migration.

chrome/browser/prefs/tracked/tracked_preferences_migration.cc

Index: chrome/browser/prefs/tracked/tracked_preferences_migration.cc
diff --git a/chrome/browser/prefs/tracked/tracked_preferences_migration.cc b/chrome/browser/prefs/tracked/tracked_preferences_migration.cc
new file mode 100644
index 0000000000000000000000000000000000000000..552a089712ed1eb473f6ff8ab4cfc0ecb74d85aa
--- /dev/null
+++ b/chrome/browser/prefs/tracked/tracked_preferences_migration.cc
@@ -0,0 +1,239 @@
+// Copyright 2014 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 "chrome/browser/prefs/tracked/tracked_preferences_migration.h"
+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/prefs/json_pref_store.h"
+#include "base/values.h"
+
+namespace {
+
+class TrackedPreferencesMigrator
+    : public base::RefCounted<TrackedPreferencesMigrator> {
+ public:
+  // Constructs a TrackedPreferencesMigrator which sets itself up as the
+  // OnFileReadInterceptor for both |unprotected_pref_store| and
+  // |protected_pref_store|. It guarantees that the most recent pref value
+  // for |unprotected_pref_names| and |protected_pref_names| are in the
+  // appropriate JsonPrefStores before those stores even sees those values.
+  TrackedPreferencesMigrator(
+      const std::set<std::string>& unprotected_pref_names,
+      const std::set<std::string>& protected_pref_names,
+      JsonPrefStore* unprotected_pref_store,
+      JsonPrefStore* protected_pref_store);
+
+ private:
+  friend class base::RefCounted<TrackedPreferencesMigrator>;
+
+  enum PrefStoreID {
+    UNPROTECTED_PREFERENCE_STORE,
+    PROTECTED_PREFERENCE_STORE
+  };
+
+  ~TrackedPreferencesMigrator();
+
+  // Stores the data coming in for the store identified by |id| into this class
+  // and then calls MigrateIfReady();
+  void OnFileRead(
+      PrefStoreID id,
+      scoped_ptr<base::DictionaryValue> prefs,
+      const JsonPrefStore::FinalizePrefsReadCallback& finalize_prefs_read);
+
+  // Proceeds with migration if both |unprotected_prefs_| and |protected_prefs_|
+  // have been set.
+  void MigrateIfReady();
+
+  const std::set<std::string> unprotected_pref_names_;
+  const std::set<std::string> protected_pref_names_;
+
+  scoped_ptr<base::DictionaryValue> unprotected_prefs_;
+  scoped_ptr<base::DictionaryValue> protected_prefs_;
+
+  JsonPrefStore::FinalizePrefsReadCallback finalize_unprotected_prefs_read_;
+  JsonPrefStore::FinalizePrefsReadCallback finalize_protected_prefs_read_;
+
+  DISALLOW_COPY_AND_ASSIGN(TrackedPreferencesMigrator);
+};
+
+// Silently removes all |keys_to_clean| from |pref_store| if the weak pointer is
+// still valid.
+void CleanupPrefStore(const base::WeakPtr<JsonPrefStore>& pref_store,
+                      const std::set<std::string>& keys_to_clean) {
+  if (pref_store) {
+    for (std::set<std::string>::const_iterator it = keys_to_clean.begin();
+         it != keys_to_clean.end(); ++it) {
+      // Silently clean the values as they have already been copied to their
+      // new authoritative store and we don't want to alert PrefStore observers
+      // about the irrelevant removal of this value which no one should be
+      // reading anymore.
+      pref_store->RemoveValueSilently(*it);
+    }
+  }
+}
+
+// If |wait_for_commit_to_destination_store|: schedules a cleanup of the
+// |keys_to_clean| from the |source_pref_store| once the
+// |destination_pref_store| they were migrated to was successfully written to
+// disk. Otherwise, executes the cleanup right away.
+void ScheduleSourcePrefStoreCleanup(
+    const base::WeakPtr<JsonPrefStore>& source_pref_store,
+    const std::set<std::string>& keys_to_clean,
+    bool wait_for_commit_to_destination_store,
+    JsonPrefStore* destination_pref_store) {
+  DCHECK(source_pref_store);
+  DCHECK(destination_pref_store);
+  DCHECK(!keys_to_clean.empty());
+  if (wait_for_commit_to_destination_store) {
+    destination_pref_store->RegisterOnNextSuccessfulWriteCallback(
+        base::Bind(&CleanupPrefStore, source_pref_store, keys_to_clean));
+  } else {
+    CleanupPrefStore(source_pref_store, keys_to_clean);
+  }
+}
+
+// Copies the value of each pref in |pref_names| which is set in |old_store|,
+// but not in |new_store| into |new_store|. Sets |old_store_needs_cleanup| to
+// true if any old duplicates remain in |old_store| and sets |new_store_altered|
+// to true if any value was copied to |new_store|.
+void MigratePrefsFromOldToNewStore(
+    const std::set<std::string>& pref_names,
+    const base::DictionaryValue* old_store,
+    base::DictionaryValue* new_store,
+    bool* old_store_needs_cleanup,
+    bool* new_store_altered) {
+  for (std::set<std::string>::const_iterator it = pref_names.begin();
+       it != pref_names.end(); ++it) {
+    const std::string& pref_name = *it;
+
+    const base::Value* value_in_old_store = NULL;
+    if (!old_store->Get(pref_name, &value_in_old_store))
+      continue;
+
+    // Whether this value ends up being copied below or was left behind by a
+    // previous incomplete migration, it should be cleaned up.
+    *old_store_needs_cleanup = true;
+
+    if (new_store->Get(pref_name, NULL))
+      continue;
+
+    // Copy the value from |old_store| to |new_store| rather than moving it to
+    // avoid data loss should |old_store| be flushed to disk without |new_store|
+    // having equivalently been successfully flushed to disk (e.g., on crash or
+    // in cases where |new_store| is read-only following a read error on
+    // startup).
+    new_store->Set(pref_name, value_in_old_store->DeepCopy());
+    *new_store_altered = true;
+  }
+}
+
+TrackedPreferencesMigrator::TrackedPreferencesMigrator(
+    const std::set<std::string>& unprotected_pref_names,
+    const std::set<std::string>& protected_pref_names,
+    JsonPrefStore* unprotected_pref_store,
+    JsonPrefStore* protected_pref_store)
+    : unprotected_pref_names_(unprotected_pref_names),
+      protected_pref_names_(protected_pref_names) {
+  // The callbacks bound below will own this TrackedPreferencesMigrator by
+  // reference.
+  unprotected_pref_store->InterceptNextFileRead(
+      base::Bind(&TrackedPreferencesMigrator::OnFileRead,
+                 this,
+                 UNPROTECTED_PREFERENCE_STORE));
+  protected_pref_store->InterceptNextFileRead(
+      base::Bind(&TrackedPreferencesMigrator::OnFileRead,
+                 this,
+                 PROTECTED_PREFERENCE_STORE));
+}
+
+TrackedPreferencesMigrator::~TrackedPreferencesMigrator() {}
+
+void TrackedPreferencesMigrator::OnFileRead(
+    PrefStoreID id,
+    scoped_ptr<base::DictionaryValue> prefs,
+    const JsonPrefStore::FinalizePrefsReadCallback& finalize_prefs_read) {
+  switch (id) {
+    case UNPROTECTED_PREFERENCE_STORE:
+      unprotected_prefs_ = prefs.Pass();
+      finalize_unprotected_prefs_read_ = finalize_prefs_read;
+      break;
+    case PROTECTED_PREFERENCE_STORE:
+      protected_prefs_ = prefs.Pass();
+      finalize_protected_prefs_read_ = finalize_prefs_read;
+      break;
+  }
+
+  MigrateIfReady();
+}
+
+void TrackedPreferencesMigrator::MigrateIfReady() {
+  // Wait for both stores to have been read before proceeding.
+  if (!protected_prefs_ || !unprotected_prefs_)
+    return;
+
+  bool protected_prefs_need_cleanup = false;
+  bool unprotected_prefs_altered = false;
+  MigratePrefsFromOldToNewStore(unprotected_pref_names_,
+                                protected_prefs_.get(),
+                                unprotected_prefs_.get(),
+                                &protected_prefs_need_cleanup,
+                                &unprotected_prefs_altered);
+  bool unprotected_prefs_need_cleanup = false;
+  bool protected_prefs_altered = false;
+  MigratePrefsFromOldToNewStore(protected_pref_names_,
+                                unprotected_prefs_.get(),
+                                protected_prefs_.get(),
+                                &unprotected_prefs_need_cleanup,
+                                &protected_prefs_altered);
+
+  // Hand the processed prefs back to their respective JsonPrefStore and keep
+  // weak pointers to those stores to make a best effort attempt at cleaning up
+  // duplicate values once those values' destination store was successfully
+  // written to disk.
+  base::WeakPtr<JsonPrefStore> unprotected_pref_store =
+      finalize_unprotected_prefs_read_.Run(unprotected_prefs_.Pass(),
+                                           unprotected_prefs_altered);
+  base::WeakPtr<JsonPrefStore> protected_pref_store =
+      finalize_protected_prefs_read_.Run(protected_prefs_.Pass(),
+                                         protected_prefs_altered);
+  DCHECK(unprotected_pref_store);
+  DCHECK(protected_pref_store);
+
+  if (unprotected_prefs_need_cleanup) {
+    // Cleanup the |protected_pref_names_| from the |unprotected_pref_store|
+    // once they were successfully written to the |protected_pref_store|.
+    ScheduleSourcePrefStoreCleanup(unprotected_pref_store,
+                                   protected_pref_names_,
+                                   protected_prefs_altered,
+                                   protected_pref_store.get());
+  }
+
+  if (protected_prefs_need_cleanup) {
+    // Cleanup the |unprotected_pref_names_| from the |protected_pref_store|
+    // once they were successfully written to the |unprotected_pref_store|.
+    ScheduleSourcePrefStoreCleanup(protected_pref_store,
+                                   unprotected_pref_names_,
+                                   unprotected_prefs_altered,
+                                   unprotected_pref_store.get());
+  }
+}
+
+}  // namespace
+
+void SetupTrackedPreferencesMigration(
+    const std::set<std::string>& unprotected_pref_names,
+    const std::set<std::string>& protected_pref_names,
+    JsonPrefStore* unprotected_pref_store,
+    JsonPrefStore* protected_pref_store) {
+  scoped_refptr<TrackedPreferencesMigrator> prefs_migrator(
+      new TrackedPreferencesMigrator(unprotected_pref_names,
+                                     protected_pref_names,
+                                     unprotected_pref_store,
+                                     protected_pref_store));
+}