Make SDCH dictionaries persistent across browser restart.

net/sdch/sdch_owner.cc

 // 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 "net/sdch/sdch_owner.h"
 
 #include "base/bind.h"
+#include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/prefs/persistent_pref_store.h"
+#include "base/prefs/value_map_pref_store.h"
 #include "base/time/default_clock.h"
+#include "base/values.h"
 #include "net/base/sdch_manager.h"
 #include "net/base/sdch_net_log_params.h"
 
+namespace net {
+
 namespace {
 
 enum DictionaryFate {
   // A Get-Dictionary header wasn't acted on.
   DICTIONARY_FATE_GET_IGNORED = 1,
 
   // A fetch was attempted, but failed.
   // TODO(rdsmith): Actually record this case.
   DICTIONARY_FATE_FETCH_FAILED = 2,
 
   // A successful fetch was dropped on the floor, no space.
   DICTIONARY_FATE_FETCH_IGNORED_NO_SPACE = 3,
 
   // A successful fetch was refused by the SdchManager.
   DICTIONARY_FATE_FETCH_MANAGER_REFUSED = 4,
 
-  // A dictionary was successfully added.
-  DICTIONARY_FATE_ADDED = 5,
+  // A dictionary was successfully added based on
+  // a Get-Dictionary header in a response.
+  DICTIONARY_FATE_ADD_RESPONSE_TRIGGERED = 5,
 
   // A dictionary was evicted by an incoming dict.
   DICTIONARY_FATE_EVICT_FOR_DICT = 6,
 
   // A dictionary was evicted by memory pressure.
   DICTIONARY_FATE_EVICT_FOR_MEMORY = 7,
 
   // A dictionary was evicted on destruction.
   DICTIONARY_FATE_EVICT_FOR_DESTRUCTION = 8,
 
-  DICTIONARY_FATE_MAX = 9
+  // A dictionary was successfully added based on
+  // persistence from a previous browser revision.
+  DICTIONARY_FATE_ADD_PERSISTENCE_TRIGGERED = 9,
+
+  DICTIONARY_FATE_MAX = 10
 };
 
+enum PersistenceFailureReason {
+  // File didn't exist; is being created.
+  PERSISTENCE_FAILURE_REASON_NO_FILE = 1,
+
+  // Error reading in information, but should be able to write.
+  PERSISTENCE_FAILURE_REASON_READ_FAILED = 2,
+
+  // Error leading to abort on attempted persistence.
+  PERSISTENCE_FAILURE_REASON_WRITE_FAILED = 3,
+
+  PERSISTENCE_FAILURE_REASON_MAX = 4
+};
+
+// Dictionaries that haven't been touched in 24 hours may be evicted
+// to make room for new dictionaries.
+const int kFreshnessLifetimeHours = 24;
+
+// Dictionaries that have never been used only stay fresh for one hour.
+const int kNeverUsedFreshnessLifetimeHours = 1;
+
 void RecordDictionaryFate(enum DictionaryFate fate) {
   UMA_HISTOGRAM_ENUMERATION("Sdch3.DictionaryFate", fate, DICTIONARY_FATE_MAX);
 }
 
+void RecordPersistenceFailure(PersistenceFailureReason failure_reason) {
+  UMA_HISTOGRAM_ENUMERATION("Sdch3.PersistenceFailureReason", failure_reason,
+                            PERSISTENCE_FAILURE_REASON_MAX);
+}
+
 void RecordDictionaryEviction(int use_count, DictionaryFate fate) {
   DCHECK(fate == DICTIONARY_FATE_EVICT_FOR_DICT ||
          fate == DICTIONARY_FATE_EVICT_FOR_MEMORY ||
          fate == DICTIONARY_FATE_EVICT_FOR_DESTRUCTION);
 
   UMA_HISTOGRAM_COUNTS_100("Sdch3.DictionaryUseCount", use_count);
   RecordDictionaryFate(fate);
 }
 
+// Schema specifications and access routines.
+
+// The persistent prefs store is conceptually shared with any other network
+// stack systems that want to persist data over browser restarts, and so
+// use of it must be namespace restricted.
+// Schema:
+//      pref_store_->GetValue(kPreferenceName) -> Dictionary {
+//          'version' -> 1 [int]
+//          'dictionaries' -> Dictionary {
+//              server_hash -> {
+//                  'url' -> URL [string]
+//                  'last_used' -> seconds since unix epoch [double]
+//                  'use_count' -> use count [int]
+//                  'size' -> size [int]
+//          }
+//      }
+const char kPreferenceName[] = "SDCH";
+const char kVersionKey[] = "version";
+const char kDictionariesKey[] = "dictionaries";
+const char kDictionaryUrlKey[] = "url";
+const char kDictionaryLastUsedKey[] = "last_used";
+const char kDictionaryUseCountKey[] = "use_count";
+const char kDictionarySizeKey[] = "size";
+
+const int kVersion = 1;
+
+// This function returns store[kPreferenceName/kDictionariesKey].  The caller
+// is responsible for making sure any needed calls to
+// |store->ReportValueChanged()| occur.
+base::DictionaryValue* GetPersistentStoreDictionaryMap(
+    WriteablePrefStore* store) {
+  base::Value* result = nullptr;
+  bool success = store->GetMutableValue(kPreferenceName, &result);
+  DCHECK(success);
+
+  base::DictionaryValue* preference_dictionary = nullptr;
+  success = result->GetAsDictionary(&preference_dictionary);
+  DCHECK(success);
+  DCHECK(preference_dictionary);
+
+  base::DictionaryValue* dictionary_list_dictionary = nullptr;
+  success = preference_dictionary->GetDictionary(kDictionariesKey,
+                                                 &dictionary_list_dictionary);
+  DCHECK(success);
+  DCHECK(dictionary_list_dictionary);
+
+  return dictionary_list_dictionary;
+}
+
+// This function initializes a pref store with an empty version of the
+// above schema, removing anything previously in the store under
+// kPreferenceName.
+void InitializePersistentStore(WriteablePrefStore* store) {
+  base::DictionaryValue* empty_store(new base::DictionaryValue);
+  empty_store->SetInteger(kVersionKey, kVersion);
+  empty_store->Set(kDictionariesKey,
+                   make_scoped_ptr(new base::DictionaryValue));
+  store->SetValue(kPreferenceName, empty_store);
+}
+
+// A class to allow iteration over all dictionaries in the pref store, and
+// easy lookup of the information associated with those dictionaries.
+// Note that this is an "Iterator" in the same sense (and for the same
+// reasons) that base::Dictionary::Iterator is an iterator--it allows
+// iterating over all the dictionaries in the preference store, but it
+// does not allow use as an STL iterator because the container it
+// is iterating over does not export begin()/end() methods.
+class DictionaryPreferenceIterator {
+ public:
+  explicit DictionaryPreferenceIterator(WriteablePrefStore* pref_store);
+
+  bool IsAtEnd() const { return dictionary_iterator_.IsAtEnd(); }
+  void Advance();
+
+  const std::string& server_hash() const { return server_hash_; }
+  GURL url() const { return url_; }
+  base::Time last_used() const { return last_used_; }
+  int use_count() const { return use_count_; }
+  int size() const { return size_; }
+
+ private:
+  void LoadDictionaryOrDie();
+
+  std::string server_hash_;
+  GURL url_;
+  base::Time last_used_;
+  int use_count_;
+  int size_;
+
+  base::DictionaryValue::Iterator dictionary_iterator_;
+};
+
+DictionaryPreferenceIterator::DictionaryPreferenceIterator(
+    WriteablePrefStore* pref_store)
+    : dictionary_iterator_(*GetPersistentStoreDictionaryMap(pref_store)) {
+  if (!IsAtEnd())
+    LoadDictionaryOrDie();
+}
+
+void DictionaryPreferenceIterator::Advance() {
+  dictionary_iterator_.Advance();
+  if (!IsAtEnd())
+    LoadDictionaryOrDie();
+}
+
+void DictionaryPreferenceIterator::LoadDictionaryOrDie() {
+  double last_used_seconds_from_epoch;
+  const base::DictionaryValue* dict = nullptr;
+  bool success =
+      dictionary_iterator_.value().GetAsDictionary(&dict);
+  DCHECK(success);
+
+  server_hash_ = dictionary_iterator_.key();
+
+  std::string url_spec;
+  success = dict->GetString(kDictionaryUrlKey, &url_spec);
+  DCHECK(success);
+  url_ = GURL(url_spec);
+
+  success = dict->GetDouble(kDictionaryLastUsedKey,
+                                     &last_used_seconds_from_epoch);
+  DCHECK(success);
+  last_used_ = base::Time::FromDoubleT(last_used_seconds_from_epoch);
+
+  success = dict->GetInteger(kDictionaryUseCountKey, &use_count_);
+  DCHECK(success);
+
+  success = dict->GetInteger(kDictionarySizeKey, &size_);
+  DCHECK(success);
+}
+
+// Triggers a ReportValueChanged() on the specified WriteablePrefStore
+// when the object goes out of scope.
+class ScopedPrefNotifier {
+ public:
+  // Caller must guarantee lifetime of |*pref_store| exceeds the
+  // lifetime of this object.
+  ScopedPrefNotifier(WriteablePrefStore* pref_store)
+      : pref_store_(pref_store) {}
+  ~ScopedPrefNotifier() { pref_store_->ReportValueChanged(kPreferenceName); }
+
+ private:
+  WriteablePrefStore* pref_store_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedPrefNotifier);
+};
+
 }  // namespace
 
-namespace net {
-
 // Adjust SDCH limits downwards for mobile.
 #if defined(OS_ANDROID) || defined(OS_IOS)
 // static
 const size_t SdchOwner::kMaxTotalDictionarySize = 1000 * 1000;
 #else
 // static
 const size_t SdchOwner::kMaxTotalDictionarySize = 20 * 1000 * 1000;
 #endif
 
 // Somewhat arbitrary, but we assume a dictionary smaller than
 // 50K isn't going to do anyone any good.  Note that this still doesn't
 // prevent download and addition unless there is less than this
 // amount of space available in storage.
 const size_t SdchOwner::kMinSpaceForDictionaryFetch = 50 * 1000;
 
 SdchOwner::SdchOwner(net::SdchManager* sdch_manager,
                      net::URLRequestContext* context)
     : manager_(sdch_manager),
-      fetcher_(context,
-               base::Bind(&SdchOwner::OnDictionaryFetched,
-                          // Because |fetcher_| is owned by SdchOwner, the
-                          // SdchOwner object will be available for the lifetime
-                          // of |fetcher_|.
-                          base::Unretained(this))),
+      fetcher_(context),
       total_dictionary_bytes_(0),
       clock_(new base::DefaultClock),
       max_total_dictionary_size_(kMaxTotalDictionarySize),
       min_space_for_dictionary_fetch_(kMinSpaceForDictionaryFetch),
+      in_memory_pref_store_(new ValueMapPrefStore()),
+      external_pref_store_(nullptr),
+      pref_store_(in_memory_pref_store_.get()),
       memory_pressure_listener_(
           base::Bind(&SdchOwner::OnMemoryPressure,
                      // Because |memory_pressure_listener_| is owned by
                      // SdchOwner, the SdchOwner object will be available
                      // for the lifetime of |memory_pressure_listener_|.
                      base::Unretained(this))) {
   manager_->AddObserver(this);
+  InitializePersistentStore(pref_store_);
 }
 
 SdchOwner::~SdchOwner() {
-  for (auto it = local_dictionary_info_.begin();
-       it != local_dictionary_info_.end(); ++it) {
-    RecordDictionaryEviction(it->second.use_count,
+  for (DictionaryPreferenceIterator it(pref_store_); !it.IsAtEnd();
+       it.Advance()) {
+    RecordDictionaryEviction(it.use_count(),
                              DICTIONARY_FATE_EVICT_FOR_DESTRUCTION);
   }
   manager_->RemoveObserver(this);
+
+  // This object only observes the external store during loading,
+  // i.e. before it's made the default preferences store.
+  if (external_pref_store_)
+    external_pref_store_->RemoveObserver(this);
+}
+
+void SdchOwner::EnablePersistentStorage(PersistentPrefStore* pref_store) {
+  DCHECK(!external_pref_store_);
+  external_pref_store_ = pref_store;
+  external_pref_store_->AddObserver(this);
+
+  if (external_pref_store_->IsInitializationComplete())
+    OnInitializationCompleted(true);
 }
 
 void SdchOwner::SetMaxTotalDictionarySize(size_t max_total_dictionary_size) {
   max_total_dictionary_size_ = max_total_dictionary_size;
 }
 
 void SdchOwner::SetMinSpaceForDictionaryFetch(
     size_t min_space_for_dictionary_fetch) {
   min_space_for_dictionary_fetch_ = min_space_for_dictionary_fetch;
 }
 
-void SdchOwner::OnDictionaryFetched(const std::string& dictionary_text,
+void SdchOwner::OnDictionaryFetched(base::Time last_used,
+                                    int use_count,
+                                    const std::string& dictionary_text,
                                     const GURL& dictionary_url,
                                     const net::BoundNetLog& net_log) {
   struct DictionaryItem {
     base::Time last_used;
     std::string server_hash;
     int use_count;
     size_t dictionary_size;
 
     DictionaryItem() : use_count(0), dictionary_size(0) {}
     DictionaryItem(const base::Time& last_used,
                    const std::string& server_hash,
                    int use_count,
                    size_t dictionary_size)
         : last_used(last_used),
           server_hash(server_hash),
           use_count(use_count),
           dictionary_size(dictionary_size) {}
     DictionaryItem(const DictionaryItem& rhs) = default;
     DictionaryItem& operator=(const DictionaryItem& rhs) = default;
     bool operator<(const DictionaryItem& rhs) const {
       return last_used < rhs.last_used;
     }
   };
 
+  // Figure out if there is space for the incoming dictionary; evict
+  // stale dictionaries if needed to make space.
+
   std::vector<DictionaryItem> stale_dictionary_list;
   size_t recoverable_bytes = 0;
-  base::Time stale_boundary(clock_->Now() - base::TimeDelta::FromDays(1));
-  for (auto used_it = local_dictionary_info_.begin();
-       used_it != local_dictionary_info_.end(); ++used_it) {
-    if (used_it->second.last_used < stale_boundary) {
-      stale_dictionary_list.push_back(
-          DictionaryItem(used_it->second.last_used, used_it->first,
-                         used_it->second.use_count, used_it->second.size));
-      recoverable_bytes += used_it->second.size;
+  base::Time now(clock_->Now());
+  base::Time stale_boundary(
+      now - base::TimeDelta::FromHours(kFreshnessLifetimeHours));
+  base::Time never_used_stale_boundary(
+      now - base::TimeDelta::FromHours(kNeverUsedFreshnessLifetimeHours));
+  for (DictionaryPreferenceIterator it(pref_store_); !it.IsAtEnd();
+       it.Advance()) {
+    if (it.last_used() < stale_boundary ||
+        (it.use_count() == 0 && it.last_used() < never_used_stale_boundary)) {
+      stale_dictionary_list.push_back(DictionaryItem(
+          it.last_used(), it.server_hash(), it.use_count(), it.size()));
+      recoverable_bytes += it.size();
     }
   }
 
   if (total_dictionary_bytes_ + dictionary_text.size() - recoverable_bytes >
       max_total_dictionary_size_) {
     RecordDictionaryFate(DICTIONARY_FATE_FETCH_IGNORED_NO_SPACE);
     net::SdchManager::SdchErrorRecovery(SDCH_DICTIONARY_NO_ROOM);
     net_log.AddEvent(net::NetLog::TYPE_SDCH_DICTIONARY_ERROR,
                      base::Bind(&net::NetLogSdchDictionaryFetchProblemCallback,
                                 SDCH_DICTIONARY_NO_ROOM, dictionary_url, true));
     return;
   }
 
-  // Evict from oldest to youngest until we have space.
-  std::sort(stale_dictionary_list.begin(), stale_dictionary_list.end());
-  size_t avail_bytes = max_total_dictionary_size_ - total_dictionary_bytes_;
-  auto stale_it = stale_dictionary_list.begin();
-  while (avail_bytes < dictionary_text.size() &&
-         stale_it != stale_dictionary_list.end()) {
-    manager_->RemoveSdchDictionary(stale_it->server_hash);
-    RecordDictionaryEviction(stale_it->use_count,
-                             DICTIONARY_FATE_EVICT_FOR_DICT);
-    local_dictionary_info_.erase(stale_it->server_hash);
-    avail_bytes += stale_it->dictionary_size;
-    ++stale_it;
-  }
-  DCHECK(avail_bytes >= dictionary_text.size());
-
+  // Add the new dictionary.  This is done before removing the stale
+  // dictionaries so that no state change will occur if dictionary addition
+  // fails.
   std::string server_hash;
   net::SdchProblemCode rv = manager_->AddSdchDictionary(
       dictionary_text, dictionary_url, &server_hash);
   if (rv != net::SDCH_OK) {
     RecordDictionaryFate(DICTIONARY_FATE_FETCH_MANAGER_REFUSED);
     net::SdchManager::SdchErrorRecovery(rv);
     net_log.AddEvent(net::NetLog::TYPE_SDCH_DICTIONARY_ERROR,
                      base::Bind(&net::NetLogSdchDictionaryFetchProblemCallback,
                                 rv, dictionary_url, true));
     return;
   }
 
-  RecordDictionaryFate(DICTIONARY_FATE_ADDED);
+  base::DictionaryValue* pref_dictionary_map =
+      GetPersistentStoreDictionaryMap(pref_store_);
+  ScopedPrefNotifier scoped_pref_notifier(pref_store_);
 
-  DCHECK(local_dictionary_info_.end() ==
-         local_dictionary_info_.find(server_hash));
+  // Remove the old dictionaries.
+  std::sort(stale_dictionary_list.begin(), stale_dictionary_list.end());
+  size_t avail_bytes = max_total_dictionary_size_ - total_dictionary_bytes_;
+  auto stale_it = stale_dictionary_list.begin();
+  while (avail_bytes < dictionary_text.size() &&
+         stale_it != stale_dictionary_list.end()) {
+    manager_->RemoveSdchDictionary(stale_it->server_hash);
+
+    DCHECK(pref_dictionary_map->HasKey(stale_it->server_hash));
+    bool success = pref_dictionary_map->RemoveWithoutPathExpansion(
+        stale_it->server_hash, nullptr);
+    DCHECK(success);
+
+    avail_bytes += stale_it->dictionary_size;
+
+    RecordDictionaryEviction(stale_it->use_count,
+                             DICTIONARY_FATE_EVICT_FOR_DICT);
+
+    ++stale_it;
+  }
+  DCHECK_GE(avail_bytes, dictionary_text.size());
+
+  RecordDictionaryFate(
+      // Distinguish between loads triggered by network responses and
+      // loads triggered by persistence.
+      last_used.is_null() ? DICTIONARY_FATE_ADD_RESPONSE_TRIGGERED
+                          : DICTIONARY_FATE_ADD_PERSISTENCE_TRIGGERED);
+
+  // If a dictionary has never been used, its dictionary addition time
+  // is recorded as its last used time.  Never used dictionaries are treated
+  // specially in the freshness logic.
+  if (last_used.is_null())
+    last_used = clock_->Now();
+
   total_dictionary_bytes_ += dictionary_text.size();
-  local_dictionary_info_[server_hash] = DictionaryInfo(
-      // Set the time last used to something to avoid thrashing, but not recent,
-      // to avoid taking too much time/space with useless dictionaries/one-off
-      // visits to web sites.
-      clock_->Now() - base::TimeDelta::FromHours(23), dictionary_text.size());
+
+  // Record the addition in the pref store.
+  scoped_ptr<base::DictionaryValue> dictionary_description(
+      new base::DictionaryValue());
+  dictionary_description->SetString(kDictionaryUrlKey, dictionary_url.spec());
+  dictionary_description->SetDouble(kDictionaryLastUsedKey,
+                                    last_used.ToDoubleT());
+  dictionary_description->SetInteger(kDictionaryUseCountKey, use_count);
+  dictionary_description->SetInteger(kDictionarySizeKey,
+                                     dictionary_text.size());
+  pref_dictionary_map->Set(server_hash, dictionary_description.Pass());
 }
 
 void SdchOwner::OnDictionaryUsed(SdchManager* manager,
                                  const std::string& server_hash) {
-  auto it = local_dictionary_info_.find(server_hash);
-  DCHECK(local_dictionary_info_.end() != it);
+  base::Time now(clock_->Now());
+  base::DictionaryValue* pref_dictionary_map =
+      GetPersistentStoreDictionaryMap(pref_store_);
+  ScopedPrefNotifier scoped_pref_notifier(pref_store_);
 
-  it->second.last_used = clock_->Now();
-  it->second.use_count++;
+  base::Value* value = nullptr;
+  bool success = pref_dictionary_map->Get(server_hash, &value);
+  // TODO(rdsmith): Is the behavior of the SdchManager certain enough for
+  // the following DCHECKs?
+  DCHECK(success);
+  base::DictionaryValue* specific_dictionary_map = nullptr;
+  success = value->GetAsDictionary(&specific_dictionary_map);
+  DCHECK(success);
+
+  double last_used_time = 0.0;
+  success = specific_dictionary_map->GetDouble(kDictionaryLastUsedKey,
+                                               &last_used_time);
+  DCHECK(success);
+  int use_count = 0;
+  success =
+      specific_dictionary_map->GetInteger(kDictionaryUseCountKey, &use_count);
+  DCHECK(success);
+
+  base::TimeDelta time_since_last_used(now -
+                                       base::Time::FromDoubleT(last_used_time));
+
+  // TODO(rdsmith): Distinguish between "Never used" and "Actually not
+  // touched for 48 hours".
+  UMA_HISTOGRAM_CUSTOM_TIMES(
+      "Sdch3.UsageInterval",
+      use_count ? time_since_last_used : base::TimeDelta::FromHours(48),
+      base::TimeDelta(), base::TimeDelta::FromHours(48), 50);
+
+  specific_dictionary_map->SetDouble(kDictionaryLastUsedKey, now.ToDoubleT());
+  specific_dictionary_map->SetInteger(kDictionaryUseCountKey, use_count + 1);
 }
 
 void SdchOwner::OnGetDictionary(net::SdchManager* manager,
                                 const GURL& request_url,
                                 const GURL& dictionary_url) {
   base::Time stale_boundary(clock_->Now() - base::TimeDelta::FromDays(1));
   size_t avail_bytes = 0;
-  for (auto it = local_dictionary_info_.begin();
-       it != local_dictionary_info_.end(); ++it) {
-    if (it->second.last_used < stale_boundary)
-      avail_bytes += it->second.size;
+  for (DictionaryPreferenceIterator it(pref_store_); !it.IsAtEnd();
+       it.Advance()) {
+    if (it.last_used() < stale_boundary)
+      avail_bytes += it.size();
   }
 
   // Don't initiate the fetch if we wouldn't be able to store any
   // reasonable dictionary.
   // TODO(rdsmith): Maybe do a HEAD request to figure out how much
   // storage we'd actually need?
   if (max_total_dictionary_size_ < (total_dictionary_bytes_ - avail_bytes +
                                     min_space_for_dictionary_fetch_)) {
     RecordDictionaryFate(DICTIONARY_FATE_GET_IGNORED);
     // TODO(rdsmith): Log a net-internals error.  This requires
     // SdchManager to forward the URLRequest that detected the
     // Get-Dictionary header to its observers, which is tricky
     // because SdchManager is layered underneath URLRequest.
     return;
   }
 
-  fetcher_.Schedule(dictionary_url);
+  fetcher_.Schedule(dictionary_url,
+                    base::Bind(&SdchOwner::OnDictionaryFetched,
+                               // SdchOwner will outlive its member variables.
+                               base::Unretained(this), base::Time(), 0));
 }
 
 void SdchOwner::OnClearDictionaries(net::SdchManager* manager) {
   total_dictionary_bytes_ = 0;
-  local_dictionary_info_.clear();
   fetcher_.Cancel();
+
+  InitializePersistentStore(pref_store_);
+}
+
+void SdchOwner::OnPrefValueChanged(const std::string& key) {
+}
+
+void SdchOwner::OnInitializationCompleted(bool succeeded) {
+  // Errors on load are self-correcting; if dictionaries were not
+  // persisted from the last instance of the browser, they will be
+  // faulted in by user action over time.  However, if a load error
+  // means that the dictionary information won't be able to be persisted,
+  // the in memory pref store is left in place.
+  switch (external_pref_store_->GetReadError()) {
+    case PersistentPrefStore::PREF_READ_ERROR_NONE:
+      if (succeeded)
+        break;
+
+    case PersistentPrefStore::PREF_READ_ERROR_NO_FILE:
+      // First time reading; the file will be created.
+      if (succeeded) {
+        RecordPersistenceFailure(PERSISTENCE_FAILURE_REASON_NO_FILE);
+        break;
+      }
+
+    case PersistentPrefStore::PREF_READ_ERROR_JSON_PARSE:
+    case PersistentPrefStore::PREF_READ_ERROR_JSON_TYPE:
+    case PersistentPrefStore::PREF_READ_ERROR_FILE_OTHER:
+    case PersistentPrefStore::PREF_READ_ERROR_FILE_LOCKED:
+    case PersistentPrefStore::PREF_READ_ERROR_JSON_REPEAT:
+    case PersistentPrefStore::PREF_READ_ERROR_LEVELDB_IO:
+    case PersistentPrefStore::PREF_READ_ERROR_LEVELDB_CORRUPTION_READ_ONLY:
+    case PersistentPrefStore::PREF_READ_ERROR_LEVELDB_CORRUPTION:
+      // If the load was marked as succeeding (meaning the directory
+      // was present), even if the file wasn't read, it can be written.
+      if (succeeded) {
+        RecordPersistenceFailure(PERSISTENCE_FAILURE_REASON_READ_FAILED);
+        break;
+      }
+
+    case PersistentPrefStore::PREF_READ_ERROR_ACCESS_DENIED:
+    case PersistentPrefStore::PREF_READ_ERROR_FILE_NOT_SPECIFIED:
+      // Unrecoverable failure; we default to the internal pref store.
+      external_pref_store_->RemoveObserver(this);
+      external_pref_store_ = nullptr;
+      RecordPersistenceFailure(PERSISTENCE_FAILURE_REASON_WRITE_FAILED);
+      return;
+
+    case PersistentPrefStore::PREF_READ_ERROR_ASYNCHRONOUS_TASK_INCOMPLETE:
+    case PersistentPrefStore::PREF_READ_ERROR_MAX_ENUM:
+      // Shouldn't ever happen.
+      NOTREACHED();
+      break;
+  }
+
+  // TODO(rdsmith): Implement versioning.
+
+  // Load in what was stored before chrome exited previously.
+  const base::Value* sdch_persistence_value = nullptr;
+  const base::DictionaryValue* sdch_persistence_dictionary = nullptr;
+
+  // The GetPersistentStore() routine above assumes data formatted
+  // according to the schema described at the top of this file.  Since
+  // this data comes from disk, to avoid disk corruption resulting in
+  // persistent chrome errors this code avoids those assupmtions.
+  if (external_pref_store_->GetValue(kPreferenceName,
+                                     &sdch_persistence_value) &&
+      sdch_persistence_value->GetAsDictionary(&sdch_persistence_dictionary)) {
+    SchedulePersistedDictionaryLoads(*sdch_persistence_dictionary);
+  }
+
+  // Reset the persistent store and update it with the accumulated
+  // information from the local store.
+  InitializePersistentStore(external_pref_store_);
+
+  ScopedPrefNotifier scoped_pref_notifier(external_pref_store_);
+  GetPersistentStoreDictionaryMap(external_pref_store_)
+      ->Swap(GetPersistentStoreDictionaryMap(in_memory_pref_store_.get()));
+
+  // This object can stop waiting on (i.e. observing) the external preference
+  // store and switch over to using it as the primary preference store.
+  pref_store_ = external_pref_store_;
+  external_pref_store_->RemoveObserver(this);
+  external_pref_store_ = nullptr;
+  in_memory_pref_store_ = nullptr;
 }
 
 void SdchOwner::SetClockForTesting(scoped_ptr<base::Clock> clock) {
   clock_ = clock.Pass();
 }
 
 void SdchOwner::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel level) {
   DCHECK_NE(base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE, level);
 
-  for (auto it = local_dictionary_info_.begin();
-       it != local_dictionary_info_.end(); ++it) {
-    RecordDictionaryEviction(it->second.use_count,
-                             DICTIONARY_FATE_EVICT_FOR_MEMORY);
+  for (DictionaryPreferenceIterator it(pref_store_); !it.IsAtEnd();
+       it.Advance()) {
+    RecordDictionaryEviction(it.use_count(), DICTIONARY_FATE_EVICT_FOR_MEMORY);
   }
 
   // TODO(rdsmith): Make a distinction between moderate and critical
   // memory pressure.
   manager_->ClearData();
 }
 
+bool SdchOwner::SchedulePersistedDictionaryLoads(
+    const base::DictionaryValue& persisted_info) {
+  // Any schema error will result in dropping the persisted info.
+  int version = 0;
+  if (!persisted_info.GetInteger(kVersionKey, &version))
+    return false;
+
+  // Any version mismatch will result in dropping the persisted info;
+  // it will be faulted in at small performance cost as URLs using
+  // dictionaries for encoding are visited.
+  if (version != kVersion)
+    return false;
+
+  const base::DictionaryValue* dictionary_set = nullptr;
+  if (!persisted_info.GetDictionary(kDictionariesKey, &dictionary_set))
+    return false;
+
+  // Any formatting error will result in skipping that particular
+  // dictionary.
+  for (base::DictionaryValue::Iterator dict_it(*dictionary_set);
+       !dict_it.IsAtEnd(); dict_it.Advance()) {
+    const base::DictionaryValue* dict_info = nullptr;
+    if (!dict_it.value().GetAsDictionary(&dict_info))
+      continue;
+
+    std::string url_string;
+    if (!dict_info->GetString(kDictionaryUrlKey, &url_string))
+      continue;
+    GURL dict_url(url_string);
+
+    double last_used;
+    if (!dict_info->GetDouble(kDictionaryLastUsedKey, &last_used))
+      continue;
+
+    int use_count;
+    if (!dict_info->GetInteger(kDictionaryUseCountKey, &use_count))
+      continue;
+
+    fetcher_.ScheduleReload(
+        dict_url, base::Bind(&SdchOwner::OnDictionaryFetched,
+                             // SdchOwner will outlive its member variables.
+                             base::Unretained(this),
+                             base::Time::FromDoubleT(last_used), use_count));
+  }
+
+  return true;
+}
+
 }  // namespace net