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diff --git a/absl/time/internal/cctz/src/time_zone_impl.cc b/absl/time/internal/cctz/src/time_zone_impl.cc
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+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "time_zone_impl.h"
+
+#include <mutex>
+#include <string>
+#include <unordered_map>
+#include <utility>
+
+#include "time_zone_fixed.h"
+
+namespace absl {
+namespace time_internal {
+namespace cctz {
+
+namespace {
+
+// time_zone::Impls are linked into a map to support fast lookup by name.
+using TimeZoneImplByName =
+ std::unordered_map<std::string, const time_zone::Impl*>;
+TimeZoneImplByName* time_zone_map = nullptr;
+
+// Mutual exclusion for time_zone_map.
+std::mutex time_zone_mutex;
+
+} // namespace
+
+time_zone time_zone::Impl::UTC() {
+ return time_zone(UTCImpl());
+}
+
+bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
+ const time_zone::Impl* const utc_impl = UTCImpl();
+
+ // First check for UTC (which is never a key in time_zone_map).
+ auto offset = sys_seconds::zero();
+ if (FixedOffsetFromName(name, &offset) && offset == sys_seconds::zero()) {
+ *tz = time_zone(utc_impl);
+ return true;
+ }
+
+ // Then check, under a shared lock, whether the time zone has already
+ // been loaded. This is the common path. TODO: Move to shared_mutex.
+ {
+ std::lock_guard<std::mutex> lock(time_zone_mutex);
+ if (time_zone_map != nullptr) {
+ TimeZoneImplByName::const_iterator itr = time_zone_map->find(name);
+ if (itr != time_zone_map->end()) {
+ *tz = time_zone(itr->second);
+ return itr->second != utc_impl;
+ }
+ }
+ }
+
+ // Now check again, under an exclusive lock.
+ std::lock_guard<std::mutex> lock(time_zone_mutex);
+ if (time_zone_map == nullptr) time_zone_map = new TimeZoneImplByName;
+ const Impl*& impl = (*time_zone_map)[name];
+ if (impl == nullptr) {
+ // The first thread in loads the new time zone.
+ Impl* new_impl = new Impl(name);
+ new_impl->zone_ = TimeZoneIf::Load(new_impl->name_);
+ if (new_impl->zone_ == nullptr) {
+ delete new_impl; // free the nascent Impl
+ impl = utc_impl; // and fallback to UTC
+ } else {
+ impl = new_impl; // install new time zone
+ }
+ }
+ *tz = time_zone(impl);
+ return impl != utc_impl;
+}
+
+const time_zone::Impl& time_zone::Impl::get(const time_zone& tz) {
+ if (tz.impl_ == nullptr) {
+ // Dereferencing an implicit-UTC time_zone is expected to be
+ // rare, so we don't mind paying a small synchronization cost.
+ return *UTCImpl();
+ }
+ return *tz.impl_;
+}
+
+void time_zone::Impl::ClearTimeZoneMapTestOnly() {
+ std::lock_guard<std::mutex> lock(time_zone_mutex);
+ if (time_zone_map != nullptr) {
+ // Existing time_zone::Impl* entries are in the wild, so we simply
+ // leak them. Future requests will result in reloading the data.
+ time_zone_map->clear();
+ }
+}
+
+time_zone::Impl::Impl(const std::string& name) : name_(name) {}
+
+const time_zone::Impl* time_zone::Impl::UTCImpl() {
+ static Impl* utc_impl = [] {
+ Impl* impl = new Impl("UTC");
+ impl->zone_ = TimeZoneIf::Load(impl->name_); // never fails
+ return impl;
+ }();
+ return utc_impl;
+}
+
+} // namespace cctz
+} // namespace time_internal
+} // namespace absl