Export of internal Abseil changes

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baf626d27ff1547776745f3b601cc42f703d4bdf by Greg Falcon <gfalcon@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 315486679

--
39d4e7f9214c5a74e4ee4b8a7f4af26479925029 by Gennadiy Rozental <rogeeff@google.com>:

Avoid order dependant test cases.

PiperOrigin-RevId: 315327948

--
8feb187703a28bb0c5cfc5dd6091e3faa90a8e61 by Gennadiy Rozental <rogeeff@google.com>:

Avoid order dependant test cases.

PiperOrigin-RevId: 315317018

--
cffd8fddad15908ec9de7f21f65b20528972e3fa by Gennadiy Rozental <rogeeff@google.com>:

Avoid order dependant test cases.

PiperOrigin-RevId: 315311587

--
48997a0ceb231b75c14eb4b1eb3b66af69e49f42 by Tom Manshreck <shreck@google.com>:

Nit: remove extra "the"

PiperOrigin-RevId: 314959763

--
9516f44a4b5d3055427c95e80c8c04a6321e4221 by Gennadiy Rozental <rogeeff@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 314925147

--
4f8691d74a4eb42ed03ef29c9588a01b78829941 by Abseil Team <absl-team@google.com>:

Uninclude util/bits/bits.h

PiperOrigin-RevId: 314858384

--
02b42a8ec5e5561b29b4e5f93cc1482c3c72ef2d by Abseil Team <absl-team@google.com>:

Google-internal changes only.

PiperOrigin-RevId: 314855667

--
146013d69dabafbe2030e23ced7b741d9e8d417c by Gennadiy Rozental <rogeeff@google.com>:

Uninclude util/bits/bits.h

PiperOrigin-RevId: 314838927
GitOrigin-RevId: baf626d27ff1547776745f3b601cc42f703d4bdf
Change-Id: I2602286fb13cf35a88bdd80fe0b44974d4388d46

9 files changed, 215 insertions, 65 deletions

diff --git a/absl/base/internal/low_level_scheduling.h b/absl/base/internal/low_level_scheduling.h
index 7c52d48c..961cc981 100644
--- a/absl/base/internal/low_level_scheduling.h
+++ b/absl/base/internal/low_level_scheduling.h
@@ -29,9 +29,6 @@ extern "C" void __google_enable_rescheduling(bool disable_result);
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
-class CondVar;
-class Mutex;
-
 namespace base_internal {
 
 class SchedulingHelper;  // To allow use of SchedulingGuard.
@@ -80,8 +77,6 @@ class SchedulingGuard {
   };
 
   // Access to SchedulingGuard is explicitly white-listed.
-  friend class absl::CondVar;
-  friend class absl::Mutex;
   friend class SchedulingHelper;
   friend class SpinLock;
 
diff --git a/absl/flags/commandlineflag_test.cc b/absl/flags/commandlineflag_test.cc
index c5afff61..585db4ba 100644
--- a/absl/flags/commandlineflag_test.cc
+++ b/absl/flags/commandlineflag_test.cc
@@ -34,6 +34,10 @@ ABSL_FLAG(std::string, string_flag, "dflt",
           absl::StrCat("string_flag", " help"));
 ABSL_RETIRED_FLAG(bool, bool_retired_flag, false, "bool_retired_flag help");
 
+// These are only used to test default values.
+ABSL_FLAG(int, int_flag2, 201, "");
+ABSL_FLAG(std::string, string_flag2, "dflt", "");
+
 namespace {
 
 namespace flags = absl::flags_internal;
@@ -94,15 +98,15 @@ TEST_F(CommandLineFlagTest, TestAttributesAccessMethods) {
 // --------------------------------------------------------------------
 
 TEST_F(CommandLineFlagTest, TestValueAccessMethods) {
-  absl::SetFlag(&FLAGS_int_flag, 301);
-  auto* flag_01 = absl::FindCommandLineFlag("int_flag");
+  absl::SetFlag(&FLAGS_int_flag2, 301);
+  auto* flag_01 = absl::FindCommandLineFlag("int_flag2");
 
   ASSERT_TRUE(flag_01);
   EXPECT_EQ(flag_01->CurrentValue(), "301");
   EXPECT_EQ(flag_01->DefaultValue(), "201");
 
-  absl::SetFlag(&FLAGS_string_flag, "new_str_value");
-  auto* flag_02 = absl::FindCommandLineFlag("string_flag");
+  absl::SetFlag(&FLAGS_string_flag2, "new_str_value");
+  auto* flag_02 = absl::FindCommandLineFlag("string_flag2");
 
   ASSERT_TRUE(flag_02);
   EXPECT_EQ(flag_02->CurrentValue(), "new_str_value");
diff --git a/absl/flags/internal/program_name_test.cc b/absl/flags/internal/program_name_test.cc
index 269142f2..aff9f631 100644
--- a/absl/flags/internal/program_name_test.cc
+++ b/absl/flags/internal/program_name_test.cc
@@ -25,7 +25,7 @@ namespace {
 
 namespace flags = absl::flags_internal;
 
-TEST(FlagsPathUtilTest, TestInitialProgamName) {
+TEST(FlagsPathUtilTest, TestProgamNameInterfaces) {
   flags::SetProgramInvocationName("absl/flags/program_name_test");
   std::string program_name = flags::ProgramInvocationName();
   for (char& c : program_name)
@@ -43,9 +43,7 @@ TEST(FlagsPathUtilTest, TestInitialProgamName) {
 
   EXPECT_TRUE(absl::EndsWith(program_name, expect_name)) << program_name;
   EXPECT_EQ(flags::ShortProgramInvocationName(), expect_basename);
-}
 
-TEST(FlagsPathUtilTest, TestProgamNameInterfaces) {
   flags::SetProgramInvocationName("a/my_test");
 
   EXPECT_EQ(flags::ProgramInvocationName(), "a/my_test");
diff --git a/absl/flags/parse_test.cc b/absl/flags/parse_test.cc
index 67ae8aad..d35a6e47 100644
--- a/absl/flags/parse_test.cc
+++ b/absl/flags/parse_test.cc
@@ -637,6 +637,10 @@ TEST_F(ParseTest, TestFlagfileInFlagfile) {
       "--flagfile=$0/parse_test.ff2",
   };
 
+  GetFlagfileFlag({{"parse_test.ff2", absl::MakeConstSpan(ff2_data)},
+                   {"parse_test.ff1", absl::MakeConstSpan(ff1_data)}},
+                      flagfile_flag);
+
   const char* in_args1[] = {
       "testbin",
       GetFlagfileFlag({{"parse_test.ff3", absl::MakeConstSpan(ff3_data)}},
diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index f833a80a..36bd84a3 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h
@@ -19,7 +19,7 @@
 //
 // The `str_format` library is a typesafe replacement for the family of
 // `printf()` string formatting routines within the `<cstdio>` standard library
-// header. Like the `printf` family, the `str_format` uses a "format string" to
+// header. Like the `printf` family, `str_format` uses a "format string" to
 // perform argument substitutions based on types. See the `FormatSpec` section
 // below for format string documentation.
 //
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 36c93d5f..1f8a696e 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -58,7 +58,6 @@
 
 using absl::base_internal::CurrentThreadIdentityIfPresent;
 using absl::base_internal::PerThreadSynch;
-using absl::base_internal::SchedulingGuard;
 using absl::base_internal::ThreadIdentity;
 using absl::synchronization_internal::GetOrCreateCurrentThreadIdentity;
 using absl::synchronization_internal::GraphCycles;
@@ -1109,7 +1108,6 @@ void Mutex::TryRemove(PerThreadSynch *s) {
 // on the mutex queue.  In this case, remove "s" from the queue and return
 // true, otherwise return false.
 ABSL_XRAY_LOG_ARGS(1) void Mutex::Block(PerThreadSynch *s) {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   while (s->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) {
     if (!DecrementSynchSem(this, s, s->waitp->timeout)) {
       // After a timeout, we go into a spin loop until we remove ourselves
@@ -1899,7 +1897,6 @@ static void CheckForMutexCorruption(intptr_t v, const char* label) {
 }
 
 void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   int c = 0;
   intptr_t v = mu_.load(std::memory_order_relaxed);
   if ((v & kMuEvent) != 0) {
@@ -2019,7 +2016,6 @@ void Mutex::LockSlowLoop(SynchWaitParams *waitp, int flags) {
 // or it is in the process of blocking on a condition variable; it must requeue
 // itself on the mutex/condvar to wait for its condition to become true.
 ABSL_ATTRIBUTE_NOINLINE void Mutex::UnlockSlow(SynchWaitParams *waitp) {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   intptr_t v = mu_.load(std::memory_order_relaxed);
   this->AssertReaderHeld();
   CheckForMutexCorruption(v, "Unlock");
@@ -2335,7 +2331,6 @@ void Mutex::Trans(MuHow how) {
 // It will later acquire the mutex with high probability.  Otherwise, we
 // enqueue thread w on this mutex.
 void Mutex::Fer(PerThreadSynch *w) {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   int c = 0;
   ABSL_RAW_CHECK(w->waitp->cond == nullptr,
                  "Mutex::Fer while waiting on Condition");
@@ -2434,7 +2429,6 @@ CondVar::~CondVar() {
 
 // Remove thread s from the list of waiters on this condition variable.
 void CondVar::Remove(PerThreadSynch *s) {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   intptr_t v;
   int c = 0;
   for (v = cv_.load(std::memory_order_relaxed);;
@@ -2595,7 +2589,6 @@ void CondVar::Wakeup(PerThreadSynch *w) {
 }
 
 void CondVar::Signal() {
-  SchedulingGuard::ScopedDisable disable_rescheduling;
   ABSL_TSAN_MUTEX_PRE_SIGNAL(nullptr, 0);
   intptr_t v;
   int c = 0;
diff --git a/absl/time/internal/cctz/src/time_zone_format.cc b/absl/time/internal/cctz/src/time_zone_format.cc
index a4428632..2e02233c 100644
--- a/absl/time/internal/cctz/src/time_zone_format.cc
+++ b/absl/time/internal/cctz/src/time_zone_format.cc
@@ -67,6 +67,48 @@ char* strptime(const char* s, const char* fmt, std::tm* tm) {
 }
 #endif
 
+// Convert a cctz::weekday to a tm_wday value (0-6, Sunday = 0).
+int ToTmWday(weekday wd) {
+  switch (wd) {
+    case weekday::sunday:
+      return 0;
+    case weekday::monday:
+      return 1;
+    case weekday::tuesday:
+      return 2;
+    case weekday::wednesday:
+      return 3;
+    case weekday::thursday:
+      return 4;
+    case weekday::friday:
+      return 5;
+    case weekday::saturday:
+      return 6;
+  }
+  return 0; /*NOTREACHED*/
+}
+
+// Convert a tm_wday value (0-6, Sunday = 0) to a cctz::weekday.
+weekday FromTmWday(int tm_wday) {
+  switch (tm_wday) {
+    case 0:
+      return weekday::sunday;
+    case 1:
+      return weekday::monday;
+    case 2:
+      return weekday::tuesday;
+    case 3:
+      return weekday::wednesday;
+    case 4:
+      return weekday::thursday;
+    case 5:
+      return weekday::friday;
+    case 6:
+      return weekday::saturday;
+  }
+  return weekday::sunday; /*NOTREACHED*/
+}
+
 std::tm ToTM(const time_zone::absolute_lookup& al) {
   std::tm tm{};
   tm.tm_sec = al.cs.second();
@@ -84,34 +126,19 @@ std::tm ToTM(const time_zone::absolute_lookup& al) {
     tm.tm_year = static_cast<int>(al.cs.year() - 1900);
   }
 
-  switch (get_weekday(al.cs)) {
-    case weekday::sunday:
-      tm.tm_wday = 0;
-      break;
-    case weekday::monday:
-      tm.tm_wday = 1;
-      break;
-    case weekday::tuesday:
-      tm.tm_wday = 2;
-      break;
-    case weekday::wednesday:
-      tm.tm_wday = 3;
-      break;
-    case weekday::thursday:
-      tm.tm_wday = 4;
-      break;
-    case weekday::friday:
-      tm.tm_wday = 5;
-      break;
-    case weekday::saturday:
-      tm.tm_wday = 6;
-      break;
-  }
+  tm.tm_wday = ToTmWday(get_weekday(al.cs));
   tm.tm_yday = get_yearday(al.cs) - 1;
   tm.tm_isdst = al.is_dst ? 1 : 0;
   return tm;
 }
 
+// Returns the week of the year [0:53] given a civil day and the day on
+// which weeks are defined to start.
+int ToWeek(const civil_day& cd, weekday week_start) {
+  const civil_day d(cd.year() % 400, cd.month(), cd.day());
+  return static_cast<int>((d - prev_weekday(civil_year(d), week_start)) / 7);
+}
+
 const char kDigits[] = "0123456789";
 
 // Formats a 64-bit integer in the given field width.  Note that it is up
@@ -355,7 +382,7 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
     if (cur == end || (cur - percent) % 2 == 0) continue;
 
     // Simple specifiers that we handle ourselves.
-    if (strchr("YmdeHMSzZs%", *cur)) {
+    if (strchr("YmdeUuWwHMSzZs%", *cur)) {
       if (cur - 1 != pending) {
         FormatTM(&result, std::string(pending, cur - 1), tm);
       }
@@ -376,6 +403,22 @@ std::string format(const std::string& format, const time_point<seconds>& tp,
           if (*cur == 'e' && *bp == '0') *bp = ' ';  // for Windows
           result.append(bp, static_cast<std::size_t>(ep - bp));
           break;
+        case 'U':
+          bp = Format02d(ep, ToWeek(civil_day(al.cs), weekday::sunday));
+          result.append(bp, static_cast<std::size_t>(ep - bp));
+          break;
+        case 'u':
+          bp = Format64(ep, 0, tm.tm_wday ? tm.tm_wday : 7);
+          result.append(bp, static_cast<std::size_t>(ep - bp));
+          break;
+        case 'W':
+          bp = Format02d(ep, ToWeek(civil_day(al.cs), weekday::monday));
+          result.append(bp, static_cast<std::size_t>(ep - bp));
+          break;
+        case 'w':
+          bp = Format64(ep, 0, tm.tm_wday);
+          result.append(bp, static_cast<std::size_t>(ep - bp));
+          break;
         case 'H':
           bp = Format02d(ep, al.cs.hour());
           result.append(bp, static_cast<std::size_t>(ep - bp));
@@ -610,6 +653,17 @@ const char* ParseTM(const char* dp, const char* fmt, std::tm* tm) {
   return dp;
 }
 
+// Sets year, tm_mon and tm_mday given the year, week_num, and tm_wday,
+// and the day on which weeks are defined to start.
+void FromWeek(int week_num, weekday week_start, year_t* year, std::tm* tm) {
+  const civil_year y(*year % 400);
+  civil_day cd = prev_weekday(y, week_start);  // week 0
+  cd = next_weekday(cd - 1, FromTmWday(tm->tm_wday)) + (week_num * 7);
+  *year += cd.year() - y.year();
+  tm->tm_mon = cd.month() - 1;
+  tm->tm_mday = cd.day();
+}
+
 }  // namespace
 
 // Uses strptime(3) to parse the given input.  Supports the same extended
@@ -659,6 +713,8 @@ bool parse(const std::string& format, const std::string& input,
   const char* fmt = format.c_str();  // NUL terminated
   bool twelve_hour = false;
   bool afternoon = false;
+  int week_num = -1;
+  weekday week_start = weekday::sunday;
 
   bool saw_percent_s = false;
   std::int_fast64_t percent_s = 0;
@@ -697,10 +753,27 @@ bool parse(const std::string& format, const std::string& input,
       case 'm':
         data = ParseInt(data, 2, 1, 12, &tm.tm_mon);
         if (data != nullptr) tm.tm_mon -= 1;
+        week_num = -1;
         continue;
       case 'd':
       case 'e':
         data = ParseInt(data, 2, 1, 31, &tm.tm_mday);
+        week_num = -1;
+        continue;
+      case 'U':
+        data = ParseInt(data, 0, 0, 53, &week_num);
+        week_start = weekday::sunday;
+        continue;
+      case 'W':
+        data = ParseInt(data, 0, 0, 53, &week_num);
+        week_start = weekday::monday;
+        continue;
+      case 'u':
+        data = ParseInt(data, 0, 1, 7, &tm.tm_wday);
+        if (data != nullptr) tm.tm_wday %= 7;
+        continue;
+      case 'w':
+        data = ParseInt(data, 0, 0, 6, &tm.tm_wday);
         continue;
       case 'H':
         data = ParseInt(data, 2, 0, 23, &tm.tm_hour);
@@ -891,6 +964,9 @@ bool parse(const std::string& format, const std::string& input,
     year += 1900;
   }
 
+  // Compute year, tm.tm_mon and tm.tm_mday if we parsed a week number.
+  if (week_num != -1) FromWeek(week_num, week_start, &year, &tm);
+
   const int month = tm.tm_mon + 1;
   civil_second cs(year, month, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);
 
diff --git a/absl/time/internal/cctz/src/time_zone_format_test.cc b/absl/time/internal/cctz/src/time_zone_format_test.cc
index 13a4227e..e625a839 100644
--- a/absl/time/internal/cctz/src/time_zone_format_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_format_test.cc
@@ -679,6 +679,34 @@ TEST(Format, RFC1123Format) {  // locale specific
   EXPECT_EQ("28 Jun 1977 09:08:07 -0700", format(RFC1123_no_wday, tp, tz));
 }
 
+TEST(Format, Week) {
+  const time_zone utc = utc_time_zone();
+
+  auto tp = convert(civil_second(2017, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2017-01-7", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2017-00-0", format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2017, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2017-53-7", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2017-52-0", format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2018, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2018-00-1", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2018-01-1", format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2018, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2018-52-1", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2018-53-1", format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2019, 1, 1, 0, 0, 0), utc);
+  EXPECT_EQ("2019-00-2", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2019-00-2", format("%Y-%W-%w", tp, utc));
+
+  tp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_EQ("2019-52-2", format("%Y-%U-%u", tp, utc));
+  EXPECT_EQ("2019-52-2", format("%Y-%W-%w", tp, utc));
+}
+
 //
 // Testing parse()
 //
@@ -1395,6 +1423,66 @@ TEST(Parse, RFC3339Format) {
   EXPECT_EQ(tp, tp4);
 }
 
+TEST(Parse, Week) {
+  const time_zone utc = utc_time_zone();
+  time_point<absl::time_internal::cctz::seconds> tp;
+
+  auto exp = convert(civil_second(2017, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2017-01-7", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2017-00-0", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2017, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2017-53-7", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2017-52-0", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2018, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2018-00-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2018-01-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2018, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2018-52-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2018-53-1", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2019, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2019-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2019-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2019-52-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2019-52-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+}
+
+TEST(Parse, WeekYearShift) {
+  // %U/%W conversions with week values in {0, 52, 53} can slip
+  // into the previous/following calendar years.
+  const time_zone utc = utc_time_zone();
+  time_point<absl::time_internal::cctz::seconds> tp;
+
+  auto exp = convert(civil_second(2019, 12, 31, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2020-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2020-00-2", utc, &tp));
+  EXPECT_EQ(exp, tp);
+
+  exp = convert(civil_second(2021, 1, 1, 0, 0, 0), utc);
+  EXPECT_TRUE(parse("%Y-%U-%u", "2020-52-5", utc, &tp));
+  EXPECT_EQ(exp, tp);
+  EXPECT_TRUE(parse("%Y-%W-%w", "2020-52-5", utc, &tp));
+  EXPECT_EQ(exp, tp);
+}
+
 TEST(Parse, MaxRange) {
   const time_zone utc = utc_time_zone();
   time_point<absl::time_internal::cctz::seconds> tp;
diff --git a/absl/time/internal/cctz/src/time_zone_impl.cc b/absl/time/internal/cctz/src/time_zone_impl.cc
index 030ae0e1..f34e3aec 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.cc
+++ b/absl/time/internal/cctz/src/time_zone_impl.cc
@@ -15,6 +15,7 @@
 #include "time_zone_impl.h"
 
 #include <deque>
+#include <memory>
 #include <mutex>
 #include <string>
 #include <unordered_map>
@@ -48,17 +49,16 @@ std::mutex& TimeZoneMutex() {
 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();
+  const Impl* const utc_impl = UTCImpl();
 
-  // First check for UTC (which is never a key in time_zone_map).
+  // Check for UTC (which is never a key in time_zone_map).
   auto offset = seconds::zero();
   if (FixedOffsetFromName(name, &offset) && offset == 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.
+  // Check whether the time zone has already been loaded.
   {
     std::lock_guard<std::mutex> lock(TimeZoneMutex());
     if (time_zone_map != nullptr) {
@@ -70,20 +70,15 @@ bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
     }
   }
 
-  // Now check again, under an exclusive lock.
+  // Load the new time zone (outside the lock).
+  std::unique_ptr<const Impl> new_impl(new Impl(name));
+
+  // Add the new time zone to the map.
   std::lock_guard<std::mutex> lock(TimeZoneMutex());
   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
-    }
+  if (impl == nullptr) {  // this thread won any load race
+    impl = new_impl->zone_ ? new_impl.release() : utc_impl;
   }
   *tz = time_zone(impl);
   return impl != utc_impl;
@@ -104,14 +99,11 @@ void time_zone::Impl::ClearTimeZoneMapTestOnly() {
   }
 }
 
-time_zone::Impl::Impl(const std::string& name) : name_(name) {}
+time_zone::Impl::Impl(const std::string& name)
+    : name_(name), zone_(TimeZoneIf::Load(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;
-  }();
+  static const Impl* utc_impl = new Impl("UTC");  // never fails
   return utc_impl;
 }