[libc++][chrono] Completes the tzdb class.

It adds the missing member functions of the tzdb class and adds the free
functions that use these member functions.

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones

Author: mordante

libcxx/include/__chrono/tzdb.h

@@ -16,6 +16,7 @@
 // Enable the contents of the header only when libc++ was built with experimental features enabled.
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
 
+#  include <__algorithm/ranges_lower_bound.h>
 #  include <__chrono/leap_second.h>
 #  include <__chrono/time_zone.h>
 #  include <__chrono/time_zone_link.h>
@@ -43,6 +44,40 @@ struct tzdb {
   vector<time_zone_link> links;
 
   vector<leap_second> leap_seconds;
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const time_zone* __locate_zone(string_view __name) const {
+    if (const time_zone* __result = __find_in_zone(__name); __result)
+      return __result;
+
+    if (auto __it = ranges::lower_bound(links, __name, {}, &time_zone_link::name);
+        __it != links.end() && __it->name() == __name)
+      if (const time_zone* __result = __find_in_zone(__it->target()); __result)
+        return __result;
+
+    return nullptr;
+  }
+
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI const time_zone* locate_zone(string_view __name) const {
+    if (const time_zone* __result = __locate_zone(__name))
+      return __result;
+
+    std::__throw_runtime_error("tzdb: requested time zone not found");
+  }
+
+  _LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_HIDE_FROM_ABI const time_zone* current_zone() const {
+    return __current_zone();
+  }
+
+private:
+  _LIBCPP_HIDE_FROM_ABI const time_zone* __find_in_zone(string_view __name) const noexcept {
+    if (auto __it = ranges::lower_bound(zones, __name, {}, &time_zone::name);
+        __it != zones.end() && __it->name() == __name)
+      return std::addressof(*__it);
+
+    return nullptr;
+  }
+
+  [[nodiscard]] _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI const time_zone* __current_zone() const;
 };
 
 } // namespace chrono

libcxx/include/__chrono/tzdb_list.h

@@ -17,6 +17,7 @@
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
 
 #  include <__availability>
+#  include <__chrono/time_zone.h>
 #  include <__chrono/tzdb.h>
 #  include <__config>
 #  include <__fwd/string.h>
@@ -74,6 +75,15 @@ _LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_HIDE_FROM_ABI inline con
   return get_tzdb_list().front();
 }
 
+_LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_HIDE_FROM_ABI inline const time_zone*
+locate_zone(string_view __name) {
+  return get_tzdb().locate_zone(__name);
+}
+
+_LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_HIDE_FROM_ABI inline const time_zone* current_zone() {
+  return get_tzdb().current_zone();
+}
+
 _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI const tzdb& reload_tzdb();
 
 _LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI string remote_version();

libcxx/include/chrono

@@ -689,6 +689,9 @@ struct tzdb {
   vector<time_zone>      zones;
   vector<time_zone_link> links;
   vector<leap_second>    leap_seconds;
+
+  const time_zone* locate_zone(string_view tz_name) const;
+  const time_zone* current_zone() const;
 };
 
 class tzdb_list {                                                                // C++20
@@ -714,6 +717,8 @@ public:
 // [time.zone.db.access], time zone database access
 const tzdb& get_tzdb();                                                          // C++20
 tzdb_list& get_tzdb_list();                                                      // C++20
+const time_zone* locate_zone(string_view tz_name);                               // C++20
+const time_zone* current_zone()                                                  // C++20
 
 // [time.zone.db.remote], remote time zone database support
 const tzdb& reload_tzdb();                                                       // C++20

libcxx/modules/std/chrono.inc

@@ -199,10 +199,10 @@ export namespace std {
     using std::chrono::tzdb_list;
 
     // [time.zone.db.access], time zone database access
-    // using std::chrono::current_zone;
+    using std::chrono::current_zone;
     using std::chrono::get_tzdb;
     using std::chrono::get_tzdb_list;
-    // using std::chrono::locate_zone;
+    using std::chrono::locate_zone;
 
     // [time.zone.db.remote], remote time zone database support
     using std::chrono::reload_tzdb;

libcxx/src/tzdb.cpp

@@ -675,6 +675,57 @@ void __init_tzdb(tzdb& __tzdb, __tz::__rules_storage_type& __rules) {
   std::ranges::sort(__tzdb.leap_seconds);
 }
 
+#ifdef _WIN32
+[[nodiscard]] static const time_zone* __current_zone_windows(const tzdb& tzdb) {
+  // TODO TZDB Implement this on Windows.
+  std::__throw_runtime_error("unknown time zone");
+}
+#else  // ifdef _WIN32
+[[nodiscard]] static const time_zone* __current_zone_posix(const tzdb& tzdb) {
+  // On POSIX systems there are several ways to configure the time zone.
+  // In order of priority they are:
+  // - TZ environment variable
+  //   https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08
+  //   The documentation is unclear whether or not it's allowed to
+  //   change time zone information. For example the TZ string
+  //     MST7MDT
+  //   this is an entry in tzdata.zi. The value
+  //     MST
+  //   is also an entry. Is it allowed to use the following?
+  //     MST-3
+  //   Even when this is valid there is no time_zone record in the
+  //   database. Since the library would need to return a valid pointer,
+  //   this means the library needs to allocate and leak a pointer.
+  //
+  // - The time zone name is the target of the symlink /etc/localtime
+  //   relative to /usr/share/zoneinfo/
+
+  // The algorithm is like this:
+  // - If the environment variable TZ is set and points to a valid
+  //   record use this value.
+  // - Else use the timezone name.
+
+  if (const char* __tz = getenv("TZ"))
+    if (const time_zone* __result = tzdb.__locate_zone(__tz))
+      return __result;
+
+  filesystem::path __path = "/etc/localtime";
+  if (!std::filesystem::exists(__path))
+    std::__throw_runtime_error("the symlink '/etc/localtime' does not exist");
+
+  if (!std::filesystem::is_symlink(__path))
+    std::__throw_runtime_error("the path '/etc/localtime' is not a symlink");
+
+  filesystem::path __tz = filesystem::read_symlink(__path);
+  string __name         = filesystem::relative(__tz, "/usr/share/zoneinfo/");
+
+  if (const time_zone* __result = tzdb.__locate_zone(__name))
+    return __result;
+
+  std::__throw_runtime_error(("the time zone '" + __name + "' is not found in the database").c_str());
+}
+#endif // _WIN32
+
 //===----------------------------------------------------------------------===//
 //                           Public API
 //===----------------------------------------------------------------------===//
@@ -684,6 +735,14 @@ _LIBCPP_NODISCARD_EXT _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI tzdb_l
   return __result;
 }
 
+[[nodiscard]] _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI const time_zone* tzdb::__current_zone() const {
+#ifdef _WIN32
+  return chrono::__current_zone_windows(*this);
+#else
+  return chrono::__current_zone_posix(*this);
+#endif
+}
+
 _LIBCPP_AVAILABILITY_TZDB _LIBCPP_EXPORTED_FROM_ABI const tzdb& reload_tzdb() {
   if (chrono::remote_version() == chrono::get_tzdb().version)
     return chrono::get_tzdb();

libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.compile.pass.cpp

@@ -38,8 +38,16 @@ void test() {
 
   std::chrono::get_tzdb_list();
   std::chrono::get_tzdb();
+  std::chrono::locate_zone("name");
+  std::chrono::current_zone();
   std::chrono::remote_version();
 
+  {
+    const std::chrono::tzdb& t = list.front();
+    t.locate_zone("name");
+    t.current_zone();
+  }
+
   {
     tz.name();
     operator==(tz, tz);

libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.verify.cpp

@@ -33,9 +33,17 @@ void test() {
   list.cbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   list.cend();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
+  {
+    const std::chrono::tzdb& t = list.front();
+    t.locate_zone("name"); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    t.current_zone();      // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  }
+
   namespace crno = std::chrono;
   crno::get_tzdb_list();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   crno::get_tzdb();       // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  crno::locate_zone("n"); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  crno::current_zone();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   crno::remote_version(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
   {

libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/current_zone.pass.cpp

@@ -0,0 +1,77 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// const time_zone* current_zone();
+
+#include <cassert>
+#include <chrono>
+#include <string_view>
+#include <stdlib.h>
+
+#include "test_macros.h"
+#include "assert_macros.h"
+#include "concat_macros.h"
+
+#ifdef _WIN32
+static void set_tz(std::string zone) {
+  // Note Windows does not have setenv, only putenv
+  // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/putenv-s-wputenv-s?view=msvc-170
+  // Unlike POSIX it does not mention the string of putenv becomes part
+  // of the environment.
+
+  int status = _putenv_s("TZ", zone.c_str(), 1);
+  assert(status == 0);
+}
+
+#else
+static void set_tz(const std::string& zone) {
+  int status = setenv("TZ", zone.c_str(), 1);
+  assert(status == 0);
+}
+#endif
+
+static void test_zone(const std::string& zone) {
+  set_tz(zone);
+  const std::chrono::time_zone* tz = std::chrono::current_zone();
+  assert(tz);
+  assert(tz->name() == zone);
+}
+
+static void test_link(const std::string& link, std::string_view zone) {
+  set_tz(link);
+  const std::chrono::time_zone* tz = std::chrono::current_zone();
+  assert(tz);
+  assert(tz->name() == zone);
+}
+
+int main(int, const char**) {
+  const std::chrono::time_zone* tz = std::chrono::current_zone();
+  // Returns a valid time zone, the value depends on the OS settings.
+  assert(tz);
+  // setting the environment to an invalid value returns the value of
+  // the OS setting.
+  set_tz("This is not a time zone");
+  assert(tz == std::chrono::current_zone());
+
+  const std::chrono::tzdb& db = std::chrono::get_tzdb();
+  for (const auto& zone : db.zones)
+    test_zone(std::string{zone.name()});
+
+  for (const auto& link : db.links)
+    test_link(std::string{link.name()}, link.target());
+
+  return 0;
+}

libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/locate_zone.pass.cpp

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// const time_zone* locate_zone(string_view tz_name);
+
+#include <cassert>
+#include <chrono>
+#include <string_view>
+
+#include "test_macros.h"
+#include "assert_macros.h"
+#include "concat_macros.h"
+
+static void test_zone(std::string_view zone) {
+  const std::chrono::time_zone* tz = std::chrono::locate_zone(zone);
+  assert(tz);
+  assert(tz->name() == zone);
+}
+
+static void test_link(std::string_view link, std::string_view zone) {
+  const std::chrono::time_zone* tz = std::chrono::locate_zone(link);
+  assert(tz);
+  assert(tz->name() == zone);
+}
+
+static void test_exception([[maybe_unused]] std::string_view zone) {
+  TEST_VALIDATE_EXCEPTION(
+      std::runtime_error,
+      [&]([[maybe_unused]] const std::runtime_error& e) {
+        std::string_view what{"tzdb: requested time zone not found"};
+        TEST_LIBCPP_REQUIRE(
+            e.what() == what,
+            TEST_WRITE_CONCATENATED("\nExpected exception ", what, "\nActual exception   ", e.what(), '\n'));
+      },
+      TEST_IGNORE_NODISCARD std::chrono::locate_zone(zone));
+}
+
+int main(int, const char**) {
+  const std::chrono::tzdb& db = std::chrono::get_tzdb();
+  for (const auto& zone : db.zones)
+    test_zone(zone.name());
+
+  for (const auto& link : db.links)
+    test_link(link.name(), link.target());
+
+  test_exception("This is not a time zone");
+
+  return 0;
+}