[llvm-branch-commits] [libcxx] [libc++][chrono] Completes the tzdb class. (PR #82157)

[Louis Dionne via llvm-branch-commits]

================
@@ -655,6 +655,57 @@ 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());
----------------
ldionne wrote:

```suggestion
  std::__throw_runtime_error(("tzdb: the time zone '" + __name + "' is not found in the database").c_str());
```

https://github.com/llvm/llvm-project/pull/82157