libstdc++: Use /etc/sysconfig/clock for std::chrono::current_zone() [PR108530]

On some systems /etc/localtime is a tzfile, not a symlink to one. We
cannot use it to determine the current time zone in that case. See if
/etc/sysconfig/clock sets the variable DEFAULT_TIMEZONE instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/108530
	* src/c++20/tzdb.cc (current_zone): Look for DEFAULT_TIMEZONE in
	/etc/sysconfig/clock.

1 files changed, 22 insertions, 2 deletions

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 20399b9..eb68111 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -62,8 +62,8 @@
 #if ! __cpp_constinit
 # if __has_cpp_attribute(clang::require_constant_initialization)
 #  define constinit [[clang::require_constant_initialization]]
-#else // YOLO
-# define constinit
+# else // YOLO
+#  define constinit
 # endif
 #endif
 
@@ -1663,6 +1663,26 @@ namespace std::chrono
 	  if (auto tz = do_locate_zone(this->zones, this->links, name))
 	    return tz;
       }
+
+    if (ifstream tzf{"/etc/sysconfig/clock"})
+      {
+	string line;
+	string_view key = "DEFAULT_TIMEZONE=";
+	while (std::getline(tzf, line))
+	  if (line.starts_with(key))
+	    {
+	      string_view name = line;
+	      name.remove_prefix(key.size());
+	      if (name.size() != 0 && name.front() == '"')
+		{
+		  name.remove_prefix(1);
+		  if (auto pos = name.find('"'); pos != name.npos)
+		    name = name.substr(0, pos);
+		}
+	      if (auto tz = do_locate_zone(this->zones, this->links, name))
+		return tz;
+	    }
+      }
 #else
     // AIX stores current zone in $TZ in /etc/environment but the value
     // is typically a POSIX time zone name, not IANA zone.