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chrono::current_zone() fails if /etc/localtime is a symlink to a zone with three components, like "America/Indiana/Indianapolis", because we only try to find "Indianapolis" and "Indiana/Indianapolis" but neither of those is a valid zone name. We need to try up to three components to handle all valid cases, such as "UTC", "America/Indianapolis", and "America/Indiana/Indianapolis". It's also possible that users could provide a custom tzdata.zi file which includes zones with names using more than three levels, so loop over all filename components of the path that /etc/localtime points to. This also replaces std::filesystem::read_symlink with a plain readlink call and find+substr operations on a std::string_view, which is approximately twice as fast as using std::filesystem::path and std::string. By default we use a fixed char[128] buffer for readlink to write into, but if that doesn't fit we use a std::string as a dynamic buffer that grows as needed. We could use ::stat to find the exact length of the symlink and avoid looping with an increasingly large std::string capacity, but it's already expected to be rare for the char[128] buffer to be exceeded, so needing to double the std::string capacity more than once (i.e. to 512 or more) should be exceedingly rare. Adding a call to ::stat would perform a third filesystem operation when two readlink calls should be sufficient for the vast majority of realistic cases. One consequence of not using filesystem::path is that redundant consecutive slashes in the pathname aren't automatically ignored, e.g. /usr/share/zoneinfo/Europe//London worked fine with the old implementation because we manually concatenated the path components, i.e. "Europe" + '/' + "London". So that this continues to work there is a new loop to remove redundant slashes from the string being processed. That adds a slower, allocating path, but is unlikely to be needed in practice (the systemd spec for /etc/localtime explicitly says it should end with a time zone name, so "Europe//London" would be invalid anyway, even if it points to a valid file). Again, this loop is expected to be rare so optimizing this case further isn't important. While manually testing this I noticed that we will interpret a bogus symlink such as /usr/share/zoneinfo/America/Europe/London as a valid timezone, even though it's a dangling symlink. We find a name match for "Europe/London" before we get to the "America" component. This seems unlikely to matter in practice, and was a pre-existing problem. There's no testcase for current_zone() correctly handling three-level names or symlinks with unusual targets. It cannot be tested without changing the target of /etc/localtime which requires root access. I'm still considering whether we want to cache the result of current_zone(), either globally or in the tzdb object. Just returning a cached variable takes 20-30ns instead of more than 700ns to access the filesystem and read the symlink. Using ::lstat to check the symlink's mtime would add some overhead though. libstdc++-v3/ChangeLog: PR libstdc++/122567 * src/c++20/tzdb.cc (tzdb::current_zone): Loop over all trailing components of /etc/localtime path. Use readlink instead of std::filesystem::read_symlink. Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.