[libc++] tzdb throws if `/etc/localtime` is not a symlink

[mgorny]

When /etc/localtime is a file rather than a symlink, tzdb throws. This can be seen e.g. in the following libc++ tests:

FAIL: llvm-libc++-shared.cfg.in :: std/time/time.zone/time.zone.db/time.zone.db.access/current_zone.pass.cpp (8284 of 9740)
******************** TEST 'llvm-libc++-shared.cfg.in :: std/time/time.zone/time.zone.db/time.zone.db.access/current_zone.pass.cpp' FAILED ********************
Exit Code: 250

Command Output (stdout):
--
# COMPILED WITH
/usr/lib/ccache/bin/i686-pc-linux-gnu-clang++ /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/current_zone.pass.cpp -pthread --target=i686-pc-linux-gnu -nostdinc++ -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/support -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings  -lc++experimental -nostdlib++ -L /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -Wl,-rpath,/var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -lc++ -latomic -o /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir/t.tmp.exe
# executed command: /usr/lib/ccache/bin/i686-pc-linux-gnu-clang++ /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/current_zone.pass.cpp -pthread --target=i686-pc-linux-gnu -nostdinc++ -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/support -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings -lc++experimental -nostdlib++ -L /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -Wl,-rpath,/var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -lc++ -latomic -o /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir/t.tmp.exe
# EXECUTED AS
/usr/bin/python3.12 /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/utils/run.py --execdir /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir --  /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir/t.tmp.exe
# executed command: /usr/bin/python3.12 /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/utils/run.py --execdir /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir -- /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.access/Output/current_zone.pass.cpp.dir/t.tmp.exe
# .---command stderr------------
# | libc++abi: terminating due to uncaught exception of type std::runtime_error: tzdb: the path '/etc/localtime' is not a symlink
# `-----------------------------
# error: command failed with exit status: 250

--

********************
FAIL: llvm-libc++-shared.cfg.in :: std/time/time.zone/time.zone.db/time.zone.db.tzdb/current_zone.pass.cpp (8298 of 9740)
******************** TEST 'llvm-libc++-shared.cfg.in :: std/time/time.zone/time.zone.db/time.zone.db.tzdb/current_zone.pass.cpp' FAILED ********************
Exit Code: 250

Command Output (stdout):
--
# COMPILED WITH
/usr/lib/ccache/bin/i686-pc-linux-gnu-clang++ /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/current_zone.pass.cpp -pthread --target=i686-pc-linux-gnu -nostdinc++ -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/support -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings  -lc++experimental -nostdlib++ -L /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -Wl,-rpath,/var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -lc++ -latomic -o /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir/t.tmp.exe
# executed command: /usr/lib/ccache/bin/i686-pc-linux-gnu-clang++ /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/current_zone.pass.cpp -pthread --target=i686-pc-linux-gnu -nostdinc++ -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/include/c++/v1 -I /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/test/support -std=c++26 -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_NONE -Werror=thread-safety -Wuser-defined-warnings -lc++experimental -nostdlib++ -L /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -Wl,-rpath,/var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/lib -lc++ -latomic -o /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir/t.tmp.exe
# EXECUTED AS
/usr/bin/python3.12 /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/utils/run.py --execdir /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir --  /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir/t.tmp.exe
# executed command: /usr/bin/python3.12 /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/libcxx/utils/run.py --execdir /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir -- /var/tmp/portage/sys-libs/libcxx-20.0.0_pre20240822/work/runtimes_build-abi_x86_32.x86/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/Output/current_zone.pass.cpp.dir/t.tmp.exe
# .---command stderr------------
# | libc++abi: terminating due to uncaught exception of type std::runtime_error: tzdb: the path '/etc/localtime' is not a symlink
# `-----------------------------
# error: command failed with exit status: 250

--

********************

While systemd requires /etc/localtime to be a symlink, there's no such requirement for non-systemd systems, and at least fresh non-systemd Gentoo installs still default to that (I'm going to work on changing that but the point remains that non-symlink /etc/localtime is still a valid scenario supported by glibc).

[mgorny]

Oh, there's another uncovered scenario: /etc/localtime is also permitted not to exist at all.

[emaste]

FreeBSD also defaults to a non-symlink /etc/localtime in existing releases.

[jrtc27]

GCC falls back on reading /etc/timezone (for Debian and derivatives) and /var/db/zoneinfo (for FreeBSD). Whether that's sensible I don't know, but it's better than doing nothing.

[mordante]

@mgorny @emaste when the file is not a symlink, does the file only contain the name of the time zone? For example "Europe/Berlin" ?

[mgorny]

At least in Gentoo, it used to be a plain copy from tzdb.

[mordante]

Thanks for all the information provided. Since both Debian (derivatives) and Gentoo support /etc/timezone I'll look at supporting that. Non-symlinked versions of /etc/localtime that contain a copy of a binary timezone file is not something we can easily support. Libc++ parses the database's text files.

I like to support FreeBSD's link to, but at the moment we don't support TZDB for FreeBSD. So that feels out of scope for this report.

[mgorny]

I'm not sure if /etc/timezone is reliable. I don't see any reference but regebro/tzlocal@786baad indicates that Debian may no longer have an up-to-date value in that file.

[mgorny]

Oh, here it is: https://salsa.debian.org/glibc-team/tzdata/-/commit/c071fbc167ad306394b576879d515e6ecbe5a734

And indeed, /etc/timezone is not updated by timedatectl on Gentoo either. I guess we should remove it.

[mordante]

I keep using /etc/localtime, when that does not exist or it is not a symlink I fall back to /etc/timezone. (This may then be useless in the future.)

@mgorny when you remove /etc/timezone what will then be the way to get the name of the current time zone on Gentoo?

[mgorny]

Ah, sorry, I misread that. Yeah, using /etc/localtime symlink should be good. I see that right now we use /etc/timezone only when /etc/localtime is a regular file, and otherwise we ensure that /etc/localtime is a symlink.