nixos/timezone: stop using mode direct-symlink for linking localtime

[NickCao]

Description of changes

This surfaces from #270727 (comment).

We are currently having an unnecessary level of indirection, namely
/etc/localtime -> /etc/zoneinfo/<tz> -> /nix/store/<tzdata>/share/zoneinfo/<tz>
Which was introduced in #26608 for no obvious reasons.

It can be reduced to
/etc/localtime -> /nix/store/<tzdata>/share/zoneinfo/<tz>
and the relevant nixos tests still pass.

This also removed the last in-tree user of the undocumented direct-symlink mode of etc files.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[lheckemann]

The indirection wasn't introduced in my PR, it was introduced before in 0762396 -- with justification.

If this turns out to be fine anyway -- given that it's the last use of direct-symlink in nixpkgs, maybe we should remove or at least deprecate the mode as well?

[NickCao]

The indirection wasn't introduced in my PR, it was introduced before in 0762396 -- with justification.

This indeed breaks systemd:

❯ timedatectl status
                Time zone: n/a (UTC, +0000)

[NickCao]

So the solution would now be keeping direct-symlink, but make perlless activation compatible with it.

[NickCao]

https://github.com/systemd/systemd/blob/db11bab38ccf1ed257f310d29070843d4c58ea01/src/basic/time-util.c#L1628

[lheckemann]

https://github.com/NixOS/nixpkgs/blob/a5ae45e45af0a697a219eb4da2827fe3cf4e9374/pkgs/os-specific/linux/systemd/0007-Change-usr-share-zoneinfo-to-etc-zoneinfo.patch

[Kiskae]

Wouldn't the following code work:

lib.optionalAttrs (config.time.timeZone != null) {
  localtime.source = "/etc/zoneinfo/${config.time.timeZone}";
  localtime.mode = "symlink";
};

This keeps the indirection because the systemd requirements, but fixes perlless activation since creating a symlink to a non-existent absolute path is allowed.

It looks like direct-symlink is a hack to get a symlink but gid or uid was set for some reason:

nixpkgs/nixos/modules/system/etc/setup-etc.pl

Lines 113 to 134 in 5fd6c33

	if (-e "$_.mode") {
	my $mode = read_file("$_.mode"); chomp $mode;
	if ($mode eq "direct-symlink") {
	atomicSymlink readlink("$static/$fn"), $target or warn "could not create symlink $target";
	} else {
	my $uid = read_file("$_.uid"); chomp $uid;
	my $gid = read_file("$_.gid"); chomp $gid;
	copy "$static/$fn", "$target.tmp" or warn;
	$uid = getpwnam $uid unless $uid =~ /^\+/;
	$gid = getgrnam $gid unless $gid =~ /^\+/;
	chown int($uid), int($gid), "$target.tmp" or warn;
	chmod oct($mode), "$target.tmp" or warn;
	unless (rename "$target.tmp", $target) {
	warn "could not create target $target";
	unlink "$target.tmp";
	}
	}
	push @copied, $fn;
	print CLEAN "$fn\n";
	} elsif (-l "$_") {
	atomicSymlink "$static/$fn", $target or warn "could not create symlink $target";
	}

Neither are the case here, so it is likely safe to remove this entirely.

[nyabinary]

Wouldn't the following code work:

lib.optionalAttrs (config.time.timeZone != null) {
  localtime.source = "/etc/zoneinfo/${config.time.timeZone}";
  localtime.mode = "symlink";
};

This keeps the indirection because the systemd requirements, but fixes perlless activation since creating a symlink to a non-existent absolute path is allowed.

It looks like direct-symlink is a hack to get a symlink but gid or uid was set for some reason:

nixpkgs/nixos/modules/system/etc/setup-etc.pl

Lines 113 to 134 in 5fd6c33

	if (-e "$_.mode") {
	my $mode = read_file("$_.mode"); chomp $mode;
	if ($mode eq "direct-symlink") {
	atomicSymlink readlink("$static/$fn"), $target or warn "could not create symlink $target";
	} else {
	my $uid = read_file("$_.uid"); chomp $uid;
	my $gid = read_file("$_.gid"); chomp $gid;
	copy "$static/$fn", "$target.tmp" or warn;
	$uid = getpwnam $uid unless $uid =~ /^\+/;
	$gid = getgrnam $gid unless $gid =~ /^\+/;
	chown int($uid), int($gid), "$target.tmp" or warn;
	chmod oct($mode), "$target.tmp" or warn;
	unless (rename "$target.tmp", $target) {
	warn "could not create target $target";
	unlink "$target.tmp";
	}
	}
	push @copied, $fn;
	print CLEAN "$fn\n";
	} elsif (-l "$_") {
	atomicSymlink "$static/$fn", $target or warn "could not create symlink $target";
	}

Neither are the case here, so it is likely safe to remove this entirely.

I believe the systemd patch will need to be updated too:

nixpkgs/pkgs/os-specific/linux/systemd/0007-Change-usr-share-zoneinfo-to-etc-zoneinfo.patch

Line 73 in a5ae45e

+ e = PATH_STARTSWITH_SET(t, "/etc/zoneinfo/", "../etc/zoneinfo/");

[nyabinary]

Any status update on this?

[devurandom]

I just ran into the same issue:

error: builder for '/nix/store/596cj5v942ms8dbfsmsz5fm69a7yghmr-etc-lowerdir.drv' failed with exit code 1;
       last 1 log lines:
       > cp: cannot stat '/etc/zoneinfo/Europe/Berlin': No such file or directory
       For full logs, run 'nix log /nix/store/596cj5v942ms8dbfsmsz5fm69a7yghmr-etc-lowerdir.drv'.

Is there a workaround? (Apart from unsetting time.timeZone.)

[nyabinary]

I just ran into the same issue:

error: builder for '/nix/store/596cj5v942ms8dbfsmsz5fm69a7yghmr-etc-lowerdir.drv' failed with exit code 1;
       last 1 log lines:
       > cp: cannot stat '/etc/zoneinfo/Europe/Berlin': No such file or directory
       For full logs, run 'nix log /nix/store/596cj5v942ms8dbfsmsz5fm69a7yghmr-etc-lowerdir.drv'.

Is there a workaround? (Apart from unsetting time.timeZone.)

I don't think so, sadly.

[jcfj]

Is there a workaround?

Setting environment.etc.localtime = lib.mkForce { source = "${pkgs.tzdata}/share/zoneinfo/${config.time.timeZone}"; }; works for that build failure, but I'm not sure the resulting system is usable.

[NickCao]

Closing in favor of #314579