Support obtaining the system timezone on old Debian-based distributions

[dkhalanskyjb]

From https://wiki.debian.org/TimeZoneChanges:

In Debian releases Sarge (3.1) and earlier, /etc/localtime was a symbolic link. It was changed to a regular file (a copy of a /usr/share/zoneinfo/ file) in Etch (4.0), and remained so until Stretch (9), at which time it was made a symbolic link again.

According to the dates from https://en.wikipedia.org/wiki/Debian_version_history, this means that some Debian releases between 2007-04-08 and 2020-07-18 used a regular file instead of a symlink for /etc/localtime, a behavior we don't currently support. Instead, the timezone name should be obtained from /etc/timezone, a Debian-specific file.

It's worth noting that the Debian releases in question are all past their long-term support guarantees and are considered old. Still, if they are still widespread, it may be worth it to include the required workaround.

[TarunVishwakarma1]

Hey, I want to confirm the logic if I'm correct on a high level. If the /etc/localtime file exists then we get the data from this file else we move to /etc/timezone file. As both the file contains the zoneID in same formate it will not affect the logic.

Am I right ?

[fzhinkin]

if they are still widespread

Are they?

[fzhinkin]

Also, there might be no /etc/localtime file at all: llvm/llvm-project#105634

[dkhalanskyjb]

Are they?

It's at least plausible that someone will encounter this problem without using a woefully outdated system. For example, Ubuntu 14.04, based on Debian 8, is still supported under the legacy LTS program: https://ubuntu.com/about/release-cycle

Also, there might be no /etc/localtime file at all

Yes, we already support this: a86e7f8

[DmitryNekrasov]

I see the next output on gentoo stage3:

docker run -i -t --platform linux/amd64 gentoo/stage3
d3b60aa6dbc7 / # ls -l /etc/localtime
lrwxrwxrwx 1 root root 29 Mar  9 17:12 /etc/localtime -> ../usr/share/zoneinfo/Factory
d3b60aa6dbc7 / # ls -la ../usr/share/zoneinfo/Factory
-rw-r--r-- 1 root root 116 Feb 23 17:15 ../usr/share/zoneinfo/Factory
d3b60aa6dbc7 / # cat ../usr/share/zoneinfo/Factory
TZif2-00TZif2-00
<-00>0
d3b60aa6dbc7 / # cat /etc/timezone
UTC

@dkhalanskyjb What do you think about this output?

[dkhalanskyjb]

@DmitryNekrasov, we can support this if someone complains. I wouldn't do that proactively, as having Factory as the time zone is both 1) non-compliant according to https://www.man7.org/linux/man-pages/man5/localtime.5.html and 2) only occurs in misconfigured systems. The problem with Debian is that, although non-compliant (so Debian is in the wrong), it's still possible without the user doing anything wrong.