[tz] Getting current tzdb version in use

[Paul G]

On 07/20/2018 10:58 AM, Paul Eggert wrote:
> Paul G wrote:
> Unfortunately it's not that simple, because in some cases the sources can disagree but neither will be "more recent"; for example, both might be based on 2018e but with different local patches applied. This is partly why I'm skeptical about attempts to use version information for anything important other than "report this string to the users, and let them figure it out".

Yes, it's not perfect, and as you mentioned in the RHEL case downstream packagers can introduce bugs, but it's certainly better than nothing. The previous status quo with *no* accessible version information meant that essentially there was no way to decide which one *claims* to be "more recent". With tzdata.zi, we can at least tell what the system claims the version is, and we can use that to check:

1. If the deployed version complies with the upstream versioning scheme
2. Whether the version differences are ambiguous (e.g. 2018e+dev23deafffa vs. 2018e+dev33471873 - which to choose?)

It's up to people who implement consumer libraries to not blindly believe the versions, of course - e.g. by default believe the versions but provide a mechanism for users to say, "Actually just always use the system version" or "Actually just always use this version", but I think that once tzdata.zi is widely deployed, in 90%+ of cases you won't go wrong by trusting the system version.

>> At this point, of course, I think the battle in the tz project is won - tzdata.zi exists and last I checked `make install` installs it into `/usr/share/zoneinfo`, so now it's time to get system distributors to make sure they include it in their distributions, I guess.
> 
> Plus, distributors need to include something better than "# version unknown", which is what Red Hat is doing unfortunately. See:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1604030

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