Re: Y2038 vs struct cache_time/time_t

["Michal Suchánek" <msuchanek@suse.de>]

On Mon, Jan 20, 2020 at 08:38:51PM +0100, Johannes Schindelin wrote:
> Team,
> 
> today, in quite an entertaining thread on Twitter
> (https://twitter.com/jxxf/status/1219009308438024200) I read about yet
> another account how the Year 2038 problem already bites people. And costs
> real amounts of money.
> 
> And after I stopped shaking my head in disbelief, I had a quick look, and
> it seems that we're safe at least until February 7th, 2106. That's not
> great, but I plan on not being around at that date anymore, so there. That
> date is when the unsigned 32-bit Unix epoch will roll over and play
> dead^W^Wwreak havoc (iff the human species manages to actually turn around
> and reverse the climate catastrophe it caused, and that's a big iff):
> https://en.wikipedia.org/wiki/Time_formatting_and_storage_bugs#Year_2106
> 
> Concretely, it looks as if we store our own timestamps on disk (in the
> index file) as uint32_t:
> 
> 	/*
> 	 * The "cache_time" is just the low 32 bits of the
> 	 * time. It doesn't matter if it overflows - we only
> 	 * check it for equality in the 32 bits we save.
> 	 */
> 	struct cache_time {
> 		uint32_t sec;
> 		uint32_t nsec;
> 	};
> 
> The comment seems to indicate that we are still safe even if 2106 comes
> around, but I am not _quite_ that sure, as I expect us to have "greater
> than" checks, not only equality checks.
> 
> But wait, we're still not quite safe. If I remember correctly, 32-bit
> Linux still uses _signed_ 32-bit integers as `time_t`, so when we render
> dates, for example, and use system-provided functions, on 32-bit Linux we
> will at least show the wrong dates starting 2038.
> 
> This got me thinking, and I put on my QA hat. Kids, try this at home:
> 
> 	$ git log --until=1.january.1960
> 
> 	$ git log --since=1.january.2200
> 
> Git does not really do what you expected, eh?
> 
> Maybe we want to do something about that, and while at it also fix the
> overflow problems, probably requiring a new index format?

Which means we can split off the timestamps to a separate file, too ;-)

Thanks

Michal