RE: Y2038 vs struct cache_time/time_t

From: Randall S. Becker <rsbecker@nexbridge.com>
Date: Jan 20, 2020, 20:23 UTC
Message-ID: <017401d5cfcf$8791d900$96b58b00$@nexbridge.com>
In-Reply-To: <nycvar.QRO.7.76.6.2001201555220.46@tvgsbejvaqbjf.bet>

On January 19, 2038 (no really January 20, 2020 2:39 PM), Johannes Schindelin wrote:

Show 6 quoted lines

> 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

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> 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_21
> 06
> 
> Concretely, it looks as if we store our own timestamps on disk (in the

index

Show 14 quoted lines

> 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

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> 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?

The preferred way of fixing this is traditionally - for those of us who have been through it (4-ish times), to convert to time64_t where available (big legacy machines, like z/OS and NonStop), or in gcc, time_t is 64 bit on 64 bit systems. It has been 64 bit on Windows since VS 2005. I have a relatively some relatively old Linux distros on 64 bit processors that also have time_t set as 64 bit in gcc. Those seem to be the standard approaches. To cover it, I suggest we move to a gittime_t which is always 64 bit (or 128 bit if you don't want to be resurrected after the sun turns into a red giant or later when we are left with evaporating black holes), no matter what the platform, and build the selection of what gittime_t is (time_t or time64_t) into our config and/or compat.h. That way, hopefully, people will rebuild their git before 2038 or before someone decides to stick a fake date into a Github repo just to mess with us.

Cheers, Randall