On January 19, 2038 (no really January 20, 2020 2:39 PM), Johannes Schindelin wrote: > 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_21 > 06 > > 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? 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
