Am 01.03.2017 um 00:10 schrieb Johannes Schindelin:
Hi René,
On Tue, 28 Feb 2017, René Scharfe wrote:
Am 28.02.2017 um 21:54 schrieb Johannes Schindelin:
On Tue, 28 Feb 2017, Junio C Hamano wrote:
René Scharfe <l.s.r@xxxxxx> writes:
Am 28.02.2017 um 15:28 schrieb Jeff King:
It looks from the discussion like the sanest path forward is our
own signed-64bit timestamp_t. That's unfortunate compared to
using the standard time_t, but hopefully it would reduce the
number of knobs (like TIME_T_IS_INT64) in the long run.
Glibc will get a way to enable 64-bit time_t on 32-bit platforms
eventually
(https://sourceware.org/glibc/wiki/Y2038ProofnessDesign). Can
platforms that won't provide a 64-bit time_t by 2038 be actually
used at that point? How would we get time information on them?
How would a custom timestamp_t help us?
That's a sensible "wait, let's step back a bit". I take it that you
are saying "time_t is just fine", and I am inclined to agree.
Right now, they may be able to have future timestamps ranging to
year 2100 and switching to time_t would limit their ability to
express future time to 2038 but they would be able to express
timestamp in the past to cover most of 20th century. Given that
these 32-bit time_t software platforms will die off before year 2038
(either by underlying hardware getting obsolete, or software updated
to handle 64-bit time_t), the (temporary) loss of 2038-2100 range
would not be too big a deal to warrant additional complexity.
You seem to assume that time_t is required to be signed. But from my
understanding that is only guaranteed by POSIX, not by ISO C.
We may very well buy ourselves a ton of trouble if we decide to switch
to `time_t` rather than to `int64_t`.
True, and time_t doesn't even have to be an integer type. But which
platforms capable of running git use something else than int32_t or
int64_t?
That kind of thinking is dangerous. We don't know what platforms are
running Git, and we have a very clear example how we got it very wrong
recently, when we broke building with musl by requiring REG_STARTEND
support [*1*].
In general that's true, and if nobody can add to the list (glibc:
int32_t on 32-bit platforms, int64_t on 64-bit platforms for now;
NetBSD, OpenBSD, Windows int64_t) then we shouldn't make assumptions
about time_t that go beyond the C standard -- at least not without
verifying them, e.g. with asserts or tests. I'd especially be
interested in hearing about platforms that use a floating point type.
Systems lacking REG_STARTEND can use compat/regex/ -- that's doesn't
sound too bad. (I didn't follow the original discussion too closely,
and don't mean to open it up again.)
So why gamble? If we switch to uint64_t, it would definitely provide the
smoothest upgrade path. It is what the code assumed implicitly when we
broke 32-bit in v2.9.1.
We can assume that time_t exists everywhere and is used by functions
like gettimeofday(2) and localtime(3). Why invent our own layer on top?
What would it be able to do that time_t alone can't? I understand a
need to handle times before 1970 for historical documents, but what good
would it do to have the ability to handle dates beyond 2038, today?
Platforms that don't use the next two decades to provide a time_t that
can store the current time by then will have bigger problems than a
crippled git.
If anybody really wants to support negative timestamps, it should be done
on top of my work. My current patch series does not even start to try to
address the ramifications of negative timestamps (see e.g. the use of
strtoull() for parsing). It is quite unreasonable to ask for such a
fundamental design change when it could very easily be done incrementally
instead, when needed, by someone who needs it.
I'm confused. Your patch series converts to time_t. Why not use it?
That would prepare ourselves for the bright future when we'll have a
64-bit time_t in every libc. :)
René