Defining timestamp_t as intmax_t instead of uintmax_t

timestamp_t is a data type defined in git-compat-util.h as uintmax_t since 28f4aee3fb (use uintmax_t for timestamps, 2017-04-26). It was introduced with dddbad728c (timestamp_t: a new data type for timestamps, 2017-04-26) to replace use of both time_t and unsigned long for time-related uses.

On Linux with glibc[1] and on Windows with MSVC[2], time_t is always a signed type. I think this is also the case on MacOS and FreeBSD. Thus, any conversion or comparison between the two types is one between a signed and an unsigned type. This also means git cannot deal with time before 1970, though I can't think of a scenario where it will have to. However, it does it to sign compare warnings from the compiler.

I propose swapping uintmax_t with intmax_t to fix this problem. I do not think that this will cause any problem unless any code sets the most significant bit of the timestamp for some special purpose. However, the fix will take some work, considering the number of references to timestamp_t. So I want to get everyone's opinions on whether this should be done before starting work on it.

[1] https://www.gnu.org/software/libc/manual/html_node/Time-Types.html [2] https://learn.microsoft.com/en-us/cpp/c-runtime-library/standard-types?view=msvc-170

Arnav Bhate <bhatearnav@gmail.com> writes:

I just tested and found that if the user enters dates before 1970 the output is incorrect, for example with git log. This error also seems to propagate to GitHub. This may or may not be related to using uintmax_t, however it seems likely.

Arnav Bhate <bhatearnav@gmail.com> writes:

I do not think the object format and the existing object parsers are prepared to handle timestamp before the epoch.

You may want to go back to the list archive for discussions around the choice of the type back then, which should have enough information to go by.

https://lore.kernel.org/git/20170228200145.ymbqmxwrbbrwagks@sigill.intra.peff.net/

might be a good starting point.

https://lore.kernel.org/git/?q=f:peff@+d:20170201..20170430+timestamp_t

gives 6 entry points to a single large thread that may be worth following before tackling this topic further.

Thanks.

Junio C Hamano <gitster@pobox.com> writes:

Looks like I bit off more than I could chew. Maybe this could be a medium-sized project in itself.

I'll go through it.