fix(prost-types): Fix date-time parsing

[mumbleskates]

Two bugs revealed so far from fuzz testing in bilrost:

• parse_two_digit_numeric does not check the length of its input before calling split_at, potentially causing a panic
• a logic error in the translation of year_to_seconds from the original musl implementation caused it to treat the year 1900 as a leap year, causing (i believe) every date from 1900-01-01 to 1900-03-01 to be offset one day into the past when converting from DateTime to Timestamp.

[mumbleskates]

Here's another suggested fix for some weirdness that fuzzing failures highlighted: mumbleskates/bilrost@2e43e02

[caspermeijn]

Thanks for your contribution

[caspermeijn]

So basically, you are saying that the fast path was incorrect for 1900, but the slow path is correct. I can't reason why that is, but I assume that you are right :-)

[mumbleskates]

The cited function in musl actually writes this condition as if (year-2ULL <= 136) {... which isn't the same thing as if year as u64 <= 138 { at all! if year is either 0 or 1 in the original function it wraps to max.

the musl code's fast-path works within i32 seconds of the epoch, which ranges from 1901-12-13 to 2038-01-18, so it actually skips the fast-path for 1901 as well because the start of that year is an underflow. for our code, we are calculating in i64 and returning an i128 anyway so it doesn't matter. (the code in this pr is still equally correct here if we write if (1..=199).contains(&year) { :) )

what DOES matter is that the fast-path ignores the century rule for leap years, because 2000 is a multiple of 400 and happens to be one! it's also the only century that occurs in the i32 epoch range. so the fast-path logic is actually WRONG when applied to any year outside 1901..=2099, even without integer wrapping.

[caspermeijn]

I understand how this could happen. That is a subtle, but important difference. Thanks for the extensive explanation!

[caspermeijn]

Here's another suggested fix for some weirdness that fuzzing failures highlighted: mumbleskates/bilrost@2e43e02

Could you create a PR for that? Thanks.