Fix TTL map wrapping sub

[jayshrivastava]

I'm pretty sure this is wrong:

datafusion-distributed/src/common/ttl_map.rs

Lines 241 to 242 in dd853a7

	let bucket_index = (time.wrapping_sub(1)) % self.buckets.len() as u64;
	self.buckets[bucket_index as usize].register_key(key);

Assume there's 10 buckets. When inserting a key at time=0, we want the key to go into bucket 9.

However, it ends up going to bucket 5.

(time.wrapping_sub(1)) % self.buckets.len() as u64
= (0 - 1) % 10
= 18,446,744,073,709,551,615 % 10
= 5

Note that time is a u64 that we increment by 1 each time and wraps around to 0.

Prematurely evicting entries in this rare case is not a significant problem, but it's a bug nonetheless.

[ruchirK]

So after reading through the code the issue is basically

((t - 1) mod 2^64) mod num_buckets) != (t - 1) mod num_buckets

which is true in cases when t < 1 and t >= 2^64. Only the first case is ever actually realizable, and I believe this is only an issue in the surrounding code because when we initialize a TTLMap we initialize it with time = 0 (see here) and instead if we initialized it with time = 1, we would be fine?

Does that analysis seem correct / reasonable?

[ruchirK]

take