Decide if Zebra is at the chain tip

[upbqdn]

Motivation

Zebra needs a way to tell when it has reached the chain tip.

Closes #2592.

Solution

This PR simply calculates the average recent sync length, and returns true if this value falls below 20. This constant (20) is based on #2592 (comment).

Reviewer Checklist

• Code implements the behavior required in Activate Mempool storage based on a list of recent syncer obtain/extend lengths #2592

[upbqdn]

The function is_close_to_tip currently returns true even when there are too many zero values returned by RecentSyncLengths. I left it this way because I'm not sure how RecentSyncLengths really behaves when Zebra is synchronized.

Also note that is_close_to_tip can return true only if there is no value greater than 80 in the list of recent sync lengths since Average([80, 0, 0, 0]) = 20.

[teor2345]

This is a good minimal implementation.
We can tweak the constants later during testing, if needed.

It looks like the waits_until_close_to_tip test is failing because it assumes that a random series of sync lengths will always be close to the tip. (Which isn't true.)

And in this PR we actually check the lengths, so the test fails.

@jvff should be able to help out here.

[oxarbitrage]

I think this looks pretty good. Should we create a specific test for it or our current tests covers it ?

[teor2345]

I think this looks pretty good. Should we create a specific test for it or our current tests covers it ?

I think the proptest is good, but I'd also like us to test some test vectors.

For example:

1. totally empty sync lengths array is not near tip
2. sync lengths array with all zeroes is near tip
3. sync lengths array with all 500s is not near tip

Those are 3 basic tests that should keep on working, even if we tweak the algorithm or constants.

[jvff]

Looks good! If you need any help adding the test vectors, let me know 👍

[oxarbitrage]

make clippy happy.

[oxarbitrage]

Tests look good to me, nice job.