simplify get_seed_for_proof()

[arvidn]

Purpose:

Use built-in function filter() instead of raw loop, checking and counting in get_seed_for_proof(). Add asserts and checks to validate my understanding of this code.

[almogdepaz]

get_seed_for_proof expects at least two sub epochs, it will throw without this.
we cannot get a sees if we dont have 2 finished sub epochs at least

we cannot change how the seed is chosen since it is verified by the verifier so we can change this without being incompatible

[arvidn]

I'm not sure what you mean by this comment. It is possible to call this function with a height that only yields a single summary height. The existing behavior is to throw a KeyError exception in this case. It took some investigation to understand what that meant. I would think this is an improvement, as a clearer error is printed to the log. I'm pretty confident returning None has essentially the same effect as the exception, we fail to respond to this weight proof request either way.

[almogdepaz]

i think this error should be thrown from inside get_seed_for_proof

i missed that this is supposed to be equivalent to the current code (i think its less readable but maybe im just familiar with the old) i always prefer not fixing something thats not broken especially critical code, we can add the error there instead of the assert thats fine

[arvidn]

yeah, it's a good point that it's always risky to change code that works. I think this especially applies to get_seed_for_proof(). I'll propose a less intrusive change to that function.

Regarding the error reporting here. Is this case materially different from the other failure cases here, to warrant an exception instead of returning None?

For example, if self.blockchain.try_block_record(tip) or self._get_recent_chain(tip_rec.height) returns None.