get_shuffling can't handle sample larger than validator_count

[djrtwo]

Issue

get_shuffling can't handle when sample larger than validator_count. The algorithm does an infinite loop. This actually occurs in code when len(active_validators) is less than ATTESTER_COUNT and get_attesters_and_signer is called.

Proposed Implementation

stub: need to think more

probably do one of the following:

• prevent this from happening with an assertion
• wrap shuffling array around to be the length of sample

[hwwhww]

It seems good to set the MINIMUM_VALIDATOR_COUNT for safety reason: https://ethresear.ch/t/safe-notary-pool-size/1728 and https://ethresear.ch/t/registrations-shard-count-and-shuffling/2129

[paulhauner]

Turns out I made a duplicate issue. I'll close it and just use this one.

FWIW, here's the suggestion I had in the duplicate issue:

if active_validators < 2:
    raise Exception()

ideal_val_count = config['attester_count'] + skip_count + 1

if ideal_val_count > active_validators:
     attesters = shuffled[:-1]
     proposer = shuffled[-1]
else:
     attesters = shuffled[:ideal_val_count - 1]
     proposer = shuffled[ideal_val_count - 1]

I agree with the MINIMUM_VALIDATOR_COUNT suggestion. I guess we'd face similar issues if skip_count gets too high, though.

EDIT: modified the code to be more sensible.

[paulhauner]

After playing around with this in Rust, my thoughts are that get_shuffling() shouldn't be concerned with "sampling" (i.e., getting some subset of shuffled active validators). That should be handled by whatever downstream function requires that sampling.

I'm only a little of the way into this state transition stuff so take that with a grain of salt. Just my thoughts so far.

[djrtwo]

get_new_shuffling in current spec does not handle sampling yay.