Merely including a vote which is not a descendent of the round target, should not disqualify a commit message

[drskalman]

This refers to these lines of the code:

finality-grandpa/src/lib.rs

Line 443 in 4c71f4b

// check that all precommits are for blocks higher than the target

We should not simply report bad commit if we encounter a vote for a block that is not a descendent of the target because the sender might have included the vote as a proof of equivocation and the equivocator might have voted on a non-descendent of the target.

[andresilva]

The reason we're doing this is that currently we wouldn't handle such vote (and note the equivocation). The first check we do when receiving a vote is whether it is a descendent of (or equal to) the current round base, otherwise we do not process it (https://github.com/paritytech/finality-grandpa/blob/master/src/voter/voting_round.rs#L341).

I guess ideally we should be able to track this vote as misbehavior, since the voter isn't following the voting rules.