#Gentoo #jobserver revealed another problem with steve in particular, and (I believe) the jobserver protocol in general: blocking clients are prioritized over polling clients.
The problem is simple: when handling blocking reads, steve can issue a job token immediately. When handling a poll, it merely indicates that a token is available, and the client must issue another read request to get it. So if tokens are scarce and there are both blocking and polling clients running, the former are likely to be taking all the incoming tokens.
My idea of working around this is to implement temporary reservations. If a client polls for a token, we reserve one for it. The reserved token can afterwards be only read by the same client. This way, both blocking and polling clients get a token — the former get it immediately, the latter get it reserved for them. And if there are no tokens available, both get into a single FIFO queue, for a poor man's round-robin (steve also throttles all reads to one token at a time).
However, polls technically don't guarantee that the client will eventually read the token, so we need to handle reservation expirations as well.
