Typecheck Hack projects on push

[jwatzman]

And reject the push if typechecking fails. In the future we may want to also
look for a Hack project entirely missing an .hhconfig file, but this will do
for now.

[pvh]

Looks good!

[RdeWilde]

Very cool! Could it problems at deploy-time causing a timeout? If so it would be usefull to have an option to disable it (or is there already).

[jwatzman]

Very cool! Could it problems at deploy-time causing a timeout? If so it would be usefull to have an option to disable it (or is there already).

To be clear, you're concerned that the typecheck will take too long, and hit some sort of timeout during deploy? How long is that timeout? Unless it is very short, I'm not too concerned. The checker is pretty efficient -- we can check Facebook's code (>10M LOC) from scratch in about 40 seconds on a 32-core machine. While Heroku is probably running the deploy on a machine an order of magnitude less beefy, projects being deployed are probably many orders of magnitude smaller. Since the time scales roughly linearly with both LOC and CPUs, I'd expect a check time of only maybe 5-10 seconds on the largest projects, with the vast majority being significantly faster than that.

I'd also prefer not to have an option to disable this, if we can get away with it. It's very important that Hack errors really are treated as real errors, the sort that you absolutely would not want to deploy with. They are definite inconsistencies in the code. Here at Facebook, a type error in trunk is considered to be "trunk is broken" -- and the only reason this is a post-commit check instead of a real commit hook is technicalities having to do with how exactly our commit process works. A type error in the production branch will block a push; this pretty regularly catches bugs from non-textual merge conflicts and the like.

Do those numbers make sense to you? It's easily possible I've mis-estimated something or am otherwise wrong here :)

PS: sorry for the delayed response, I somehow missed the github notification for this!