Use ghc -j when building a single package

[Blaisorblade]

cabal doesn't pass -j to GHC, to avoid launching too many processes. But that's the case even when it builds one package, in particular, the last one, after all dependencies are done. That's annoying when running cabal install on big packages, say Agda.

And while this is clearly a hack, it guarantees that not too many processes are created.

By #976 (comment), cabal build can pass -j but cabal install doesn't.

[nh2]

See also #976 (comment)

[Blaisorblade]

Cc @ezyang @23Skidoo.

[ezyang]

To generalize the hack, what if we pass -j to any package which is explicitly specified as a target?

[Blaisorblade]

It seems to me, if I specify two packages explicitly, that leads to too many jobs​. And if I use --only-dependencies, that leads to too few jobs.
The "hack idea" is "ensure trivially that at most one ghc -j is running", where "trivially" means "with limited code changes" and with a trivial proof.

A generalization consistent with this would be to use GHC -j also during the build, if there's a single package in the package queue. That can happen (seldom) before the end when all further packages depend on one. Is this easy/feasible to implement?

[phadej]

I'd rather introduce --j-ghc or some other explicit flag.

There is a clash between user and script interfaces; in latter you don't want any magical hacks to change behavior