When `max-jobs = 0`, our async semaphores have null capacity and
therefore no slot token can be acquired.
In addition, a derivation with `preferLocalBuild = true` with `max-jobs
= 0` which is not a built-in derivation is not considered suitable for
being possible to build.
Our documentation in `max-jobs` writes:
> `0` is useful when using remote builders to prevent any local builds
(except for preferLocalBuild derivation attribute which executes locally
regardless).
A valid interpretation is that `preferLocalBuild` derivations (all of
them) will always execute even if `max-jobs = 0`.
This CL achieves this promise by enabling `preferLocalBuild = true`
derivations to draw in priority capacity from a `preferLocalBuild`-only
pool and then fall back to the build pool (`maxBuildJobs`).
If there's non-zero max-jobs, we make the `preferLocalBuild`-only pool
empty.
If there's zero max-jobs, we make the `preferLocalBuild`-only pool
non-empty (capacity: 1).
Remark 1: if `max-jobs = 0`, no non-`preferLocalBuild = true`
derivations can be built without a remote builder and `preferLocalBuild
= true` derivations can be built one at a time if `extra-local-jobs` is
not overridden.
Remark 2: if `max-jobs != 0`, no `preferLocalBuild = true` derivations
can bypass capacity limits, i.e. `max-jobs` unless `extra-local-jobs` is
overridden.
Fixes#855.
Change-Id: I00a0acac7375a881aec6281ebb13a4530b26e460
Signed-off-by: Raito Bezarius <raito@lix.systems>
Raito Bezarius
092064db4c