this halves sandbox launch overhead, reducing the build time for 3000
trivial runCommand derivations on our machine from 80 seconds to 44s.
as a nice side effect we also get better error message propagation in
some cases, most notably setgroups failing when run in lix sandboxes.
Change-Id: Ia7c50a844915d9b8a20475b90b2d0179fd2fff34
Sandboxed builds are now much cheaper to launch on Linux with 50% lower management
overhead. This will mostly be noticeable when building derivation trees containing
many small derivations like nixpkgs' writeFile or runCommand with scripts that
very quickly. In synthetic tests we have seen build times of 3000 small runCommand
drop from 80 seconds to 44 seconds, which is the most optimistic case in practice.
eldritch horrors
56988d8605