this allows us great flexibility in how children are launched (since the
actual launching is done by a separate executable), makes fork no longer
needed in the core codebase (outside of runProgram, anyway), and we even
get to use linux vfork to its full potential to decrease the launch cost
of sandboxes to a constant factor (previously it was O(#drvs + #deps) of
the build graph, which obviously goes to n² quickly if you are unlucky.)
Change-Id: I66e2d1b20242dc24d708666ef325fb8725bd9296
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
eldritch horrors