Running "nix run" with a diverted store, e.g.
$ nix run --store local?root=/tmp/nix nixpkgs.hello
stopped working when Nix became multithreaded, because
unshare(CLONE_NEWUSER) doesn't work in multithreaded processes. The
obvious solution is to terminate all other threads first, but 1) there
is no way to terminate Boehm GC marker threads; and 2) it appears that
the kernel has a race where unshare(CLONE_NEWUSER) will still fail for
some indeterminate amount of time after joining other threads.
So instead, "nix run" will now exec() a single-threaded helper ("nix
__run_in_chroot") that performs the actual unshare()/chroot()/exec().
Thus, instead of ‘--option <name> <value>’, you can write ‘--<name>
<value>’. So
--option http-connections 100
becomes
--http-connections 100
Apart from brevity, the difference is that it's not an error to set a
non-existent option via --option, but unrecognized arguments are
fatal.
Boolean options have special treatment: they're mapped to the
argument-less flags ‘--<name>’ and ‘--no-<name>’. E.g.
--option auto-optimise-store false
becomes
--no-auto-optimise-store
