The kill(2) in Apple's libc follows POSIX semantics, which means that
kill(-1, SIGKILL) will kill the calling process too. Since nix has no
way to distinguish between the process successfully killing everything
and the process being killed by a rogue builder in that case, it can't
safely conclude that killUser was successful.
Luckily, the actual kill syscall takes a parameter that determines
whether POSIX semantics are followed, so we can call that syscall
directly and avoid the issue on Apple.
Signed-off-by: Shea Levy <shea@shealevy.com>
For instance, it's pointless to keep copy-from-other-stores running if
there are no other stores, or download-using-manifests if there are no
manifests. This also speeds things up because we don't send queries
to those substituters.
For example, given a derivation with outputs "out", "man" and "bin":
$ nix-build -A pkg
produces ./result pointing to the "out" output;
$ nix-build -A pkg.man
produces ./result-man pointing to the "man" output;
$ nix-build -A pkg.all
produces ./result, ./result-man and ./result-bin;
$ nix-build -A pkg.all -A pkg2
produces ./result, ./result-man, ./result-bin and ./result-2.
This is required on systemd, which mounts filesystems as "shared"
subtrees. Changes to shared trees in a private mount namespace are
propagated to the outside world, which is bad.
In a private PID namespace, processes have PIDs that are separate from
the rest of the system. The initial child gets PID 1. Processes in
the chroot cannot see processes outside of the chroot. This improves
isolation between builds. However, processes on the outside can see
processes in the chroot and send signals to them (if they have
appropriate rights).
Since the builder gets PID 1, it serves as the reaper for zombies in
the chroot. This might turn out to be a problem. In that case we'll
need to have a small PID 1 process that sits in a loop calling wait().
I was bitten one time too many by Python modifying the Nix store by
creating *.pyc files when run as root. On Linux, we can prevent this
by setting the immutable bit on files and directories (as in ‘chattr
+i’). This isn't supported by all filesystems, so it's not an error
if setting the bit fails. The immutable bit is cleared by the garbage
collector before deleting a path. The only tricky aspect is in
optimiseStore(), since it's forbidden to create hard links to an
immutable file. Thus optimiseStore() temporarily clears the immutable
bit before creating the link.
unreachable paths. This matters when using --max-freed etc.:
unreachable paths could become reachable again, so it's nicer to
keep them if there is "real" garbage to be deleted. Also, don't use
readDirectory() but read the Nix store and delete invalid paths in
parallel. This reduces GC latency on very large Nix stores.
exception handler, otherwise throw an exception. We need to ignore
write errors in exception handlers to ensure that cleanup code runs
to completion if the other side of stderr has been closed
unexpectedly.
it requires a certain feature on the build machine, e.g.
requiredSystemFeatures = [ "kvm" ];
We need this in Hydra to make sure that builds that require KVM
support are forwarded to machines that have KVM support. Probably
this should also be enforced for local builds.
NixOS evaluation errors in particular look intimidating and
generally aren't very useful. Ideally the builtins.throw messages
should be self-contained.
nix-store -r (or some other operation) is started via ssh, it will
at least have a chance of terminating quickly when the connection is
killed. Right now it just runs to completion, because it never
notices that stderr is no longer connected to anything. Of course
it would be better if sshd would just send a SIGHUP, but it doesn't
(https://bugzilla.mindrot.org/show_bug.cgi?id=396).
~/.nix-defexpr, otherwise the attribute cannot be selected with the
`-A' option. Useful if you want to stick a Nix expression directly
in ~/.nix-defexpr.
