Incremental optimisation requires creating links in /nix/store/.links
to all files in the store. However, this means that if we delete a
store path, no files are actually deleted because links in
/nix/store/.links still exists. So we need to check /nix/store/.links
for files with a link count of 1 and delete them.
optimiseStore() now creates persistent, content-addressed hard links
in /nix/store/.links. For instance, if it encounters a file P with
hash H, it will create a hard link
P' = /nix/store/.link/<H>
to P if P' doesn't already exist; if P' exist, then P is replaced by a
hard link to P'. This is better than the previous in-memory map,
because it had the tendency to unnecessarily replace hard links with a
hard link to whatever happened to be the first file with a given hash
it encountered. It also allows on-the-fly, incremental optimisation.
To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5"). (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.) However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory. queryPathFromHashPart() prevents this by doing a cheap
database lookup.
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().
In chroot builds, set the host name to "localhost" and the domain name
to "(none)" (the latter being the kernel's default). This improves
determinism a bit further.
P.S. I have to idea what UTS stands for.
This improves isolation a bit further, and it's just one extra flag in
the unshare() call.
P.S. It would be very cool to use CLONE_NEWPID (to put the builder in
a private PID namespace) as well, but that's slightly more risky since
having a builder start as PID 1 may cause problems.
On Linux it's possible to run a process in its own network namespace,
meaning that it gets its own set of network interfaces, disjunct from
the rest of the system. We use this to completely remove network
access to chroot builds, except that they get a private loopback
interface. This means that:
- Builders cannot connect to the outside network or to other processes
on the same machine, except processes within the same build.
- Vice versa, other processes cannot connect to processes in a chroot
build, and open ports/connections do not show up in "netstat".
- If two concurrent builders try to listen on the same port (e.g. as
part of a test), they no longer conflict with each other.
This was inspired by the "PrivateNetwork" flag in systemd.
Systemd can start the Nix daemon on demand when the Nix daemon socket
is first accessed. This is signalled through the LISTEN_FDS
environment variable, so all we need to do is check for that and then
use file descriptor 3 as the listen socket instead of creating one
ourselves.
