Currently machine specification (`/etc/nix/machine`) parser fails
with a vague exception if the file had incorrect format.
This commit adds verbose exceptions and unit-tests for the parser.
This is already used by Hydra, and is very useful when materializing
a remote builder list from service discovery. This allows the service
discovery tool to only sync one file instead of two.
This seems more correct. It also means one can specify the features a
store should support with --store and remote-store=..., which is useful.
I use this to clean up the build remotes test.
You can now include files via the "builders" option, using the syntax
"@<filename>". Having only one option makes it easier to override
builders completely.
For backward compatibility, the default is "@/etc/nix/machines", or
"@<filename>" for each file name in NIX_REMOTE_SYSTEMS.
This is useful for one-off situations where you want to specify a
builder on the command line instead of having to mess with
nix.machines. E.g.
$ nix-build -A hello --argstr system x86_64-darwin \
--option builders 'root@macstadium1 x86_64-darwin'
will perform the specified build on "macstadium1".
It also removes the need for a separate nix.machines file since you
can specify builders in nix.conf directly. (In fact nix.machines is
yet another hack that predates the general nix.conf configuration
file, IIRC.)
Note: this option is supported by the daemon for trusted users. The
fact that this allows trusted users to specify paths to SSH keys to
which they don't normally have access is maybe a bit too much trust...
