Only restart key unit if keyfile has actually changed

[cprussin]

This is perhaps a weaker but simpler optimization as #34.

Right now, when deploying with colmena, a whole bunch of services get restarted unnecessarily, because they depend on keyfiles and even though keys aren't changing, the keys are uploaded and so the systemd units for the keys are restarted (as are any units that depend on those).

#34 is probably quite challenging (but maybe there's a simple way to do it?) so I propose as a simpler solution let's just change

colmena/src/nix/host/key_uploader.template.sh

Line 21 in 19384f3

mv "$tmp" "$destination"

to:

if ! $(diff "$tmp" "$destination" > /dev/null); then
    mv "$tmp" "$destination"
else
    rm "$tmp"
fi

That may not prevent the network overhead of transferring unchanged keys, but at least it will prevent the unnecessary service restarts, and it's presumably much simpler than #34 .

Thoughts @zhaofengli ? If I put in a PR with this change would you accept it?

[cprussin]

hey @zhaofengli 👋 friendly nudge here, if you just tell me this is something you'd be willing to accept I'd be happy to put in a PR for it!