The uid split a while back caused the web interface to create GC roots in /nix/var/nix/gcroots/per-user/hydra-www, where they wouldn't be purged by hydra-update-gc-roots. Thus restarted builds would accumulate forever. The fix is to keep the roots in a shared directory with gid=hydra.
To start hacking on Hydra, run:
$ ./dev-shell
$ ./configure $configureFlags --prefix=/opt/hydra
$ make
$ make install
Or, if you just want to build from source (on x86_64-linux):
$ nix-build -A build.x86_64-linux release.nix
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