No wonder our disk was filling up

Dependencies

• [2] SZPBGW2N Fix a race condition in hydra-update-gc-roots
• [3] KJQWSRCC Speed up hydra-update-gc-roots
• [4] WZ3AEJ67 * hydra_update_gc_roots.pl registers build outputs that should be kept

Change contents

• replacement in src/script/hydra-update-gc-roots at line 139
  [3.2864]→[2.384:463](∅→∅)
  − #unlink "$gcRootsDir/$link" or warn "cannot remove $gcRootsDir/$link";
  [3.2864]

  [3.2942]
  + unlink "$gcRootsDir/$link" or warn "cannot remove $gcRootsDir/$link";