Currently Hydra duplicates lots of functionality that is in Nix. This is bad because it leads to either the Nix or Hydra versions of a feature being undertested and broken (e.g. remote builds are very finicky), or missing features on one side (Hydra doesn't support ssh-ng://).

The solution is to dedup, fixing both problems, and lowing our maintenance burden across the board.

I will try to break this down into steps, updating this list accordingly

Easier

• Generic SSH logic

  • Nix: src/libstore/ssh.cc in Nix
  • Hydra: top of src/hydra-queue-runner/build-remote.cc

  Notable differences: Hydra passes a number of extra flags to ssh, evidently based on sysadmin war stories.

• Legacy SSH protocol

  • Nix: src/libstore/legacy-ssh-store.cc in Nix
  • Hydra: Much of src/hydra-queue-runner/build-remote.cc

  Notable differences: Hydra doesn't issue builds blocking and synchronously, so it can cancel them and handle logs as they occur. Probably other things, TBD.

  • Split up logic so subsequent refactors are easier to both write and review Split the buildRemote function #1180

  • ServeProto::Command::QueryValidPath

    • Nix: Factoring out parts of the serve protocol for Hydra to share nix#6134
    • Hydra: Begin factoring out the protocol code #1165

  • ServeProto::Command::ImportPaths

  • ServeProto::Command::BuildDerivation

    • Hydra part-way: Use factored-out BuildResult serializer #1318

  • ServeProto::Command::QueryPathInfos

    • Nix: Factor out ServeProto::Serialiser<UnkeyedValidPathInfo> and test nix#9560

  • ServeProto::Command::DumpStorePath

• Use Store abstraction after previous two are complete. Support arbitrary stores for building #1200

• Use the same notion of a Machine config for remote builders. NixOS/nix@ebc9f36 was a Nix-side refactor to prepare for this, but then the Hydra side was not done.

  • Use Nix's Machine type in a minimal way #1341 Use Nix's Machine type
  • Dedup with nix: use nix::Machine::parseConfig #1386 Use Nix's Machine parser

  See also Why is machine loading rounded? #484

  See also Triage and break up the global settings nix#5638, where we would like more of the machine stuff to go in the Store config. That ties this to the previous checkbox.

Farther off

• Duplicate Scheduling logic, Nix's build/*.cc vs Hydra

CC @rickynils @Ma27 @grahamc