hydra-evaluator: Rewrite in Rust - #1608
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@Conni2461 Have an opinion? The LLM was imitating what you had done in the other crates :) (if I am interpreting the 7d9f536 squash correctly)
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Port hydra-evaluator from C++ to Rust, removing the last C++ executable from the project. The new implementation uses `sqlx` and `tokio`, matching the async patterns already established by hydra-queue-runner and hydra-builder. The scheduling logic is a fairly direct port; what gets explicit treatment is the error-recovery story, which did not survive translation on its own. The C++ evaluator leaned on pqxx throwing on a broken connection and the process exiting into systemd's `Restart=always`, which resynced all state; `sqlx`'s conveniences paper over connection loss instead. So the `db` crate's listener now disables `sqlx`'s eager reconnect so that connection loss surfaces as a stream error (see the comment there — silently dropped NOTIFYs would otherwise strand user-triggered evaluations), and both the main loop and the DB monitor exit for a supervisor restart on a broken connection. Other behaviors deliberately carried over from the C++: forgetting a disabled or deleted jobset kills its in-flight `hydra-eval-jobset` (the C++ `Pid` destructor did this), a failed spawn runs the same cleanup as a failed eval so `startTime` cannot stick forever, and the reaper holds the scheduler lock across its cleanup transaction so a freshly restarted eval cannot have its `startTime` clobbered. The SQL lives in `src/queries.rs` as compile-time-checked `sqlx::query!` macros on an extension trait over `db::Connection` — kept in this crate rather than the shared `db` crate because this slice of the schema only matters to the evaluator. The `db` crate contributes pooled acquisition (`Database::from_env`, reading `HYDRA_DATABASE_URL` via shared constants the queue runner now also uses) and a `raw()` escape hatch for exactly this pattern. Timestamps are `i32` to match the schema's `INT4` (see the comment on `JobsetSchedulingInfo`). Regenerating the `.sqlx` cache also uncovered a latent bug in the db crate: `get_drv_path_from_build_step` treated `drvPath` as nullable, but the schema declares it `not null`. Build/packaging: the C++ build is removed from `subprojects/hydra/meson.build`; the new `subprojects/hydra-evaluator/package.nix` follows the same per-binary pattern as `hydra-queue-runner` and `hydra-builder` (properly sharing the dep crates between these builds is left for later); the NixOS web-app module gains an `evaluatorExecutable` option matching how the queue runner and builder are discovered via their own modules; and stale references in `hydra-tests/meson.build` and `dev-shell.nix` are cleaned up. Co-authored-by: Jörg Thalheim <joerg@thalheim.io> Co-authored-by: Claude <noreply@anthropic.com>
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Port hydra-evaluator from C++ to Rust, removing the last C++ executable from the project. The new implementation uses sqlx and tokio, matching the async patterns already established by hydra-queue-runner and hydra-builder.
Build/packaging changes:
subprojects/hydra/meson.buildhydra-queue-runner/package.nixtosubprojects/rust-package.nixwith named outputs (queue_runner,builder,evaluator)evaluatorExecutableoption to the NixOS web-app module, matching how the queue runner and builder are discovered via their own moduleshydra-tests/meson.buildanddev-shell.nix