# P1-2: Host Resource Requirement Select — Investigation

## Status
- **C layer (P1-2a): COMPLETE** — `eval_select_simple` wired in `xce_select.c`
  (PEG grammar → AST → CSE, memoized eval). Scheduler integration (Phase 6) done.
  See `src/docs/resreq-ast-cse-design.md` for architecture reference.
- **Go bridge (P1-2b): PENDING** — `EvalSelectForHost` remains a stub returning
  `errNoCGO`. CGO boundary, resource value lookup from in-memory store, and
  `FilterHostByResReqSelect` are still to implement.

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## Background

The `select` field in an LSF-style resource requirement string (e.g.
`select[mem>4096 && type==X86_64]`) filters candidate hosts before slot
allocation. Currently `EvalSelectForHost` is a no-CGO stub that always returns
`errNoCGO`.

The full feature requires two layers:

1. **C parser + evaluator** — parse the boolean select expression into an
   AST and evaluate it against a host's resource values.
2. **Go bridge** — call the C evaluator via CGO and wire it into
   `FilterHostByResReqSelect` in the controllers layer.

---

## Sub-tasks

### #11 P1-2a: C select expression parser (PEG + eval) ✓ COMPLETE

**Files implemented:**
- `include/resreq.h` — expression-tree types, enums, `extern` declarations.
- `lib/xce_select.peg` — PEG grammar for select boolean expressions.
- `lib/xce_select.c` — `parse_select()`, `eval_select_simple()`, CSE tree.
- `lib/meson.build` — `xce_select.peg` in build.

### #12 P1-2b: Go EvalSelectForHost + FilterHostByResReqSelect (remaining)

**Files to touch:**
- `backend/clib.go` — Implement `EvalSelectForHost(hostname, selectExpr string) error`
  using `cgo.Handle` for callback context; add `#include "resreq.h"`, extern C
  declarations, `xce_select_destroy` helper.
- `controllers/hosts.go` — Implement `FilterHostByResReqSelect(hosts []models.HostInfo, expr string)`,
  update `preCheckHostRequest()` limit from 4094 → 1024 bytes.

**Open questions:**
- Does `EvalSelectForHost` look up live resource values from the in-memory
  store, or are values passed directly from C?
- `cgo.Handle` usage pattern — per-call or session-level?
- Input validation: max expression length (1024 bytes), allowed characters.