---
id: "bug-1883"
status: "open"
status_updated: "2026-09-29"
status_note: "newly logged"
component: "scheduler"
severity: "wrong-result"
timestamp: "2026-09-29T04:07:19Z"
last_seen: "2026-09-29T04:07:19Z"
occurrences: 1
files: ["src/scheduler/src/plugins/seqjob_allocator.c", "src/scheduler/src/plugin.c", "src/scheduler/src/schedobj.c", "src/scheduler/src/host_iter.c"]
tags: ["scheduler", "allocation", "compound", "seqjob"]
related: ["bug-1768", "bug-1766"]
gitea_issue: 334
gitea_synced_status: "open"
gitea_synced_hash: "12cdc1100496"
---

# bug-1883: xce_scheduler seqjob reuses a stale per-host want from an earlier COMPOUND/ALTER primary

## Description
**Observed:** (by code reading; no failing run captured yet)
When a COMPOUND or ALTER job has a `maxTasks == 1` primary after another
primary in the same attempt, seqjob's host search for the later primary
starts from the per-host want (`ctx->alloc_req.host`) left behind by the
earlier primary, with `slots:1` appended rather than replacing it:

```
.host on entry to seqjob host loop (2nd+ primary, shared ctx)
  = <previous primary's last per-host want, e.g. slots:N + rusage> + slots:1
```

- `plugin_prepare_allocation()` resets only `alloc_req.shared` in the
  global pass (`src/scheduler/src/plugin.c:617-619`), never `.host`.
- `seqjob_prepare_allocation()` appends `slots=1`
  (`src/scheduler/src/plugins/seqjob_allocator.c:118`), and
  `xvec_idval_store` appends without dedup (`src/lib/xvec.c:247-270`).
- The host iterator pre-filters candidates against this stale vector
  (`src/scheduler/src/host_iter.c:136,158`), silently dropping hosts that
  would satisfy the later primary's real `{slots:1}` want.
- A global limit on `slots` reads the first match via `get_resval_by_id`
  (`src/scheduler/src/resource.c:951-968`), so it sees the stale N, not 1.

parjob does not have this problem because it clears `.host` per bucket
(`src/scheduler/src/plugins/parjob_allocator.c:478-479`).

**Expected:** every primary's per-host want is computed from that primary's
own resource request only (the bug-1768 principle), so the later seqjob
primary is pre-filtered and limit-checked against `{slots:1}` plus its own
rusage.
**Impact:** wrong result. A later single-task primary of a COMPOUND/ALTER job
can pend (or pick a later host) although a host that fits it is free; pend
reasons can list hosts filtered on resources the primary never asked for.
Plain single-primary jobs are unaffected (fresh ctx).
**Found by:** review. Discovery/requirements research for spec
`scheduler-allocator-runner` (2026-09-28). That spec deliberately keeps this
behaviour (Requirement 3 AC 6) so the refactor stays behaviour-preserving;
fix here, separately, with its own test.

## Environment
- OS: no /etc/os-release; Docker container; NixOS kernel; kernel Linux 6.18.33 x86_64
- libc: glibc 2.42-47 (interpreter /nix/store/wb6rhpznjfczwlwx23zmdrrw74bayxw4-glibc-2.42-47/lib/ld-linux-x86-64.so.2)
- Toolchain: gcc (GCC) 15.2.0, meson 1.9.1
- Build: asan (xce.build)
- Code: scheduler-framework @ 51736fa4-dirty
- Deployment: n/a; found by code reading, not on a deployment. Affects any topology where the scheduler dispatches a COMPOUND/ALTER job with a `maxTasks == 1` primary after another primary.

## Reproduction
1. `git checkout 51736fa4` (branch `scheduler-framework`).
2. Not yet reproduced by a test. Proposed white-box unit test (same
   harness as `src/scheduler/testsuite/test_parjob_e2e_multislot.c`'s
   compound tests): one shared `xce_sched_ctx_t`; primary A `maxTasks=4`
   dispatched through parjob onto host-1 (4 slots) so `ctx->alloc_req.host`
   ends as `slots:4`; then `sched_ctx_set_primary()` for primary B
   `maxTasks=1`, and run seqjob against host-2 with exactly 1 free slot.
3. Expected RED: host-2 is pre-filtered out (stale `slots:4`) and B pends.
   Expected GREEN after a fix: B is placed on host-2.
- Frequency: deterministic by code reading (not yet executed)
- Evidence: not kept (no run yet). Code-reading trace in the research
  summary for spec `.kiro/specs/scheduler-allocator-runner/` (requirements
  Requirement 3 AC 6).

## Root cause
TODO: required for fixed/partial

## Fix
TODO: required for fixed/partial

## Verification
- Test: TODO: name + file; RED before the fix, GREEN after
- Suites: TODO: suite results and build profile
- Live: TODO: soak/deploy run, or n/a and why

## References
- Related: bug-1768 (per-primary sizing), bug-1766 (COMPOUND publish-only-if-all-succeed)
- Spec: `.kiro/specs/scheduler-allocator-runner/` (keeps this behaviour; out of scope there)