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+---
+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)