fix(scheduler): reset the per-host want at each primary's job-level prepare pass (bug-1883)

A COMPOUND/ALTER job's primaries share one ctx. The job-level prepare pass
reset only alloc_req.shared, and the allocators append their slots want
into alloc_req.host, so a later primary's host pre-filter (and a global
limit on slots) read the previous primary's last per-host want: a 1-task
primary after a 4-task one was filtered on slots:4 and pended although a
host with its one free slot existed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/scheduler/src/plugin.c

@@ -615,8 +615,15 @@ int plugin_prepare_allocation(xce_sched_core_t* core, xce_sched_ctx_t* ctx)
     }
 
     if (ctx->worker == NULL) {
+        /* bug-1883: the job-level pass also resets .host. The allocators'
+         * hooks store this primary's job-level slots want there (the host
+         * pre-filter and a global limit on slots read it); a COMPOUND's
+         * previous primary left its last per-host want in .host, which the
+         * append-only store would otherwise extend, not replace. */
         xvec_idval_fini(&(ctx->alloc_req.shared));
         xvec_idval_init(&(ctx->alloc_req.shared), 8);
+        xvec_idval_fini(&(ctx->alloc_req.host));
+        xvec_idval_init(&(ctx->alloc_req.host), 8);
     } else {
         xvec_idval_fini(&(ctx->alloc_req.host));
         xvec_idval_init(&(ctx->alloc_req.host), 8);

src/scheduler/testsuite/test_parjob_e2e_multislot.c

@@ -875,6 +875,56 @@ static void test_failed_compound_leaves_no_partial_allocation(void)
     TEST_ASSERT_NULL(sjob->allocs);
 }
 
+/*
+ * bug-1883: a COMPOUND's primaries share one ctx, and the job-level prepare
+ * pass reset only alloc_req.shared, so a later 1-task (seqjob) primary's
+ * host pre-filter ran against the previous primary's per-host want with
+ * slots:1 appended. Host A (4 slots) takes the 4-task primary, leaving
+ * slots:4 behind; host B (1 slot) fits the 1-task primary, but the stale
+ * slots:4 rejected it and the job pended.
+ */
+static void test_compound_later_seqjob_primary_ignores_previous_want(void)
+{
+    xce_scheduler_t* sched = start_sched();
+    xce_core_event_t ev;
+    xce_worker_t*    worker_ptrs[2];
+    xce_decision_t*  decision;
+    int              i;
+
+    memset(&ev, 0, sizeof(ev));
+    ev.type                 = XCE_CORE_EVENT_QUEUE_UPDATE;
+    ev.u.queue_update.queue = make_wire_queue("cmp5_queue", 1);
+    TEST_ASSERT_TRUE(xce_core_push_events(&(sched->core), &ev, 1) >= 0);
+
+    worker_ptrs[0] = make_wire_worker(WORKER_A_NAME, 4.0f);
+    worker_ptrs[1] = make_wire_worker(WORKER_B_NAME, 1.0f);
+    memset(&ev, 0, sizeof(ev));
+    ev.type                    = XCE_CORE_EVENT_WORKERS_PUSH;
+    ev.u.workers_push.nworkers = 2;
+    ev.u.workers_push.workers  = worker_ptrs;
+    TEST_ASSERT_TRUE(xce_core_push_events(&(sched->core), &ev, 1) >= 0);
+
+    decision = e2e_dispatch_resreq(sched, JOB_ID + 8, "cmp5_queue",
+                                   "4*{rusage[mem=1]} + 1*{rusage[mem=1]}");
+    TEST_ASSERT_NOT_NULL(decision);
+    TEST_ASSERT_EQUAL_INT_MESSAGE(XCE_DECISION_DISPATCH, decision->op,
+        "the 1-task primary fits host B's free slot -- it must not be "
+        "filtered on the 4-task primary's per-host want");
+    TEST_ASSERT_EQUAL_FLOAT(5.0f,
+        (float)e2e_alloc_total_slots(decision->u.alloc));
+    TEST_ASSERT_EQUAL_INT(2, decision->u.alloc->nhosts);
+    for (i = 0; i < decision->u.alloc->nhosts; i++) {
+        xce_job_alloc_t one = *(decision->u.alloc);
+
+        one.nhosts = 1;
+        one.allocs = &(decision->u.alloc->allocs[i]);
+        TEST_ASSERT_EQUAL_FLOAT(
+            (strcmp(decision->u.alloc->allocs[i]->host, WORKER_A_NAME) == 0)
+                ? 4.0f : 1.0f,
+            (float)e2e_alloc_total_slots(&one));
+    }
+}
+
 int main(void)
 {
     UNITY_BEGIN();
@@ -887,5 +937,6 @@ int main(void)
     RUN_TEST(test_compound_allocation_includes_every_primary);
     RUN_TEST(test_compound_primaries_on_same_host_merge);
     RUN_TEST(test_failed_compound_leaves_no_partial_allocation);
+    RUN_TEST(test_compound_later_seqjob_primary_ignores_previous_want);
     return UNITY_END();
 }