state store: persist NextAllocation correctly when upserting allocations (#25799)

---------

Co-authored-by: Tim Gross <tgross@hashicorp.com>

.changelog/25799.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+reconnecting client: fix issue where reconcile strategy was sometimes ignored
+```

nomad/state/state_store.go

@@ -4185,6 +4185,11 @@ func (s *StateStore) upsertAllocsImpl(index uint64, allocs []*structs.Allocation
 			alloc.ModifyIndex = index
 			alloc.AllocModifyIndex = index
 
+			// Carry over NextAllocation from existing
+			if exist.NextAllocation != "" {
+				alloc.NextAllocation = exist.NextAllocation
+			}
+
 			// Keep the clients task states
 			alloc.TaskStates = exist.TaskStates
 

nomad/state/state_store_test.go

@@ -6587,6 +6587,31 @@ func TestStateStore_UpsertAlloc_ChildJob(t *testing.T) {
 	require.False(t, watchFired(ws))
 }
 
+func TestStateStore_UpsertAlloc_NextAllocation(t *testing.T) {
+	ci.Parallel(t)
+
+	state := testStateStore(t)
+
+	alloc1 := mock.Alloc()
+	alloc2 := mock.Alloc()
+	alloc2.PreviousAllocation = alloc1.ID
+
+	err := state.UpsertAllocs(structs.MsgTypeTestSetup, 1000, []*structs.Allocation{alloc1, alloc2})
+	must.NoError(t, err)
+
+	// alloc1 should have the correct NextAllocation
+	actual, err := state.AllocByID(nil, alloc1.ID)
+	must.Eq(t, actual.NextAllocation, alloc2.ID)
+
+	err = state.UpsertAllocs(structs.MsgTypeTestSetup, 1001, []*structs.Allocation{alloc2, alloc1})
+	must.NoError(t, err)
+
+	// upsert in a different order, alloc1 should still have the correct NextAllocation
+	actual, err = state.AllocByID(nil, alloc1.ID)
+	must.NoError(t, err)
+	must.Eq(t, actual.NextAllocation, alloc2.ID)
+}
+
 func TestStateStore_UpdateAlloc_Alloc(t *testing.T) {
 	ci.Parallel(t)