agent: task groups in system jobs do not support scaling stanzas.

command/agent/job_endpoint.go

@@ -391,6 +391,18 @@ func (s *HTTPServer) jobUpdate(resp http.ResponseWriter, req *http.Request,
 		}
 	}
 
+	// GH-8481. Jobs of type system can only have a count of 1 and therefore do
+	// not support scaling. Even though this returns an error on the first
+	// occurrence, the error is generic but detailed enough that an operator
+	// can fix the problem across multiple task groups.
+	if *args.Job.Type == api.JobTypeSystem {
+		for _, tg := range args.Job.TaskGroups {
+			if tg.Scaling != nil {
+				return nil, CodedError(400, "Task groups with job type system do not support scaling stanzas")
+			}
+		}
+	}
+
 	sJob, writeReq := s.apiJobAndRequestToStructs(args.Job, req, args.WriteRequest)
 	regReq := structs.JobRegisterRequest{
 		Job:            sJob,

command/agent/job_endpoint_test.go

@@ -448,6 +448,36 @@ func TestHTTP_JobQuery_Payload(t *testing.T) {
 	})
 }
 
+func TestHTTP_jobUpdate_systemScaling(t *testing.T) {
+	t.Parallel()
+	httpTest(t, nil, func(s *TestAgent) {
+		// Create the job
+		job := MockJob()
+		job.Type = helper.StringToPtr("system")
+		job.TaskGroups[0].Scaling = &api.ScalingPolicy{Enabled: helper.BoolToPtr(true)}
+		args := api.JobRegisterRequest{
+			Job: job,
+			WriteRequest: api.WriteRequest{
+				Region:    "global",
+				Namespace: api.DefaultNamespace,
+			},
+		}
+		buf := encodeReq(args)
+
+		// Make the HTTP request
+		req, err := http.NewRequest("PUT", "/v1/job/"+*job.ID, buf)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		respW := httptest.NewRecorder()
+
+		// Make the request
+		obj, err := s.Server.JobSpecificRequest(respW, req)
+		assert.Nil(t, obj)
+		assert.Equal(t, CodedError(400, "Task groups with job type system do not support scaling stanzas"), err)
+	})
+}
+
 func TestHTTP_JobUpdate(t *testing.T) {
 	t.Parallel()
 	httpTest(t, nil, func(s *TestAgent) {