backport of commit 1f94419

.changelog/24363.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+core: add the possibility to scale system jobs between 0 and 1
+```

e2e/scaling/input/namespace_default_system.nomad

@@ -0,0 +1,25 @@
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: BUSL-1.1
+
+job "system_job" {
+  type = "system"
+
+  group "system_job_group" {
+
+    task "system_task" {
+      driver = "docker"
+
+      config {
+        image = "busybox:1"
+
+        command = "/bin/sh"
+        args    = ["-c", "sleep 15000"]
+      }
+
+      env {
+        version = "1"
+      }
+    }
+  }
+}
+

e2e/scaling/scaling.go

@@ -11,6 +11,7 @@ import (
 	"github.com/hashicorp/nomad/e2e/framework"
 	"github.com/hashicorp/nomad/helper/pointer"
 	"github.com/hashicorp/nomad/helper/uuid"
+	"github.com/hashicorp/nomad/nomad/structs"
 )
 
 type ScalingE2ETest struct {
@@ -27,7 +28,6 @@ func init() {
 			new(ScalingE2ETest),
 		},
 	})
-
 }
 
 func (tc *ScalingE2ETest) BeforeAll(f *framework.F) {
@@ -165,3 +165,101 @@ func (tc *ScalingE2ETest) TestScalingNamespaces(f *framework.F) {
 		"Nomad e2e testing", false, nil, &aWriteOpts)
 	f.NoError(err)
 }
+
+// TestScalingBasic performs basic scaling e2e tests within a single namespace using
+// using a SystemScheduler.
+func (tc *ScalingE2ETest) TestScalingBasicWithSystemSchedule(f *framework.F) {
+	t := f.T()
+	nomadClient := tc.Nomad()
+
+	// Register a system job with a scaling policy without a group count, it should
+	// default to 1 per node.
+
+	jobID := "test-scaling-" + uuid.Generate()[0:8]
+	e2eutil.RegisterAndWaitForAllocs(t, nomadClient, "scaling/input/namespace_default_system.nomad", jobID, "")
+
+	jobs := nomadClient.Jobs()
+	initialAllocs, _, err := jobs.Allocations(jobID, true, nil)
+	f.NoError(err)
+
+	nodeStubList, _, err := nomadClient.Nodes().List(&api.QueryOptions{Namespace: "default"})
+	f.NoError(err)
+
+	// A system job will spawn an allocation per node, we need to know how many nodes
+	// there are to know how many allocations to expect.
+	numberOfNodes := len(nodeStubList)
+
+	f.Equal(numberOfNodes, len(initialAllocs))
+	allocIDs := e2eutil.AllocIDsFromAllocationListStubs(initialAllocs)
+
+	// Wait for allocations to get past initial pending state
+	e2eutil.WaitForAllocsNotPending(t, nomadClient, allocIDs)
+
+	// Try to scale beyond 1
+	testMeta := map[string]interface{}{"scaling-e2e-test": "value"}
+	scaleResp, _, err := tc.Nomad().Jobs().Scale(jobID, "system_job_group", pointer.Of(3),
+		"Nomad e2e testing", false, testMeta, nil)
+
+	f.Error(err)
+	f.Nil(scaleResp)
+
+	// The same allocs should be running.
+	jobs = nomadClient.Jobs()
+	allocs1, _, err := jobs.Allocations(jobID, true, nil)
+	f.NoError(err)
+
+	f.Equal(len(initialAllocs), len(allocs1))
+
+	for i, a := range allocs1 {
+		f.Equal(a.ID, initialAllocs[i].ID)
+	}
+
+	// Scale down to 0
+	testMeta = map[string]interface{}{"scaling-e2e-test": "value"}
+	scaleResp, _, err = tc.Nomad().Jobs().Scale(jobID, "system_job_group", pointer.Of(0),
+		"Nomad e2e testing", false, testMeta, nil)
+	f.NoError(err)
+	f.NotEmpty(scaleResp.EvalID)
+
+	// Assert job is still up but no allocs are running
+	stopedAllocs, _, err := jobs.Allocations(jobID, false, nil)
+	f.NoError(err)
+
+	f.Equal(numberOfNodes, len(filterAllocsByDesiredStatus(structs.AllocDesiredStatusStop, stopedAllocs)))
+	f.Equal(numberOfNodes, len(stopedAllocs))
+
+	// Scale up to 1 again
+	testMeta = map[string]interface{}{"scaling-e2e-test": "value"}
+	scaleResp, _, err = tc.Nomad().Jobs().Scale(jobID, "system_job_group", pointer.Of(1),
+		"Nomad e2e testing", false, testMeta, nil)
+	f.NoError(err)
+	f.NotEmpty(scaleResp.EvalID)
+
+	// Wait for new allocation to get past initial pending state
+	e2eutil.WaitForAllocsNotPending(t, nomadClient, allocIDs)
+
+	// Assert job is still running and there is a running allocation again
+	allocs, _, err := jobs.Allocations(jobID, true, nil)
+	f.NoError(err)
+	f.Equal(numberOfNodes*2, len(allocs))
+
+	f.Equal(numberOfNodes, len(filterAllocsByDesiredStatus(structs.AllocDesiredStatusStop, allocs)))
+	f.Equal(numberOfNodes, len(filterAllocsByDesiredStatus(structs.AllocDesiredStatusRun, allocs)))
+
+	// Remove the job.
+	_, _, err = tc.Nomad().Jobs().Deregister(jobID, true, nil)
+	f.NoError(err)
+	f.NoError(tc.Nomad().System().GarbageCollect())
+}
+
+func filterAllocsByDesiredStatus(status string, allocs []*api.AllocationListStub) []*api.AllocationListStub {
+	res := []*api.AllocationListStub{}
+
+	for _, a := range allocs {
+		if a.DesiredStatus == status {
+			res = append(res, a)
+		}
+	}
+
+	return res
+}

nomad/job_endpoint.go

@@ -1027,8 +1027,9 @@ func (j *Job) Scale(args *structs.JobScaleRequest, reply *structs.JobRegisterRes
 	if job == nil {
 		return structs.NewErrRPCCoded(404, fmt.Sprintf("job %q not found", args.JobID))
 	}
-	if job.Type == structs.JobTypeSystem {
-		return structs.NewErrRPCCoded(http.StatusBadRequest, `cannot scale jobs of type "system"`)
+
+	if job.Type == structs.JobTypeSystem && *args.Count > 1 {
+		return structs.NewErrRPCCoded(http.StatusBadRequest, `jobs of type "system" can only be scaled between 0 and 1`)
 	}
 
 	// Since job is going to be mutated we must copy it since state store methods

nomad/job_endpoint_test.go

@@ -8271,20 +8271,43 @@ func TestJobEndpoint_Scale_SystemJob(t *testing.T) {
 	mockSystemJob := mock.SystemJob()
 	must.NoError(t, state.UpsertJob(structs.MsgTypeTestSetup, 10, nil, mockSystemJob))
 
+	// Scale to 0
 	scaleReq := &structs.JobScaleRequest{
 		JobID: mockSystemJob.ID,
 		Target: map[string]string{
 			structs.ScalingTargetGroup: mockSystemJob.TaskGroups[0].Name,
 		},
-		Count: pointer.Of(int64(13)),
+		Count: pointer.Of(int64(0)),
 		WriteRequest: structs.WriteRequest{
 			Region:    DefaultRegion,
 			Namespace: mockSystemJob.Namespace,
 		},
 	}
-	var resp structs.JobRegisterResponse
+
+	resp := structs.JobRegisterResponse{}
+	err := msgpackrpc.CallWithCodec(codec, "Job.Scale", scaleReq, &resp)
+	must.NoError(t, err)
+
+	// Scale to a negative number
+	scaleReq.Count = pointer.Of(int64(-5))
+
+	resp = structs.JobRegisterResponse{}
+	must.ErrorContains(t, msgpackrpc.CallWithCodec(codec, "Job.Scale", scaleReq, &resp),
+		`400,scaling action count can't be negative`)
+
+	// Scale back to 1
+	scaleReq.Count = pointer.Of(int64(1))
+
+	resp = structs.JobRegisterResponse{}
+	err = msgpackrpc.CallWithCodec(codec, "Job.Scale", scaleReq, &resp)
+	must.NoError(t, err)
+
+	// Scale beyond 1
+	scaleReq.Count = pointer.Of(int64(13))
+
+	resp = structs.JobRegisterResponse{}
 	must.ErrorContains(t, msgpackrpc.CallWithCodec(codec, "Job.Scale", scaleReq, &resp),
-		`400,cannot scale jobs of type "system"`)
+		`400,jobs of type "system" can only be scaled between 0 and 1`)
 }
 
 func TestJobEndpoint_Scale_BatchJob(t *testing.T) {

nomad/structs/structs.go

@@ -7055,10 +7055,6 @@ func (tg *TaskGroup) Canonicalize(job *Job) {
 		tg.EphemeralDisk = DefaultEphemeralDisk()
 	}
 
-	if job.Type == JobTypeSystem && tg.Count == 0 {
-		tg.Count = 1
-	}
-
 	if tg.Scaling != nil {
 		tg.Scaling.Canonicalize(job, tg, nil)
 	}