cli: fix panic when starting stopped jobs with no scaling policies (#26131)

Restoring scaling policies during the start of a stopped job did not account for
jobs that didn't have any scaling policies, and led to a panic when users tried
to restart such jobs.

.changelog/26131.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+cli: Fix panic when restarting stopped job with no scaling policies
+```

command/job_start.go

@@ -159,7 +159,13 @@ func (c *JobStartCommand) Run(args []string) int {
 
 			sps := lastJob.GetScalingPoliciesPerTaskGroup()
 			for _, tg := range job.TaskGroups {
-				tg.Scaling.Enabled = sps[*tg.Name].Enabled
+				// guard for nil values in case the tg doesn't have any scaling policy
+				if sps[*tg.Name] != nil {
+					if tg.Scaling == nil {
+						tg.Scaling = &api.ScalingPolicy{}
+					}
+					tg.Scaling.Enabled = sps[*tg.Name].Enabled
+				}
 			}
 		}
 	}

command/job_start_test.go

@@ -24,20 +24,24 @@ var _ cli.Command = (*JobStartCommand)(nil)
 func TestStartCommand(t *testing.T) {
 	ci.Parallel(t)
 
-	srv, _, addr := testServer(t, true, func(c *agent.Config) {
-		c.DevMode = true
-	})
-	defer srv.Shutdown()
-
-	ui := cli.NewMockUi()
-	cmd := &JobStartCommand{
-		Meta: Meta{
-			Ui:          ui,
-			flagAddress: addr,
-		},
+	testSetup := func() (*agent.TestAgent, *JobStartCommand, string) {
+		srv, _, addr := testServer(t, true, func(c *agent.Config) {
+			c.DevMode = true
+		})
+		ui := cli.NewMockUi()
+		cmd := &JobStartCommand{
+			Meta: Meta{
+				Ui:          ui,
+				flagAddress: addr,
+			},
+		}
+		return srv, cmd, addr
 	}
 
 	t.Run("succeeds when starting a stopped job", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
 		job := testJob(uuid.Generate())
 
 		client, err := cmd.Meta.Client()
@@ -72,6 +76,9 @@ func TestStartCommand(t *testing.T) {
 	})
 
 	t.Run("succeeds when starting a stopped job with disabled scaling policies and no submissions", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
 		job := testJob(uuid.Generate())
 
 		client, err := cmd.Meta.Client()
@@ -100,6 +107,9 @@ func TestStartCommand(t *testing.T) {
 	})
 
 	t.Run("succeeds when starting a stopped job with enabled scaling policies", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
 		job := testJob(uuid.Generate())
 
 		client, err := cmd.Meta.Client()
@@ -135,7 +145,47 @@ func TestStartCommand(t *testing.T) {
 
 	})
 
+	t.Run("succeeds when starting a stopped job with no scaling policies", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
+		job := testJob(uuid.Generate())
+
+		client, err := cmd.Meta.Client()
+		must.NoError(t, err)
+
+		job.TaskGroups[0].Scaling = nil
+
+		jsonBytes, err := json.Marshal(job)
+		must.NoError(t, err)
+
+		_, _, err = client.Jobs().RegisterOpts(job, &api.RegisterOptions{
+			Submission: &api.JobSubmission{
+				Source: string(jsonBytes),
+				Format: "json",
+			},
+		}, nil)
+		must.NoError(t, err)
+
+		waitForJobAllocsStatus(t, client, *job.ID, api.AllocClientStatusRunning, "")
+
+		_, _, err = client.Jobs().Deregister(*job.ID, false, nil)
+		must.Nil(t, err)
+
+		waitForJobAllocsStatus(t, client, *job.ID, api.AllocClientStatusComplete, "")
+
+		res := cmd.Run([]string{"-address", addr, *job.ID})
+		must.Zero(t, res)
+
+		pol, _, err := client.Scaling().ListPolicies(nil)
+		must.NoError(t, err)
+		must.Zero(t, len(pol))
+	})
+
 	t.Run("fails to start a job not previously stopped", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
 		job := testJob(uuid.Generate())
 
 		client, err := cmd.Meta.Client()
@@ -151,6 +201,9 @@ func TestStartCommand(t *testing.T) {
 	})
 
 	t.Run("fails to start a non-existant job", func(t *testing.T) {
+		srv, cmd, addr := testSetup()
+		defer srv.Shutdown()
+
 		res := cmd.Run([]string{"-address", addr, "non-existant"})
 		must.Eq(t, 1, res)
 	})