structs: Fix job canonicalization for array type fields (#20522)

Co-authored-by: Piotr Kazmierczak <470696+pkazmierczak@users.noreply.github.com>

.changelog/20522.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+structs: Fix job canonicalization for array type fields
+```

nomad/structs/structs.go

@@ -4510,12 +4510,24 @@ func (j *Job) Canonicalize() {
 		return
 	}
 
-	// Ensure that an empty and nil map are treated the same to avoid scheduling
+	// Ensure that an empty and nil map or array are treated the same to avoid scheduling
 	// problems since we use reflect DeepEquals.
 	if len(j.Meta) == 0 {
 		j.Meta = nil
 	}
 
+	if len(j.Constraints) == 0 {
+		j.Constraints = nil
+	}
+
+	if len(j.Affinities) == 0 {
+		j.Affinities = nil
+	}
+
+	if len(j.Spreads) == 0 {
+		j.Spreads = nil
+	}
+
 	// Ensure the job is in a namespace.
 	if j.Namespace == "" {
 		j.Namespace = DefaultNamespace
@@ -6730,12 +6742,24 @@ func (tg *TaskGroup) Copy() *TaskGroup {
 
 // Canonicalize is used to canonicalize fields in the TaskGroup.
 func (tg *TaskGroup) Canonicalize(job *Job) {
-	// Ensure that an empty and nil map are treated the same to avoid scheduling
+	// Ensure that an empty and nil map or array are treated the same to avoid scheduling
 	// problems since we use reflect DeepEquals.
 	if len(tg.Meta) == 0 {
 		tg.Meta = nil
 	}
 
+	if len(tg.Constraints) == 0 {
+		tg.Constraints = nil
+	}
+
+	if len(tg.Affinities) == 0 {
+		tg.Affinities = nil
+	}
+
+	if len(tg.Spreads) == 0 {
+		tg.Spreads = nil
+	}
+
 	// Set the default restart policy.
 	if tg.RestartPolicy == nil {
 		tg.RestartPolicy = NewRestartPolicy(job.Type)
@@ -7794,7 +7818,7 @@ func (t *Task) Copy() *Task {
 
 // Canonicalize canonicalizes fields in the task.
 func (t *Task) Canonicalize(job *Job, tg *TaskGroup) {
-	// Ensure that an empty and nil map are treated the same to avoid scheduling
+	// Ensure that an empty and nil map or array are treated the same to avoid scheduling
 	// problems since we use reflect DeepEquals.
 	if len(t.Meta) == 0 {
 		t.Meta = nil
@@ -7805,6 +7829,17 @@ func (t *Task) Canonicalize(job *Job, tg *TaskGroup) {
 	if len(t.Env) == 0 {
 		t.Env = nil
 	}
+	if len(t.Constraints) == 0 {
+		t.Constraints = nil
+	}
+
+	if len(t.Affinities) == 0 {
+		t.Affinities = nil
+	}
+
+	if len(t.VolumeMounts) == 0 {
+		t.VolumeMounts = nil
+	}
 
 	for _, service := range t.Services {
 		service.Canonicalize(job.Name, tg.Name, t.Name, job.Namespace)

nomad/structs/structs_test.go

@@ -766,6 +766,38 @@ func TestJob_Copy(t *testing.T) {
 	}
 }
 
+func TestJob_Canonicalize(t *testing.T) {
+	ci.Parallel(t)
+	cases := []struct {
+		job *Job
+	}{
+		{
+			job: testJob(),
+		},
+		{
+			job: &Job{},
+		},
+		{
+			job: &Job{
+				Datacenters: []string{},
+				Constraints: []*Constraint{},
+				Affinities:  []*Affinity{},
+				Spreads:     []*Spread{},
+				TaskGroups:  []*TaskGroup{},
+				Meta:        map[string]string{},
+			},
+		},
+	}
+
+	for _, c := range cases {
+		c.job.Canonicalize()
+		copied := c.job.Copy()
+		if !reflect.DeepEqual(c.job, copied) {
+			t.Fatalf("Canonicalize() returned a Job that changed after copy; before %#v; after %#v", c.job, copied)
+		}
+	}
+}
+
 func TestJob_IsPeriodic(t *testing.T) {
 	ci.Parallel(t)
 
@@ -1977,6 +2009,41 @@ func TestTaskGroupNetwork_Validate(t *testing.T) {
 	}
 }
 
+func TestTaskGroup_Canonicalize(t *testing.T) {
+	ci.Parallel(t)
+	job := testJob()
+	cases := []struct {
+		tg *TaskGroup
+	}{
+		{
+			tg: job.TaskGroups[0],
+		},
+		{
+			tg: &TaskGroup{},
+		},
+		{
+			tg: &TaskGroup{
+				Constraints: []*Constraint{},
+				Tasks:       []*Task{},
+				Meta:        map[string]string{},
+				Affinities:  []*Affinity{},
+				Spreads:     []*Spread{},
+				Networks:    []*NetworkResource{},
+				Services:    []*Service{},
+				Volumes:     map[string]*VolumeRequest{},
+			},
+		},
+	}
+
+	for _, c := range cases {
+		c.tg.Canonicalize(job)
+		copied := c.tg.Copy()
+		if !reflect.DeepEqual(c.tg, copied) {
+			t.Fatalf("Canonicalize() returned a TaskGroup that changed after copy; before %#v; after %#v", c.tg, copied)
+		}
+	}
+}
+
 func TestTask_Validate(t *testing.T) {
 	ci.Parallel(t)
 
@@ -2148,6 +2215,46 @@ func TestTask_Validate_Resources(t *testing.T) {
 	}
 }
 
+func TestTask_Canonicalize(t *testing.T) {
+	ci.Parallel(t)
+	job := testJob()
+	tg := job.TaskGroups[0]
+	cases := []struct {
+		task *Task
+	}{
+		{
+			task: tg.Tasks[0],
+		},
+		{
+			task: &Task{},
+		},
+		{
+			task: &Task{
+				Config:          map[string]interface{}{},
+				Env:             map[string]string{},
+				Services:        []*Service{},
+				Templates:       []*Template{},
+				Constraints:     []*Constraint{},
+				Affinities:      []*Affinity{},
+				Meta:            map[string]string{},
+				Artifacts:       []*TaskArtifact{},
+				VolumeMounts:    []*VolumeMount{},
+				ScalingPolicies: []*ScalingPolicy{},
+				Identities:      []*WorkloadIdentity{},
+				Actions:         []*Action{},
+			},
+		},
+	}
+
+	for _, c := range cases {
+		c.task.Canonicalize(job, tg)
+		copied := c.task.Copy()
+		if !reflect.DeepEqual(c.task, copied) {
+			t.Fatalf("Canonicalize() returned a Task that changed after copy; before %#v; after %#v", c.task, copied)
+		}
+	}
+}
+
 func TestNetworkResource_Copy(t *testing.T) {
 	ci.Parallel(t)