deployments: canary=0 is implicitly autopromote (#11013)

In a multi-task-group job, treat 0 canary groups as auto-promote.

This change fixes an edge case where Nomad requires a manual promotion,
if the job had any group with canary=0 and rest of groups having
auto_promote set.

Co-authored-by: Michael Schurter <mschurter@hashicorp.com>

.changelog/11013.txt

@@ -0,0 +1,4 @@
+
+```release-note:bug
+deployments: Fixed a bug where multi-group deployments don't get auto-promoted when one group has no canaries.
+```

nomad/structs/structs.go

@@ -4302,19 +4302,24 @@ func (j *Job) Warnings() error {
 	var mErr multierror.Error
 
 	// Check the groups
-	ap := 0
+	hasAutoPromote, allAutoPromote := false, true
+
 	for _, tg := range j.TaskGroups {
 		if err := tg.Warnings(j); err != nil {
 			outer := fmt.Errorf("Group %q has warnings: %v", tg.Name, err)
 			mErr.Errors = append(mErr.Errors, outer)
 		}
-		if tg.Update != nil && tg.Update.AutoPromote {
-			ap += 1
+
+		if u := tg.Update; u != nil {
+			hasAutoPromote = hasAutoPromote || u.AutoPromote
+
+			// Having no canaries implies auto-promotion since there are no canaries to promote.
+			allAutoPromote = allAutoPromote && (u.Canary == 0 || u.AutoPromote)
 		}
 	}
 
 	// Check AutoPromote, should be all or none
-	if ap > 0 && ap < len(j.TaskGroups) {
+	if hasAutoPromote && !allAutoPromote {
 		err := fmt.Errorf("auto_promote must be true for all groups to enable automatic promotion")
 		mErr.Errors = append(mErr.Errors, err)
 	}
@@ -8877,7 +8882,7 @@ func (d *Deployment) HasAutoPromote() bool {
 		return false
 	}
 	for _, group := range d.TaskGroups {
-		if !group.AutoPromote {
+		if group.DesiredCanaries > 0 && !group.AutoPromote {
 			return false
 		}
 	}

nomad/structs/structs_test.go

@@ -200,6 +200,27 @@ func TestJob_Warnings(t *testing.T) {
 					{
 						Update: &UpdateStrategy{
 							AutoPromote: false,
+							Canary:      1,
+						},
+					},
+				},
+			},
+		},
+		{
+			Name:     "no error for mixed but implied AutoPromote",
+			Expected: []string{},
+			Job: &Job{
+				Type: JobTypeService,
+				TaskGroups: []*TaskGroup{
+					{
+						Update: &UpdateStrategy{
+							AutoPromote: true,
+						},
+					},
+					{
+						Update: &UpdateStrategy{
+							AutoPromote: false,
+							Canary:      0,
 						},
 					},
 				},