MRD: deregister regions that are dropped on update (#8763)

This changeset is the OSS hooks for what will be implemented in ENT.

nomad/job_endpoint.go

@@ -395,6 +395,12 @@ func (j *Job) Register(args *structs.JobRegisterRequest, reply *structs.JobRegis
 	args.Job.JobModifyIndex = reply.JobModifyIndex
 
 	if eval == nil {
+		// For dispatch jobs we return early, so we need to drop regions
+		// here rather than after eval for deployments is kicked off
+		err = j.multiregionDrop(args, reply)
+		if err != nil {
+			return err
+		}
 		return nil
 	}
 
@@ -424,6 +430,14 @@ func (j *Job) Register(args *structs.JobRegisterRequest, reply *structs.JobRegis
 		if err != nil {
 			return err
 		}
+		// We drop any unwanted regions only once we know all jobs have
+		// been registered and we've kicked off the deployment. This keeps
+		// dropping regions close in semantics to dropping task groups in
+		// single-region deployments
+		err = j.multiregionDrop(args, reply)
+		if err != nil {
+			return err
+		}
 	}
 
 	return nil

nomad/job_endpoint_oss.go

@@ -25,6 +25,12 @@ func (j *Job) multiregionStart(args *structs.JobRegisterRequest, reply *structs.
 	return nil
 }
 
+// multiregionDrop is used to deregister regions from a previous version of the
+// job that are no longer in use
+func (j *Job) multiregionDrop(args *structs.JobRegisterRequest, reply *structs.JobRegisterResponse) error {
+	return nil
+}
+
 // interpolateMultiregionFields interpolates a job for a specific region
 func (j *Job) interpolateMultiregionFields(args *structs.JobPlanRequest) error {
 	return nil