feat(master,agent): deploy digest-pinned refs and route them to protocol 3 agents (#9)

internal/master/hub/route.go

@@ -14,9 +14,10 @@ type candidate struct {
 	hello   protocol.Hello
 }
 
-// Route picks the connection a job for svc goes to. It returns
-// model.ErrAgentOffline or model.ErrAgentTooOld when there is none.
-func (h *Hub) Route(svc model.Service) (model.Route, error) {
+// Route picks the connection a job for svc goes to, among agents that speak
+// at least protocol version minProtocol. It returns model.ErrAgentOffline or
+// model.ErrAgentTooOld when there is none.
+func (h *Hub) Route(svc model.Service, minProtocol int) (model.Route, error) {
 	h.mu.Lock()
 	conns := make([]candidate, 0, len(h.conns))
 	for id, c := range h.conns {
@@ -25,7 +26,7 @@ func (h *Hub) Route(svc model.Service) (model.Route, error) {
 		}
 	}
 	h.mu.Unlock()
-	return route(svc, conns)
+	return route(svc, conns, minProtocol)
 }
 
 // route picks the connection a job for svc goes to. Compose services and
@@ -33,7 +34,7 @@ func (h *Hub) Route(svc model.Service) (model.Route, error) {
 // run on the reporting agent if it qualifies, else on any other connected
 // manager of the same cluster (lowest agent ID first). Too-old wins over
 // offline only when a connection would otherwise have qualified.
-func route(svc model.Service, conns []candidate) (model.Route, error) {
+func route(svc model.Service, conns []candidate, minProtocol int) (model.Route, error) {
 	sort.Slice(conns, func(a, b int) bool { return conns[a].agentID < conns[b].agentID })
 	tooOld := false
 	try := func(c candidate) bool {
@@ -42,7 +43,7 @@ func route(svc model.Service, conns []candidate) (model.Route, error) {
 				return false
 			}
 		}
-		if c.hello.ProtocolVersion < protocol.MinJobVersion {
+		if c.hello.ProtocolVersion < minProtocol {
 			tooOld = true
 			return false
 		}