fix(master): say why a pinned pick fails on a protocol 2 agent (#9)

internal/master/hub/route.go

@@ -16,7 +16,8 @@ type candidate struct {
 
 // 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.
+// model.ErrAgentTooOld (model.ErrAgentTooOldForPinned when minProtocol asks
+// for a pinned ref) 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))
@@ -64,6 +65,9 @@ func route(svc model.Service, conns []candidate, minProtocol int) (model.Route,
 		}
 	}
 	if tooOld {
+		if minProtocol >= protocol.MinPinnedRefVersion {
+			return model.Route{}, model.ErrAgentTooOldForPinned
+		}
 		return model.Route{}, model.ErrAgentTooOld
 	}
 	return model.Route{}, model.ErrAgentOffline

internal/master/hub/route_test.go

@@ -30,7 +30,7 @@ func TestRoute(t *testing.T) {
 		{"compose, own agent too old", composeSvc, []candidate{{2, 20, protocol.Hello{}}}, jobs, model.Route{}, model.ErrAgentTooOld},
 		{"compose, own agent missing", composeSvc, nil, jobs, model.Route{}, model.ErrAgentOffline},
 		{"compose, only another agent online", composeSvc, []candidate{{5, 50, v2}}, jobs, model.Route{}, model.ErrAgentOffline},
-		{"compose, v2 agent and a pinned ref", composeSvc, []candidate{{2, 20, v2}}, pinned, model.Route{}, model.ErrAgentTooOld},
+		{"compose, v2 agent and a pinned ref", composeSvc, []candidate{{2, 20, v2}}, pinned, model.Route{}, model.ErrAgentTooOldForPinned},
 		{"compose, v3 agent and a pinned ref", composeSvc, []candidate{{2, 20, v3}}, pinned, model.Route{AgentID: 2, ConnID: 20}, nil},
 		{"swarm on reporting agent", swarmSvc, []candidate{{1, 10, mgrV2}, {3, 30, mgrV2}}, jobs, model.Route{AgentID: 1, ConnID: 10}, nil},
 		{"swarm, reporting agent offline, other manager", swarmSvc, []candidate{{3, 30, mgrV2}}, jobs, model.Route{AgentID: 3, ConnID: 30}, nil},
@@ -47,7 +47,7 @@ func TestRoute(t *testing.T) {
 			[]candidate{{1, 10, protocol.Hello{ProtocolVersion: 2, SwarmManager: true}}}, jobs, model.Route{}, model.ErrAgentOffline},
 		{"swarm, pinned ref falls back to a v3 manager when the reporting one is v2", swarmSvc,
 			[]candidate{{1, 10, mgrV2}, {3, 30, mgrV3}}, pinned, model.Route{AgentID: 3, ConnID: 30}, nil},
-		{"swarm, pinned ref and only v2 managers", swarmSvc, []candidate{{1, 10, mgrV2}}, pinned, model.Route{}, model.ErrAgentTooOld},
+		{"swarm, pinned ref and only v2 managers", swarmSvc, []candidate{{1, 10, mgrV2}}, pinned, model.Route{}, model.ErrAgentTooOldForPinned},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

internal/master/jobs/jobs.go

@@ -156,8 +156,9 @@ func (s *Service) TriggerVersion(ctx context.Context, serviceID, versionID int64
 }
 
 // Start starts an update of svc. It returns the job, which may already be
-// failed ("agent offline", "agent too old for updates", the latter also when
+// failed ("agent offline", "agent too old for updates", or "agent too old to
-// the ref has a digest or no tag and the agent speaks protocol version 2); a
+// deploy a pinned version" when the ref has a digest or no tag and the agent
+// speaks protocol version 2); a
 // *model.NotUpdatableError when the service cannot be updated (no job
 // created); the active job and model.ErrJobActive when one is queued or
 // running.

internal/master/jobs/jobs_test.go

@@ -83,6 +83,9 @@ type fakeHub struct {
 func (h *fakeHub) Route(_ model.Service, minProtocol int) (model.Route, error) {
 	h.mins = append(h.mins, minProtocol)
 	if h.agentProtocol != 0 && minProtocol > h.agentProtocol {
+		if minProtocol >= protocol.MinPinnedRefVersion {
+			return model.Route{}, model.ErrAgentTooOldForPinned
+		}
 		return model.Route{}, model.ErrAgentTooOld
 	}
 	return h.route, h.routeErr
@@ -481,7 +484,7 @@ func TestTriggerVersion_RequiresPinnedProtocol(t *testing.T) {
 	pinned, err := s.TriggerVersion(context.Background(), 1, 10, "sub-1")
 	require.NoError(t, err)
 	assert.Equal(t, model.JobFailed, pinned.Status)
-	assert.Equal(t, "agent too old for updates", pinned.Error)
+	assert.Equal(t, "agent too old to deploy a pinned version (needs protocol 3)", pinned.Error)
 	assert.Empty(t, h.starts, "nothing sent to the old agent")
 
 	plain, err := s.TriggerVersion(context.Background(), 1, 11, "sub-1")

internal/master/model/job.go

@@ -15,8 +15,20 @@ var (
 	// ErrAgentTooOld means the only agent that could run the job speaks a
 	// protocol version without jobs.
 	ErrAgentTooOld = errors.New("agent too old for updates")
+	// ErrAgentTooOldForPinned means the only agent that could run the job
+	// speaks protocol version 2, which cannot deploy a digest-pinned or
+	// tagless ref. errors.Is(err, ErrAgentTooOld) is true for it as well.
+	ErrAgentTooOldForPinned error = agentTooOldForPinned{}
 )
 
+type agentTooOldForPinned struct{}
+
+func (agentTooOldForPinned) Error() string {
+	return "agent too old to deploy a pinned version (needs protocol 3)"
+}
+
+func (agentTooOldForPinned) Is(target error) bool { return target == ErrAgentTooOld }
+
 // NotUpdatableError says why a service cannot be updated. Reason is shown
 // in the UI as is.
 type NotUpdatableError struct {