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

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
-// the ref has a digest or no tag and the agent speaks protocol version 2); a
+// failed ("agent offline", "agent too old for updates", or "agent too old to
+// 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")