Fix NextJob panic (#411)

* Check for nil job
* Write error header
* Add case for NextJob returning nil

tracker/handler.go

@@ -113,6 +113,10 @@ func (h *Handler) nextJob(resp http.ResponseWriter, req *http.Request) *JobWithT
 		return nil
 	}
 	jt := h.jobservice.NextJob(req.Context())
+	if jt == nil {
+		resp.WriteHeader(http.StatusInternalServerError)
+		return nil
+	}
 
 	// Check for empty job (no job found with files)
 	if jt.Job.Date.Equal(time.Time{}) {

tracker/handler_test.go

@@ -31,6 +31,10 @@ type fakeJobService struct {
 }
 
 func (f *fakeJobService) NextJob(ctx context.Context) *tracker.JobWithTarget {
+	if f.calls >= len(f.jobs) {
+		// Return nil if there are no more jobs.
+		return nil
+	}
 	j := f.jobs[f.calls]
 	f.calls++
 	return &tracker.JobWithTarget{Job: j}
@@ -280,4 +284,7 @@ func TestNextJobV2Handler(t *testing.T) {
 
 	// This one should fail because the fakeJobService returns a duplicate job.
 	postAndExpect(t, &url, http.StatusInternalServerError)
+
+	// Get nil result.
+	postAndExpect(t, &url, http.StatusInternalServerError)
 }