private function loadCoverage(DifferentialChangeset $changeset)
 {
     $target_phids = $changeset->getDiff()->getBuildTargetPHIDs();
     if (!$target_phids) {
         return null;
     }
     $unit = id(new HarbormasterBuildUnitMessage())->loadAllWhere('buildTargetPHID IN (%Ls)', $target_phids);
     if (!$unit) {
         return null;
     }
     $coverage = array();
     foreach ($unit as $message) {
         $test_coverage = $message->getProperty('coverage');
         if ($test_coverage === null) {
             continue;
         }
         $coverage_data = idx($test_coverage, $changeset->getFileName());
         if (!strlen($coverage_data)) {
             continue;
         }
         $coverage[] = $coverage_data;
     }
     if (!$coverage) {
         return null;
     }
     return ArcanistUnitTestResult::mergeCoverage($coverage);
 }