Program: 5546
Project: 5546
Buggy source:
@Override public Integer apply(Table table) throws Exception {
  int deletedColumns=0;
  try (Scanner scanner=table.scan(ROW_KEY_PREFIX,ROW_KEY_PREFIX_END)){
    Row row;
    while ((row=scanner.next()) != null) {
      byte[] rowKey=row.getRow();
      final NamespaceId namespaceId=getNamespaceId(rowKey);
      String namespacedLogDir=impersonator.doAs(namespaceId,new Callable<String>(){
        @Override public String call() throws Exception {
          return LoggingContextHelper.getNamespacedBaseDirLocation(namespacedLocationFactory,logBaseDir,namespaceId,impersonator).toString();
        }
      }
);
      for (      final Map.Entry<byte[],byte[]> entry : row.getColumns().entrySet()) {
        try {
          byte[] colName=entry.getKey();
          URI file=new URI(Bytes.toString(entry.getValue()));
          if (LOG.isDebugEnabled()) {
            LOG.debug("String_Node_Str",file,Bytes.toLong(colName));
          }
          Location fileLocation=impersonator.doAs(namespaceId,new Callable<Location>(){
            @Override public Location call() throws Exception {
              return rootLocationFactory.create(new URI(Bytes.toString(entry.getValue())));
            }
          }
);
          if (!fileLocation.exists()) {
            LOG.warn("String_Node_Str",file);
            table.delete(rowKey,colName);
            deletedColumns++;
          }
 else           if (fileLocation.lastModified() < tillTime) {
            callback.handle(namespaceId,fileLocation,namespacedLogDir);
            table.delete(rowKey,colName);
            deletedColumns++;
          }
        }
 catch (        Exception e) {
          LOG.error("String_Node_Str",Bytes.toString(entry.getValue()),e);
        }
      }
    }
  }
   return deletedColumns;
}

Fixed version (ground truth):
@Override public Integer apply(Table table) throws Exception {
  int deletedColumns=0;
  try (Scanner scanner=table.scan(ROW_KEY_PREFIX,ROW_KEY_PREFIX_END)){
    Row row;
    while ((row=scanner.next()) != null) {
      byte[] rowKey=row.getRow();
      final NamespaceId namespaceId=getNamespaceId(rowKey);
      String namespacedLogDir=null;
      try {
        namespacedLogDir=impersonator.doAs(namespaceId,new Callable<String>(){
          @Override public String call() throws Exception {
            return LoggingContextHelper.getNamespacedBaseDirLocation(namespacedLocationFactory,logBaseDir,namespaceId,impersonator).toString();
          }
        }
);
      }
 catch (      Exception e) {
        if (e instanceof NamespaceNotFoundException) {
          LOG.warn("String_Node_Str",namespaceId.getEntityName(),e);
        }
 else {
          LOG.warn("String_Node_Str",namespaceId.getEntityName(),e);
          continue;
        }
      }
      for (      final Map.Entry<byte[],byte[]> entry : row.getColumns().entrySet()) {
        try {
          byte[] colName=entry.getKey();
          URI file=new URI(Bytes.toString(entry.getValue()));
          if (LOG.isDebugEnabled()) {
            LOG.debug("String_Node_Str",file,Bytes.toLong(colName));
          }
          if (Strings.isNullOrEmpty(namespacedLogDir)) {
            LOG.warn("String_Node_Str" + "String_Node_Str",Bytes.toString(entry.getValue()),namespaceId.getEntityName());
            table.delete(rowKey,colName);
            deletedColumns++;
            continue;
          }
          Location fileLocation=impersonator.doAs(namespaceId,new Callable<Location>(){
            @Override public Location call() throws Exception {
              return rootLocationFactory.create(new URI(Bytes.toString(entry.getValue())));
            }
          }
);
          if (!fileLocation.exists()) {
            LOG.warn("String_Node_Str",file);
            table.delete(rowKey,colName);
            deletedColumns++;
          }
 else           if (fileLocation.lastModified() < tillTime) {
            callback.handle(namespaceId,fileLocation,namespacedLogDir);
            table.delete(rowKey,colName);
            deletedColumns++;
          }
        }
 catch (        Exception e) {
          if (e instanceof NamespaceNotFoundException) {
            LOG.warn("String_Node_Str",Bytes.toString(entry.getValue()),namespaceId.getEntityName());
          }
 else {
            LOG.error("String_Node_Str",Bytes.toString(entry.getValue()),e);
          }
        }
      }
    }
  }
   return deletedColumns;
}