title: main/delta split marker with sphinx using xmlpipe2?
body: <p>we are going to have sphinx running main+delta with nosql source. So we're going to use xmlpipe2. To implement main+delta strategy we have to maintain a marker to distinguish "main rows" from "new rows". </p>
  
  <p>The problem is that, unlike sql sources, with xmlpipe2 we can't tell (at least don't know how) if the indexing was successful or not. If we simply update the marker at the end of the main feed generator, and indexing fails for whatever reason, the setup is going to be in inconsistent state. </p>
  
  <p>For SQL sources we have <strong>sql_query_post_index</strong> hook, how can we achieve similar thing with <strong>xmlpipe2</strong>?</p>