Comment 2 for bug 1787977

Reviewed: https://review.openstack.org/592698
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=0a88916911e2b02055a2a707bda026c975f4472c
Submitter: Zuul
Branch: master

commit 0a88916911e2b02055a2a707bda026c975f4472c
Author: Dan Smith <email address hidden>
Date: Thu Aug 16 13:52:01 2018 -0700

    Batch results per cell when doing cross-cell listing

    This extends the multi_cell_list module with batching support to avoid
    querying N*$limit total results when listing resources across cells.
    Instead, if our total limit is over a given threshold, we should query
    smaller batches in the per-cell thread until we reach the total limit
    or are stopped because the sort feeder has found enough across all cells
    to satisfy the requirements. In many cases, this can drop the total number
    of results we load and process from N*$limit to (best case) $limit+$batch
    or (usual case) $limit+(N*$batch).

    Since we return a generator from our scatter-gather function, this should
    mean we basically finish the scatter immediately after the first batch query
    to each cell database, keeping the threads alive until they produce all the
    results possible from their cell, or are terminated in the generator loop
    by the master loop hitting the total_limit condition. As a result, the
    checking over results that we do immediately after the scatter finishes
    will no longer do anything since we start running the query code for the
    first time as heapq.merge() starts hitting the generators. So, this brings
    a query_wrapper() specific to the multi_cell_list code which can mimic the
    timeout and error handling abilities of scatter_gather_cells, but inline
    as we're processing so that we don't interrupt the merge sort for a
    failure.

    Related-Bug: #1787977
    Change-Id: Iaa4759822e70b39bd735104d03d4deec988d35a1