Zygo Blaxell 03f809bf22 roots: reimplement scan modes using virtual base and methods ...

Split each scan mode into two distinct phases:

    1.  A heavy discovery phase, where we search the entire filesystem
    for something (new items in subvol trees in this case).

    2.  A light consuming phase, where we fetch extents to dedupe
    from places that we found in the discovery phase.

Part 1 recomputes the subvol ordering every time there is a new transid.
For some scan modes this computation is quite expensive, far too costly
to pay for every extent, so we do it no more than once per transaction.

Part 2 is run every time a worker thread hits the crawl_more Task.
It simply pulls one extent from the first crawler off a sorted list,
removing the crawler from the list when the crawler runs out of data.

Part 1 creates a new structure and swaps it into place, while Part 2
continues to run using the previous strucuture.  Neither of these
need to block the other, so they don't.

The separate class and base pointer also make it easer to add new scan
modes that are not based on subvol trees or that don't use BeesCrawl.

While we're here, fix up some method visibility in BeesRoots.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

2022-12-20 20:51:01 -05:00

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