[Bug debuginfod/28514] New: limit grooming time for many stale files

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28514

            Bug ID: 28514
           Summary: limit grooming time for many stale files
           Product: elfutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: debuginfod
          Assignee: unassigned at sourceware dot org
          Reporter: fche at redhat dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

When a big debuginfod server starts grooming, and starts finding stale data
(archives or files being removed), its self-cleaning efforts can take a long
time.  It's been observed to take O(seconds) to do a single sqlite query.  In
the metrics, see the sqlite3_milliseconds_count...{"nuke..."} ones.  And the
groom() function will check every file for staleness, until interrupted by a
SIGUSR1, so that O(50000) stale files could take a whole day.

During all this time, the server can service buildid requests, so it's not that
bad, but it cannot scan for new files.

We should investigate whether a more time-bounded groom operation could serve
about as well.  We could limit groom to a certain percentage of time, like 1
hr/day, then abort.  (We'd have to traverse the file list in some stateful or
random way in order not to just recheck the same ones over and over.)  The
post-loop cleanup ops ("nuke orphan buildids" ... end of function) are
relatively quick and not worth worrying about at this time.

Alternately, there may be a way to accelerate the individual nuke queries,
maybe with more indexes, at the cost of more storage.

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