grok-pull.post_update_hook: flock(2) before SQLite check
Unlike DBD::SQLite, the sqlite3(1) CLI does not have a default
busy timeout enabled, so it easily times out while acquiring a
SHARED lock for read-only queries. We can avoid battery-wasting
polling from the SQLite timeout handler by relying on flock(2)
as we do in our Perl code.
Furthermore, this avoids triggering some locking problems[1]
from a long "SELECT COUNT(*) ..." query and reindex.
While there may be other SQLite-related parallelism issues[1],
this works around one of them by relying on flock(2).
[1] https://public-inbox.org/meta/
20200825001204.GA840@dcvr/