borrow_worker db f
f db'
db'
db
f
with_lock m (fun () -> ... with_lock m (fun () -> ... ))
fold db f a expr ...
f (... (f (f a r1) r2) ...) rN
rN
expr
select_f db f expr ...
select db expr ...
select_one db expr ...
select_one_f db f expr ...
select_one_f_maybe db expr ...
select_one_maybe db expr ...
set_max_threads n
max n current_thread_count
Sqlite3.db
steal_worker db f