g10: Avoid gratuitious SQLite aborts and starving writers.
* g10/tofu.c: Include <time.h>, <utime.h>, <fcntl.h> and <unistd.h>. (tofu_dbs_s): Add fields want_lock_file and want_lock_file_ctime. (begin_transaction): Only yield if DBS->WANT_LOCK_FILE_CTIME has changed since we took the lock. Don't use gpgrt_yield to yield, but sleep for 100ms. After taking the batch lock, update DBS->WANT_LOCK_FILE_CTIME. Also take the batch lock the first time we take the real lock. When taking the real lock, use immediate not deferred mode to avoid gratuitious aborts. (end_transaction): When dropping the outermost real lock, drop the batch lock. (busy_handler): New function. (opendbs): Set the busy handler to it when opening the DB. Initialize CTRL->TOFU.DBS->WANT_LOCK_FILE. (tofu_closedbs): Free DBS->WANT_LOCK_FILE.
By default, SQLite defers transactions until they are actually needed.
A consequence of this is that if we have two readers and both decide
to do a write, then one has to abort. To avoid this problem, we can
make the outermost transaction an immediate transaction. This has the
disadvantage that we only allow a single reader at a time, but at
least we don't have gratuitous aborts anymore.
A second problem is that SQLite apparently doesn't actually create a
queue of waiters. The result is that doing a sched_yield between
dropping and retaking the batch transaction is not enough to allow the
other process to make progress. Instead, we need to wait a
while (emperically: 100ms seems reasonable). To avoid waiting when
there is no contention, we use a new file's timestamp to signal that
there is a waiter.
- Signed-off-by: Neal H. Walfield <email@example.com>