SQLite

** the associated write-lock on the database released, so the implementation 
** may read, write or [checkpoint] the database as required.
**
** ^The first parameter passed to the callback function when it is invoked
** is a copy of the third parameter passed to sqlite3_wal_hook() when
** registering the callback. ^The second is a copy of the database handle.
** ^The third parameter is the name of the database that was written to -
** either "main" or the name of an [ATTACH]-ed database. ^The fourth parameter
** is the number of pages currently in the write-ahead log file,
** including those that were just committed.
**
** The callback function should normally return [SQLITE_OK].  ^If an error
** code is returned, that error will propagate back up through the
** SQLite code base to cause the statement that provoked the callback
** to report an error, though the commit will have still occurred. If the
** callback returns [SQLITE_ROW] or [SQLITE_DONE], or if it returns a value
** that does not correspond to any valid SQLite error code, the results
** are undefined.
**
** A single database handle may have at most a single write-ahead log callback 
** registered at one time. ^Calling [sqlite3_wal_hook()] replaces any
** previously registered write-ahead log callback. ^Note that the
** [sqlite3_wal_autocheckpoint()] interface and the
** [wal_autocheckpoint pragma] both invoke [sqlite3_wal_hook()] and will
** those overwrite any prior [sqlite3_wal_hook()] settings.
*/
void *sqlite3_wal_hook(
  sqlite3*, 
  int(*)(void *,sqlite3*,const char*,int),