libSQL WAL API

[penberg]

The libSQL has the following WAL API functions, which are useful for
syncing the WAL between databases:

• libsql_wal_disable_checkpoint -- Disable checkpointing, including on database close.
• libsql_wal_frame_count -- Get the number of frames in the WAL.
• libsql_wal_get_frame -- Get a frame from the WAL.
• libsql_wal_insert_begin -- Begin WAL insertion.
• libsql_wal_insert_frame -- Insert a frame into the WAL.
• libsql_wal_insert_end -- End WAL insertion.

The purpose of this API is to allow syncing the frames of one database WAL onto another. Example usage looks as follows (omitting error handling):

static void sync_db(sqlite3 *db_primary, sqlite3 *db_backup){
  unsigned int max_frame;

  libsql_wal_frame_count(db_primary, &max_frame);
  libsql_wal_begin_commit(db_backup);
  for(int i=1; i<=max_frame; i++){
    char frame[4096+24];
    libsql_wal_get_frame(db_primary, i, frame, sizeof(frame));
    libsql_wal_insert_frame(db_backup, i, frame, sizeof(frame));
  }
  libsql_wal_end_commit(db_backup);
}

Possible future work:

• Do we need to make the WAL salt the same across synced databases?
• How can we detect that frames belong to a database that it is synced to?

Fixes #200

[justjake]

Is the sqlite3_mutex_enter seen here disabled outside of SQLITE_CONFIG_SERIALIZED? Or is this mutex only disabled with SQLITE_CONFIG_SINGLETHREAD? Is grabbing a mutex around each read typical in the rest of sqlite? For a potential bulk-copy need like WAL I would think amortizing the mutex by batching multiple read calls per mutex enter would be helpful.