close[t:4684] Add comment. Closes #4684.

int toku_txn_prepare_txn (TOKUTXN txn, GID gid) {

    assert(txn->state==TOKUTXN_LIVE);

    txn->state = TOKUTXN_PREPARING;

    txn->state = TOKUTXN_PREPARING; // This state transition must be protected against begin_checkpoint.  Right now it uses the ydb lock.

    if (txn->parent) return 0; // nothing to do if there's a parent.

    // Do we need to do an fsync?

    txn->do_fsync = (txn->force_fsync_on_commit || txn->num_rollentries>0);

    txn->gid.gid = toku_memdup(gid.gid, DB_GID_SIZE);

    // This list will go away with #4683, so we wn't need the ydb lock for this anymore.

    toku_list_push(&txn->logger->prepared_txns, &txn->prepared_txns_link);

    return toku_log_xprepare(txn->logger, &txn->do_fsync_lsn, 0, txn->txnid64, gid);

}