SPI_rollback rolls back the current transaction. It is approximately equivalent to running the SQL command ROLLBACK. After a transaction is rolled back, a new transaction has to be started using SPI_start_transaction before further database actions can be executed.
SPI_rollback_and_chain is the same, but a new transaction is immediately started with the same transaction characteristics as the just finished one, like with the SQL command ROLLBACK AND CHAIN.
These functions can only be executed if the SPI connection has been set as nonatomic in the call to SPI_connect_ext.
The man page SPI_rollback_and_chain(3) is an alias of SPI_rollback(3).