SPI_connect man page
SPI_connect — connect a procedure to the SPI manager
SPI_connect opens a connection from a procedure invocation to the SPI manager. You must call this function if you want to execute commands through SPI. Some utility SPI functions can be called from unconnected procedures.
If your procedure is already connected, SPI_connect will return the error code SPI_ERROR_CONNECT. This could happen if a procedure that has called SPI_connect directly calls another procedure that calls SPI_connect. While recursive calls to the SPI manager are permitted when an SQL command called through SPI invokes another function that uses SPI, directly nested calls to SPI_connect and SPI_finish are forbidden. (But see SPI_push and SPI_pop.)