Recursion and SPI

Does the SPI interface support recursion? That is, can a function
use SPI to make a query which involves calling another function which
uses SPI?

The documentation suggests not, saying that if a function which uses
SPI calls another function which uses SPI, it won't work, and calling
that ``bad practice.''

However, in spi.c I note that there is a stack, and a variable
_SPI_curid, and the undocumented functions SPI_push and SPI_pop. If
that works to support recursion, then why does the documentation
recommend against it?

Are there any restrictions on when SPI_connect may be called? The
executor can presumably call a function at any time during the
execution of a plan if the function is used in a WHERE clause. If
that function turns around and calls the executor via SPI, are there
any possibilities for deadlock?

Ian