The current DirectFunctionCall functions use NULL as the flinfo in
initializing the FunctionCallInfoData for the call. That means the
called function has no fn_mcxt or fn_extra to work with, and attempting
to do so will result in an access violation. These functions instead use
the provided flinfo, which will usually be the caller's own flinfo. The
caller needs to ensure that it doesn't use the fn_extra in way that is
incompatible with the way the called function will use it. The called
function should not rely on anything else in the provided context, as it
will be relevant to the caller, not the callee.

Original code from Tom Lane.

Discussion: https://postgr.es/m/db2b70a4-78d7-294a-a315-8e7f506c5978@2ndQuadrant.com