PL/Python failed if a PL/Python function was invoked recursively via SPI,
since arguments are passed to the function in its global dictionary
(a horrible decision that's far too ancient to undo) and it would delete
those dictionary entries on function exit, leaving the outer recursion
level(s) without any arguments.  Not deleting them would be little better,
since the outer levels would then see the innermost level's arguments.

Since PL/Python uses ValuePerCall mode for evaluating set-returning
functions, it's possible for multiple executions of the same SRF to be
interleaved within a query.  PL/Python failed in such a case, because
it stored only one iterator per function, directly in the function's
PLyProcedure struct.  Moreover, one interleaved instance of the SRF
would see argument values that should belong to another.

Hence, invent code for saving and restoring the argument entries.  To fix
the recursion case, we only need to save at recursive entry and restore
at recursive exit, so the overhead in non-recursive cases is negligible.
To fix the SRF case, we have to save when suspending a SRF and restore
when resuming it, which is potentially not negligible; but fortunately
this is mostly a matter of manipulating Python object refcounts and
should not involve much physical data copying.

Also, store the Python iterator and saved argument values in a structure
associated with the SRF call site rather than the function itself.  This
requires adding a memory context deletion callback to ensure that the SRF
state is cleaned up if the calling query exits before running the SRF to
completion.  Without that we'd leak a refcount to the iterator object in
such a case, resulting in session-lifespan memory leakage.  (In the
pre-existing code, there was no memory leak because there was only one
iterator pointer, but what would happen is that the previous iterator
would be resumed by the next query attempting to use the SRF.  Hardly the
semantics we want.)

We can buy back some of whatever overhead we've added by getting rid of
PLy_function_delete_args(), which seems a useless activity: there is no
need to delete argument entries from the global dictionary on exit,
since the next time anyone would see the global dict is on the next
fresh call of the PL/Python function, at which time we'd overwrite those
entries with new arg values anyway.

Also clean up some really ugly coding in the SRF implementation, including
such gems as returning directly out of a PG_TRY block.  (The only reason
that failed to crash hard was that all existing call sites immediately
exited their own PG_TRY blocks, popping the dangling longjmp pointer before
there was any chance of it being used.)

In principle this is a bug fix; but it seems a bit too invasive relative to
its value for a back-patch, and besides the fix depends on memory context
callbacks so it could not go back further than 9.5 anyway.

Alexey Grishchenko and Tom Lane