Re: Memory leak in plpython3u (with testcase and patch)

I wrote:
> However, I don't really see why we need to use that scratch context.
> PLy_spi_execute_plan runs a subtransaction, so we could perfectly well
> use the subtransaction's CurTransactionContext.

Nah, I'm wrong about that: I forgot CurTransactionContext goes away on
subtransaction abort, but not subtransaction commit. So we really
need to make our own context with suitable lifespan.

I also noticed that the code is moving heaven and earth to store
the argument Datums ... but for some reason not the associated
isnull flags ... in the PLyPlanObject. This is just nuts, because
the values don't need to survive past the functions that are computing
them. In the cursor case, the values will be copied into the cursor
portal, so they still don't need to survive. We can drop all of
that.

So I arrive at the attached. (This is just for HEAD. In the back
branches, we'd better leave the PLyPlanObject.values field in place
for ABI safety, but we don't have to use it.)

regards, tom lane