possible memory leak with SRFs

Hi,

I saw this behavior with latest GIT head:

create table xlarge(val numeric(19,0));
insert into xlarge values(generate_series(1,5));

The above generate series will return an int8 which will then be
casted to numeric (via int8_to_numericvar) before being inserted into
the table. I observed that the ExprContext memory associated with
econtext->ecxt_per_tuple_memory is slowly bloating up till the end of
the insert operation.

This becomes significant the moment we try to insert a significant
number of entries using this SRF. I can see the memory being consumed
by the PG backend slowly grow to a large percentage.

I see that the executor (take ExecResult as an example) does not reset
the expression context early if an SRF is churning out tuples. What
could be a good way to fix this?

Regards,
Nikhils
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