回复：SnapBuildSerialize function forgot pfree variable ondisk_c

If we create logical slot frequently, every time when creating a logical slot, it will generate a LogStandbySnapshot wal record.

Session A:
do language plpgsql $$
declare
v_text text := 'a';
begin
for i in 1..290000 loop
execute $_$select pg_create_logical_replication_slot('logical_slot4', 'test_decoding')$_$;
execute $_$select pg_drop_replication_slot('logical_slot4')$_$;
end loop;
exception when others then
raise notice 'execute failed.';
end;$$;

Session B:
pg_recvlogical -d postgres --start -S test -f test.log

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发件人：Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
发送时间：2020年11月6日(星期五) 11:21
收件人：范孝剑(康贤) <funnyxj(dot)fxj(at)alibaba-inc(dot)com>
抄　送：pgsql-bugs <pgsql-bugs(at)lists(dot)postgresql(dot)org>
主　题：Re: SnapBuildSerialize function forgot pfree variable ondisk_c

On Thu, Nov 5, 2020 at 2:06 PM 范孝剑(康贤) <funnyxj(dot)fxj(at)alibaba-inc(dot)com> wrote:
>
> Hi all,
> When I tested logical decoding, I found that walsender process memory usage grow very high, by debugging, I found SnapBuildSerialize palloc memory for variable ondisk_c, but don't pfree it at last.
>

By looking at code, it is clear that it is good to free the memory
allocated for variable ondisk_c.

> So when master LogStandbySnapshot() too frequently, walsender memory will grow very high and OOM finally.
>

Is there any particular scenario where you are seeing this behavior?
Do you have any reproducible test case? Have you confirmed that after
freeing that memory your problem is solved? It is not clear to me why
other users of Logical Replication are not facing this issue?

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With Regards,
Amit Kapila.