RE: Logical replication keepalive flood

From Tuesday, September 14, 2021 1:39 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> However, the problem I found is that, with the patch applied, there is
> a test failure when running “make check-world”:
>
> t/006_logical_decoding.pl ............ 4/14
> # Failed test 'pg_recvlogical acknowledged changes'
> # at t/006_logical_decoding.pl line 117.
> # got: 'BEGIN
> # table public.decoding_test: INSERT: x[integer]:5 y[text]:'5''
> # expected: ''
> # Looks like you failed 1 test of 14.
> t/006_logical_decoding.pl ............ Dubious, test returned 1 (wstat
> 256, 0x100) Failed 1/14 subtests
>
>

After applying the patch,
I saw the same problem and can reproduce it by the following steps:

1) execute the SQLs.
-----------SQL-----------
CREATE TABLE decoding_test(x integer, y text);
SELECT pg_create_logical_replication_slot('test_slot', 'test_decoding');
INSERT INTO decoding_test(x,y) SELECT s, s::text FROM generate_series(1,4) s;

-- use the lsn here to execute pg_recvlogical later
SELECT lsn FROM pg_logical_slot_peek_changes('test_slot', NULL, NULL) ORDER BY lsn DESC LIMIT 1;
INSERT INTO decoding_test(x,y) SELECT s, s::text FROM generate_series(5,50) s;
----------------------

2) Then, if I execute the following command twice:
# pg_recvlogical -E lsn -d postgres -S 'test_slot' --start --no-loop -f -
BEGIN 708
table public.decoding_test: INSERT: x[integer]:1 y[text]:'1'
table public.decoding_test: INSERT: x[integer]:2 y[text]:'2'
table public.decoding_test: INSERT: x[integer]:3 y[text]:'3'
table public.decoding_test: INSERT: x[integer]:4 y[text]:'4'
COMMIT 708

# pg_recvlogical -E lsn -d postgres -S 'test_slot' --start --no-loop -f -
BEGIN 709

It still generated ' BEGIN 709' in the second time execution.
But it will output nothing in the second time execution without the patch.

Best regards,
Hou zj