REPACK (CONCURRENTLY) can lose data in pg_dump output

Hi,

If pg_dump starts while REPACK (CONCURRENTLY) is waiting for its final
lock on a table, it dumps that table with no rows, and still exits
successfully:

CREATE TABLE dumptest (id int PRIMARY KEY, v text);
INSERT INTO dumptest SELECT x, 'row' || x FROM generate_series(1,1000) x;

Session 1:
BEGIN;
SELECT count(*) FROM dumptest;

Session 2:
REPACK (CONCURRENTLY) dumptest; -- waits for session 1

Terminal:
pg_dump -t dumptest --data-only -- waits behind REPACK

Session 1:
COMMIT;

The dump then contains:

COPY public.dumptest (id, v) FROM stdin;
\.

Using VACUUM FULL or plain REPACK in session 2 instead dumps all 1000 rows.

I realise REPACK (CONCURRENTLY) is documented as not being MVCC-safe,
and this is just demonstrating that, but the pg_dump docs don't
mention it, and as REPACK (CONCURRENTLY) is intended to be run while
the database is in use, it seems quite likely to coincide with a
backup at some point, and nothing indicates that the backup is missing
the table's data.

Should this at least be documented on the pg_dump page, or could
pg_dump detect that a table has been rewritten since its snapshot was
taken and fail instead?

Regards

Thom