pgsql: Improve REPACK (CONCURRENTLY) error when wal_level < replica

Improve REPACK (CONCURRENTLY) error when wal_level < replica

The error emitted when REPACK (CONCURRENTLY) is run with too low a
wal_level is thrown by CheckSlotRequirements(), which is a bit
mysterious when the user doesn't know what's up. Add an upfront check
in check_concurrent_repack_requirements() for a more explicit, REPACK-
centered report, which is easier to understand -- this also saves
starting the worker just to have it die immediately.

Author: Baji Shaik <baji(dot)pgdev(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+fm-ROdgh0rEVuXoViBk4TVgjodrN=MTR_RYuOuKLZ9voX4YA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/43649b6a53e9b3964e921cc962f5c977bce688fd

Modified Files
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src/backend/commands/repack.c | 9 +++++++++
1 file changed, 9 insertions(+)