Re: Minimal logical decoding on standbys

On Thu, May 23, 2019 at 8:08 AM Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> This made me think more of the race conditions. For instance, in
> pg_create_logical_replication_slot(), just after
> CheckLogicalDecodingRequirements and before actually creating the
> slot, suppose concurrently Controlfile->wal_level is changed from
> logical to replica. So suppose a new slot does get created. Later the
> slot is read, so in pg_logical_slot_get_changes_guts(),
> CheckLogicalDecodingRequirements() is called where it checks
> ControlFile->wal_level value. But just before it does that,
> ControlFile->wal_level concurrently changes back to logical, because
> of replay of another param-change record. So this logical reader will
> think that the wal_level is sufficient, and will proceed to read the
> records, but those records are *before* the wal_level change, so these
> records don't have logical data.
>
> Do you think this is possible, or I am missing something?

wal_level is PGC_POSTMASTER.

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Robert Haas
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