Horton

Re: snapbuild woes

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: pgsql-hackers@postgresql.org
Date: 2017-06-09T07:25:34Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> wrote:

> Looking at 0001:
> - GetOldestSafeDecodingTransactionId() only guarantees to return an xid
>   safe for decoding (note how procArray->replication_slot_catalog_xmin
>   is checked), not one for the initial snapshot - so afaics this whole
>   exercise doesn't guarantee much so far.

I happen to use CreateInitDecodingContext() in an extension, so I had to think
what the new argumen exactly means (as for the incompatibility between PG
9.6.2 and 9.6.3, I suppose preprocessor directives can handle it).

One thing I'm failing to understand is: if TRUE is passed for
need_full_snapshot, then slot->effective_xmin receives the result of

    GetOldestSafeDecodingTransactionId(need_full_snapshot)

but this does include "catalog xmin".

If the value returned is determined by an existing slot which has valid
effective_catalog_xmin and invalid effective_xmin (i.e. that slot only
protects catalog tables from VACUUM but not the regular ones), then the new
slot will think it (i.e. the new slot) protects even non-catalog tables, but
that's no true.

Shouldn't the xmin_horizon be computed by this call instead?

    GetOldestSafeDecodingTransactionId(!need_full_snapshot);

(If so, I think "considerCatalog" argument name would be clearer than
"catalogOnly".)

-- 
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at

Commits

  1. Fix thinko introduced in 2bef06d516460 et al.

  2. Avoid superfluous work for commits during logical slot creation.

  3. Fix race condition leading to hanging logical slot creation.

  4. Don't use on-disk snapshots for exported logical decoding snapshot.

  5. Preserve required !catalog tuples while computing initial decoding snapshot.