Re: snapbuild woes

On 2017-06-09 09:25:34 +0200, Antonin Houska wrote:
> Andres Freund <andres(at)anarazel(dot)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".)

Good catch. Pushing a fix. Afaict it's luckily inconsequential atm
because fo the way we wait for concurrent snapshots when creating a
slot. But it obviously nevertheless needs tobe fixed.

Greetings,

Andres Freund