On 2017-05-29 23:49:33 +0200, Petr Jelinek wrote:
> I am not quite sure I understand (both the vxid suggestion and for the
> session dying badly). Maybe we can discuss bit more when you get to
> computer so it's easier for you to expand on what you mean.
The xid interlock when exporting a snapshot is required because
otherwise it's not generally guaranteed that all resourced required for
the snapshot are reserved. In the logical replication case we could
guarantee that otherwise, but there'd be weird-ish edge cases when
erroring out just after exporting a snapshot.
The problem with using the xid as that interlock is that it requires
acquiring an xid - which is something we're going to block against when
building a new catalog snapshot. Afaict that's not entirely required -
all that we need to verify is that the snapshot in the source
transaction is still running. The easiest way for the importer to check
that the source is still alive seems to be export the virtual
transaction id instead of the xid. Normally we can't store things like
virtual xids on disk, but that concern isn't valid here because exported
snapshots are ephemeral, there's also already a precedent in
predicate.c.
It seems like it'd be fairly easy to change things around that way, but
maybe I'm missing something.
- Andres
