Re: Create replication slot in pg_basebackup if requested and not yet present

Hi,

On Thu, Mar 23, 2017 at 12:41:54PM +0100, Magnus Hagander wrote:
> On Tue, Mar 21, 2017 at 8:34 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > So I tend to think that there should always be some explicit user
> > action to cause the creation of a slot, like --create-slot-if-needed
> > or --create-slot=name. That still won't prevent careless use of that
> > option but it's less dangerous than assuming that a user who refers to
> > a nonexistent slot intended to create it when, perhaps, they just
> > typo'd it.
>
> Well, the explicit user action would be "--slot". But sure, I can
> definitely live with a --create-if-not-exists.

Can we just make that option create slot and don't worry if one exists
already? CreateReplicationSlot() can be told to not mind about already
existing slots, and I don't see a huge point in erroring out if the slot
exists already, unless somebody can show that this leads to data loss
somehow.

> The important thing, to me, is that you can run it as a single
> command, which makes it a lot easier to work with. (And not like we
> currently have for pg_receivewal which requires a separate command to
> create the slot)

Oh, that is how it works with pg_receivewal, I have to admit I've never
used it so was a bit confused about this when I read its code.

So in that case I think we don't necessarily need to have the same user
interface at all. I first thought about just adding "-C, --create" (as
in "--create --slot=foo"), but this on second thought this looked a bit
shortsighted - who knows what flashy thing pg_basebackup might create in
5 years... So I settled on --create-slot, which is only slightly more to
type (albeit repetive, "--create-slot --slot=foo"), but adding a short
option "-C" would be fine I thinkg "-C -S foo".

So attached is a patch with adds that option. If people really think it
should be --create-slot-if-not-exists instead I can update the patch, of
course.

I again added a second patch with some further refactoring which makes
it print a message on temporary slot creation in verbose mode.

Michael

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