Re: Create subscription with `create_slot=false` and incorrect slot name

On 25/05/17 23:26, Peter Eisentraut wrote:
> On 5/24/17 21:41, Robert Haas wrote:
>>> This came up in a previous thread. It is up to the publishing end what
>>> slot names it accepts. So running the validation locally is incorrect.
>>
>> That argument seems pretty tenuous; surely both ends are PostgreSQL,
>> and the rules for valid slot names aren't likely to change very often.
>
> Remember that this could be used for upgrades and side-grades, so the
> local rules could change or be more restricted in the future or
> depending on compilation options.
>
>> But even if we accept it as true, I still don't like the idea that a
>> DROP can just fail, especially with no real guidance as to how to fix
>> things so it doesn't fail. Ideas:
>>
>> 1. If we didn't create the slot (and have never connected to it?),
>> don't try to drop it.
>
> That would conceptually be nice, but it would probably create a bunch of
> problems of its own. For one, we would need an interlock so that the
> first $anything that connects to the slot registers it in the local
> catalog as "it's mine now".
>
>> 2. Emit some kind of a HINT telling people about ALTER SUBSCRIPTION ..
>> SET (slot_name = NONE).
>
> The reported error is just one of many errors that can happen when DROP
> SUBSCRIPTION tries to drop the slot (doens't exist, still active, no
> permission, etc.). We don't want to give the hint that is effectively
> "just forget about the slot then" for all of them. So we would need
> some way to distinguish "you can't do this right now" from "this would
> never work" (400 vs 500 errors).
>

This specific error returns ERRCODE_UNDEFINED_OBJECT error code so we
could check for it and offer hint only for this case.

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services