Re: backup manifests

On Wed, Jan 01, 2020 at 08:57:11PM -0500, Robert Haas wrote:
> On Wed, Jan 1, 2020 at 7:46 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > David Fetter <david(at)fetter(dot)org> writes:
> > > On Wed, Jan 01, 2020 at 01:43:40PM -0500, Robert Haas wrote:
> > >> So, if someone can suggest to me how I could read JSON from a tool in
> > >> src/bin without writing a lot of code, I'm all ears.
> >
> > > Maybe I'm missing something obvious, but wouldn't combining
> > > pg_read_file() with a cast to JSONB fix this, as below?
> >
> > Only if you're prepared to restrict the use of the tool to superusers
> > (or at least people with whatever privilege that function requires).
> >
> > Admittedly, you can probably feed the data to the backend without
> > use of an intermediate file; but it still requires a working backend
> > connection, which might be a bit of a leap for backup-related tools.
> > I'm sure Robert was envisioning doing this processing inside the tool.
>
> Yeah, exactly. I don't think verifying a backup should require a
> running server, let alone a running server on the same machine where
> the backup is stored and for which you have superuser privileges.

Thanks for clarifying the context.

> AFAICS, the only options to make that work with JSON are (1) introduce
> a new hand-coded JSON parser designed for frontend operation, (2) add
> a dependency on an external JSON parser that we can use from frontend
> code, or (3) adapt the existing JSON parser used in the backend so
> that it can also be used in the frontend.
>
> I'd be willing to do (1) -- it wouldn't be the first time I've written
> JSON parser for PostgreSQL -- but I think it will take an order of
> magnitude more code than using a file with tab-separated columns as
> I've proposed, and I assume that there will be complaints about having
> two JSON parsers in core. I'd also be willing to do (2) if that's the
> consensus, but I'd vote against such an approach if somebody else
> proposed it because (a) I'm not aware of a widely-available library
> upon which we could depend and

I believe jq has an excellent one that's available under a suitable
license.

Making jq a dependency seems like a separate discussion, though. At
the moment, we don't use git tools like submodel/subtree, and deciding
which (or whether) seems like a gigantic discussion all on its own.

> (b) introducing such a dependency for a minor feature like this
> seems fairly unpalatable to me, and it'd probably still be more code
> than just using a tab-separated file. I'd be willing to do (3) if
> somebody could explain to me how to solve the problems with porting
> that code to work on the frontend side, but the only suggestion so
> far as to how to do that is to port memory contexts, elog/report,
> and presumably encoding handling to work on the frontend side.

This port has come up several times recently in different contexts.
How big a chunk of work would it be? Just so we're clear, I'm not
suggesting that this port should gate this feature.

> That seems to me to be an unreasonably large lift, especially given
> that we have lots of other files that use ad-hoc formats already,
> and if somebody ever gets around to converting all of those to JSON,
> they can certainly convert this one at the same time.

Would that require some kind of file converter program, or just a
really loud notice in the release notes?

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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