Re: Weird failure with latches in curculio on v15

On Wed, Feb 08, 2023 at 04:24:15PM -0500, Robert Haas wrote:
> On Wed, Feb 8, 2023 at 12:43 PM Nathan Bossart  wrote:
>> I think this could be a good approach if we decide not to bake too much
>> into PostgreSQL itself (e.g., such as creating multiple archive workers
>> that each call out to the module).  Archive module authors would
>> effectively need to write their own archiver processes.  That sounds super
>> flexible, but it also sounds like it might be harder to get right.
> 
> Yep. That's a problem, and I'm certainly open to better ideas.
> 
> However, if we assume that the archive module is likely to be doing
> something like juggling a bunch of file descriptors over which it is
> speaking HTTP, what other model works, really? It might be juggling
> those file descriptors indirectly, or it might be relying on an
> intermediate library like curl or something from Amazon that talks to
> S3 or whatever, but only it knows what resources it's juggling, or
> what functions it needs to call to manage them. On the other hand, we
> don't really need a lot from it. We need it to CHECK_FOR_INTERRUPTS()
> and handle that without leaking resources or breaking the world in
> some way, and we sort of need it to, you know, actually archive stuff,
> but apart from that I guess it can do what it likes (unless I'm
> missing some other important function of the archiver?).
> 
> It's probably a good idea if the archiver function returns when it's
> fully caught up and there's no more work to do. Then we could handle
> decisions about hibernation in the core code, rather than having every
> archive module invent its own way of doing that. But when there's work
> happening, as far as I can see, the archive module needs to have
> control pretty nearly all the time, or it's not going to be able to do
> anything clever.
> 
> Always happy to hear if you see it differently....

These are all good points.  Perhaps there could be a base archiver
implementation that shell_archive uses (and that other modules could use if
desired, which might be important for backward compatibility with the
existing callbacks).  But if you want to do something fancier than
archiving sequentially, you could write your own.

In any case, I'm not really wedded to any particular approach at the
moment, so I am likewise open to better ideas.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com