Re: Postgres restart in the middle of exclusive backup and the presence of backup_label file

On 2/28/22 23:51, Nathan Bossart wrote:
> On Sat, Feb 26, 2022 at 02:06:14PM -0800, Nathan Bossart wrote:
>> On Sat, Feb 26, 2022 at 04:48:52PM +0000, Chapman Flack wrote:
>>> Assuming the value is false, so no error is thrown, is it practical to determine
>>> from flinfo->fn_expr whether the value was defaulted or supplied? If so, I would
>>> further suggest reporting a deprecation WARNING if it was explicitly supplied,
>>> with a HINT that the argument can simply be removed at the call site, and will
>>> become unrecognized in some future release.
>>
>> This is a good point. I think I agree with your proposed changes. I
>> believe it is possible to add a deprecation warning only when 'exclusive'
>> is specified. If anything, we can create a separate function that accepts
>> the 'exclusive' parameter and that always emits a NOTICE or WARNING.
>
> I've spent some time looking into this, and I haven't found a clean way to
> emit a WARNING only if the "exclusive" parameter is supplied (and set to
> false). AFAICT flinfo->fn_expr doesn't tell us whether the parameter was
> supplied or the default value was used. I was able to get it working by
> splitting pg_start_backup() into 3 separate internal functions (i.e.,
> pg_start_backup_1arg(), pg_start_backup_2arg(), and
> pg_start_backup_3arg()), but this breaks calls such as
> pg_start_backup('mylabel', exclusive => false), and it might complicate
> privilege management for users.
>
> Without a WARNING, I think it will be difficult to justify removing the
> "exclusive" parameter in the future. We would either need to leave it
> around forever, or we would have to risk unnecessarily breaking some
> working backup scripts. I wonder if we should just remove it now and make
> sure that this change is well-documented in the release notes.

Personally, I am in favor of removing it. We change/rename
functions/tables/views when we need to, and this happens in almost every
release.

What we need to do is make sure that an older installation won't
silently work in a broken way, i.e. if we remove the exclusive flag
somebody expecting the pre-9.6 behavior might not receive an error and
think everything is OK. That would not be good.

One option might be to rename the functions. Something like
pg_backup_start/stop.

Regards,
-David