Re: min_safe_lsn column in pg_replication_slots view

On 2020-Jul-09, Tom Lane wrote:

> and even the most cursory look at the code confirms that there's a
> real bug here.  KeepLogSeg expects *logSegNo to be defined on entry,
> but GetWALAvailability hasn't bothered to initialize oldestSlotSeg.
> It is not clear to me which one is in the wrong; the comment for
> KeepLogSeg isn't particularly clear on this.

Oh, so I introduced the bug when I removed the initialization in this
fix.  That one was using the wrong datatype, but evidently it achieved
the right effect.  And KeepLogSeg is using the wrong datatype Invalid
macro also.

I think we should define InvalidXLogSegNo to be ~((uint64)0) and add a
macro to test for that.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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