Re: Review for GetWALAvailability()

On 2020-Jun-25, Fujii Masao wrote:

>     /*
>      * Find the oldest extant segment file. We get 1 until checkpoint removes
>      * the first WAL segment file since startup, which causes the status being
>      * wrong under certain abnormal conditions but that doesn't actually harm.
>      */
>     oldestSeg = XLogGetLastRemovedSegno() + 1;
> 
> I see the point of the above comment, but this can cause wal_status to be
> changed from "lost" to "unreserved" after the server restart. Isn't this
> really confusing? At least it seems better to document that behavior.

Hmm.

> Or if we *can ensure* that the slot with invalidated_at set always means
> "lost" slot, we can judge that wal_status is "lost" without using fragile
> XLogGetLastRemovedSegno(). Thought?

Hmm, this sounds compelling -- I think it just means we need to ensure
we reset invalidated_at to zero if the slot's restart_lsn is set to a
correct position afterwards.  I don't think we have any operation that
does that, so it should be safe -- hopefully I didn't overlook anything?
Neither copy nor advance seem to work with a slot that has invalid
restart_lsn.

> Or XLogGetLastRemovedSegno() should be fixed so that it returns valid
> value even after the restart?

This seems more work to implement.

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