I recently submitted PR #672 to the xeps repo

https://github.com/xsf/xeps/pull/672

to make users of RSM, like MAM, aware whether the result is exact or
not. It received some scepticism from the council members in today's
council meeting. I am to blame here as I thought the abstract motivation
in the commit message was enough. It appears it wasn't.

While I think multiple applications could exploit that information, my
particular motivation was MAM. Consider the scenario where you have a
master archive and a local archive. The local archive may have multiple
holes at unknown locations. Now you want to sync your local archive from
the master using MAM/RSM.

If you don't know whether or not the MAM RSM results are exact, you need
resort to basically re-syncing the complete archive from the master. But
if you know that the results are exact, you could use count-only MAM RSM
queries (XEP-0059 § 2.7) and the unique-and-stable IDs of the archived
messages to effectively determine the holes and fill them up, using a
simple bisection algorithm which compares the message count with the
expect unique-and-stable archive message ID. This saves a lot of
roundtrips and especially transferred data when syncing the archive.

I also like to point out that the changes in #672 are backwards
compatible. Not namespace bump required.

I'm off to barbecue, but I'm still looking forward to your feedback.

- Florian

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