mvcc catalo gsnapshots and TopTransactionContext

Hi,

since the mvcc catalog patch has gone in we require all users of
systable_* to be in a valid transaction since the snapshot is copied via
CopySnapshot() in RegisterSnapshot(). Which we call in
systable_beginscan(). CopySnapshot() allocates the copied snapshot in
TopTransactionContext.

There doesn't seem be an explicitly stated rule that we cannot use the
syscaches outside of a transaction - but effectively that's required
atm.

Not having investigated at all so far I am not sure how much in core
code that breaks, but it certainly broke some out of tree code of mine
(bidirectional replication stuff, bdr branch on git.pg.o).

Is that acceptable?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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