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Check SQN order fold does not fold beyond end of snapshot

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the Journla snapshot is not a true snapshot, in that the active file in the snapshot can still be taking appends.  So when getting a snapshot it is necessary to check if folding over the snapshot that the SQN is <= JournalSQN when the snapshot is taken.

Normally consistency of the snapshot is managed as the operation depends on the penciller, and the penciller *is* a snapshot.  Not in this case, as the penciller will return true on a sqn check if the pcl SQN is behind the Journal.  So the Journal folder, has been given an additionla check to stop at the JournalSQN.

This is perhaps a fault in the pcl check sqn, which should only return true on an exact match?  I'm nervous about changing this though, so we have a less pure fix for now.
This commit is contained in:
Martin Sumner 2019-02-14 21:14:11 +00:00
parent 5799f06452
commit db1486fa36
4 changed files with 102 additions and 14 deletions
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5
src/leveled_penciller.erl
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@ -1482,6 +1482,11 @@ compare_to_sqn(Obj, SQN) ->
SQNToCompare > SQN ->
false;
true ->
% Normally we would expect the SQN to be equal here, but
% this also allows for the Journal to have a more advanced
% value. We return true here as we wouldn't want to
% compact thta more advanced value, but this may cause
% confusion in snapshots.
true
end
end.