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Don't use on-disk snapshots for exported logical decoding snapshot.

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Logical decoding stores historical snapshots on disk, so that logical
decoding can restart without having to reconstruct a snapshot from
scratch (for which the resources are not guaranteed to be present
anymore).  These serialized snapshots were also used when creating a
new slot via the walsender interface, which can export a "full"
snapshot (i.e. one that can read all tables, not just catalog ones).

The problem is that the serialized snapshots are only useful for
catalogs and not for normal user tables.  Thus the use of such a
serialized snapshot could result in an inconsistent snapshot being
exported, which could lead to queries returning wrong data.  This
would only happen if logical slots are created while another logical
slot already exists.

Author: Petr Jelinek
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/f37e975c-908f-858e-707f-058d3b1eb214@2ndquadrant.com
Backport: 9.4, where logical decoding was introduced.
This commit is contained in:
Andres Freund
2017-04-27 15:28:24 -07:00
parent 7834d20b57
commit 56e19d938d
3 changed files with 22 additions and 9 deletions
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3
src/include/replication/snapbuild.h
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@@ -54,7 +54,8 @@ struct xl_running_xacts;
extern void CheckPointSnapBuild(void);
extern SnapBuild *AllocateSnapshotBuilder(struct ReorderBuffer *cache,
TransactionId xmin_horizon, XLogRecPtr start_lsn);
TransactionId xmin_horizon, XLogRecPtr start_lsn,
bool need_full_snapshot);
extern void FreeSnapshotBuilder(SnapBuild *cache);
extern void SnapBuildSnapDecRefcount(Snapshot snap);