Here is the patch for snapshot management I currently have. Below is a complete description of the changes in this patch.
The most important change is on the use of CopySnapshot and the games with ActiveSnapshot. On the new code, whenever a piece of code needs a snapshot to persist, it is "registered" by using RegisterSnapshot. As soon as it is done with it, it must be unregistered with UnregisterSnapshot. For ActiveSnapshot, we deal with a stack structure. Callers needing an Active snap set call PushActiveSnapshot, and PopActiveSnapshot when they no longer need it. GetSnapshotData still creates initial versions of each snapshot in static memory. There's a new boolean on the SnapshotData struct (set to false by GetSnapshotData), which indicates whether the snapshot has been copied out of static memory. When a snapshot is Registered or PushedActive, this bit is checked: if false, then a copy of the snapshot is made on the dedicated SnapshotContext, and the "copied" flag is flipped. SnapshotData also carries two new reference counts: one for the Register list, another one for the ActiveSnapshot stack. Whenever a snap is Unregistered or Popped, both refcounts are checked. If both are found to be zero then memory can be released. On Unregister and Pop, after deleting the snapshot, we also verify the whole lists. If they are empty, it means no more live snapshot remain for this transaction. In this case we can reset MyProc->xmin to InvalidXid. Thus, GetSnapshotData must now recalculate MyProc->xmin each time it finds xmin to be Invalid, which can not only happen for the serializable snapshot but at any time. A serializable transaction Registers its snapshot as first thing, and keeps it registered until transaction end. Note: I had previously made noises about changing the semantics of MyProc->xmin or introducing a new VacuumXmin. I have refrained from doing so in this patch. Those are the high-level changes. Some coding changes: - SerializableSnapshot and LatestSnapshot as global symbols are no more. - A couple of PG_TRY blocks have been removed. Particularly, on plancache.c this means the do_planning() routine is now pointless and has been folded back into its sole caller. - Three CopySnapshot call sites remain outside snapmgr.c: DoCopy() on copy.c, ExplainOnePlan() on explain.c and _SPI_execute_plan() on spi.c. They are there because they grab the current ActiveSnapshot, modify it, and then use the resulting snapshot. There is no corresponding FreeSnapshot, because it's not needed. - CommandCounterIncrement now calls into snapmgr.c to set the CID of the static snapshots. - CREATE INDEX CONCURRENTLY, VACUUM FULL, and CLUSTER must now explicitely Pop the ActiveSnapshot set by PortalRunUtility before calling CommitTransactionCommand. -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc.
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