[Reposting with compressed patch] Okay, I think I've fixed most of the issues in the reviewed patch. Updated patch attached.
The most interesting change is that I've done away with CopySnapshot as a public routine in favor of a new PushUpdatedSnapshot which does the copy-update-push sequence. Also I added a refcount to RegdSnapshotElt as suggested, and changed the subxact logic to substract that exact amount on abort. There's something I'm not sure what to do about: Tom Lane wrote: > Also, I think that the whole snapshot-sharing mechanism is not working > as intended except for the serializable case; otherwise sequences > like > x = RegisterSnapshot(GetTransactionSnapshot()); > y = RegisterSnapshot(GetTransactionSnapshot()); > will result in x and y being separate copies. Or are you assuming > that this just isn't worth optimizing? It's not that I don't think it's worth optimizing, but I think it's a bit away from the scope of this patch. The problem here is how to notice that two consecutive GetTransactionSnapshot calls should really return different snapshots, considering that shared state may change in between. Perhaps there's an easy way to optimize that; I don't know. What does work is to get (say) a registered snapshot and push it as active snapshot. That results in a successfully shared snapshot. For example PortalStart does that for cursors, etc. (FWIW another thing which is probably worth rethinking is the handling of snapshots around PortalStart. Some callers pass the currently active snapshot; Others pass InvalidSnapshot. Another passes an arbitrary snapshot. When it's Invalid, PortalStart calls GetTransactionSnapshot, otherwise it uses the passed snap for PushActiveSnapshot. So this is all a bit confusing and wasteful and could use some clean up. This is material for a new patch however.) -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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