Re: snapshot too old, configured by time

On Tue, Mar 22, 2016 at 5:05 AM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
> Thanks to all for the feedback; I will try to respond later this
> week. First I'm trying to get my reviews for other patches posted.

I have been looking at 4a, the test module, and things are looking
good IMO. Something that I think would be adapted would be to define
the options for isolation tests in a variable, like ISOLATION_OPTS to
allow MSVC scripts to fetch those option values more easily.

+submake-test_decoding:
+ $(MAKE) -C $(top_builddir)/src/test/modules/snapshot_too_old
The target name here is incorrect. This should refer to snapshot_too_old.

Regarding the main patch:
+ <primary><varname>old_snapshot_threshold</> configuration
parameter</primary>
snapshot_valid_limit?

page = BufferGetPage(buf);
+ TestForOldSnapshot(scan->xs_snapshot, rel, page);
This is a sequence repeated many times in this patch, a new routine,
say BufferGetPageExtended with a uint8 flag, one flag being used to
test old snapshots would be more adapted. But this would require
creating a header dependency between the buffer manager and
SnapshotData.. Or more simply we may want a common code path when
fetching a page that a backend is going to use to fetch tuples. I am
afraid of TestForOldSnapshot() being something that could be easily
forgotten in code paths introduced in future patches...

+ if (whenTaken < 0)
+ {
+ elog(DEBUG1,
+ "MaintainOldSnapshotTimeMapping called with negative
whenTaken = %ld",
+ (long) whenTaken);
+ return;
+ }
+ if (!TransactionIdIsNormal(xmin))
+ {
+ elog(DEBUG1,
+ "MaintainOldSnapshotTimeMapping called with xmin = %lu",
+ (unsigned long) xmin);
+ return;
+ }
Material for two assertions?
--
Michael