On 25 August 2014 02:57, Alvaro Herrera <[email protected]> wrote:
> Thomas Munro wrote:
>> The difficulty of course will be testing all these racy cases reproducibly...
>
> Does this help?
> http://www.postgresql.org/message-id/[email protected]
> The useful trick there is forcing a query to get its snapshot and then
> go to sleep before actually doing anything, by way of an advisory lock.
Yes it does, thanks Alvaro and Craig. I think the attached spec
reproduces the problem using that trick, ie shows NOWAIT blocking,
presumably in EvalPlanQualFetch (though I haven't stepped through it
with a debugger yet). I'm afraid I'm out of Postgres hacking cycles
for a few days, but next weekend I should have a new patch that fixes
this by teaching EvalPlanQualFetch about wait policies, with isolation
tests for NOWAIT and SKIP LOCKED.
Best regards,
Thomas Munro
# Test NOWAIT with an updated tuple chain.
setup
{
CREATE TABLE foo (
id int PRIMARY KEY,
data text NOT NULL
);
INSERT INTO foo VALUES (1, 'x');
}
teardown
{
DROP TABLE foo;
}
session "s1"
setup { BEGIN; }
step "s1a" { SELECT * FROM foo WHERE pg_advisory_lock(0) IS NOT NULL FOR UPDATE NOWAIT; }
step "s1b" { COMMIT; }
session "s2"
step "s2a" { SELECT pg_advisory_lock(0); }
step "s2b" { UPDATE foo SET data = data; }
step "s2c" { BEGIN; }
step "s2d" { UPDATE foo SET data = data; }
step "s2e" { SELECT pg_advisory_unlock(0); }
step "s2f" { COMMIT; }
permutation "s2a" "s1a" "s2b" "s2c" "s2d" "s2e" "s1b" "s2f"
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