On Fri, Jul 12, 2013 at 11:42:23AM +0200, Andres Freund wrote:
> On 2013-07-11 15:09:45 -0400, Tom Lane wrote:
> > It never has been, and never will be, allowed to call the catcache code
> > without being in a transaction. What do you think will happen if the
> > requested row isn't in cache? A table access, that's what, and that
> > absolutely requires being in a transaction.
> I'd like to add an Assert like in the attached patch making sure we're
> in a transaction. Otherwise it's far too easy not to hit an error during
> development because everything is cached - and syscache usage isn't
> always obvious from the outside to the naive or the tired.
The following test case reliably hits this new assertion:
create table t (c int);
begin;
create index on t (c);
savepoint q;
insert into t values (1);
select 1/0;
Stack trace:
#2 0x0000000000761159 in ExceptionalCondition (conditionName=<value optimized out>, errorType=<value optimized out>,
fileName=<valueoptimized out>, lineNumber=<value optimized out>) at assert.c:54
#3 0x000000000074fc53 in SearchCatCache (cache=0xc53048, v1=139479, v2=0, v3=0, v4=0) at catcache.c:1072
#4 0x000000000075c011 in SearchSysCache (cacheId=0, key1=369539, key2=6, key3=18446744073709551615, key4=0) at
syscache.c:909
#5 0x0000000000757e57 in RelationReloadIndexInfo (relation=0x7f2dd5fffea0) at relcache.c:1770
#6 0x000000000075817b in RelationClearRelation (relation=0x7f2dd5fffea0, rebuild=1 '\001') at relcache.c:1926
#7 0x00000000007588c6 in RelationFlushRelation (relationId=139479) at relcache.c:2076
#8 RelationCacheInvalidateEntry (relationId=139479) at relcache.c:2138
#9 0x000000000075185d in LocalExecuteInvalidationMessage (msg=0xcde778) at inval.c:546
#10 0x0000000000751185 in ProcessInvalidationMessages (hdr=0xc0d390, func=0x7517d0 <LocalExecuteInvalidationMessage>)
atinval.c:422
#11 0x0000000000751a3f in AtEOSubXact_Inval (isCommit=0 '\000') at inval.c:973
#12 0x00000000004cb986 in AbortSubTransaction () at xact.c:4250
#13 0x00000000004cbf05 in AbortCurrentTransaction () at xact.c:2863
#14 0x00000000006968f6 in PostgresMain (argc=1, argv=0x7fff7cf71610, dbname=0xc13b60 "test", username=0xbedc88 "nm") at
postgres.c:3848
#15 0x000000000064c2b5 in BackendRun () at postmaster.c:4044
#16 BackendStartup () at postmaster.c:3733
#17 ServerLoop () at postmaster.c:1571
#18 0x000000000064fa8d in PostmasterMain (argc=1, argv=0xbe84a0) at postmaster.c:1227
#19 0x00000000005e2dcd in main (argc=1, argv=0xbe84a0) at main.c:196
When we call AtEOSubXact_Inval() or AtEOXact_Inval() with a relation still
open, we can potentially get a relcache rebuild and therefore a syscache
lookup as shown above. CommitSubTransaction() is also potentially affected,
though I don't have an SQL-level test case for that. It calls
CommandCounterIncrement() after moving to TRANS_COMMIT. That CCI had better
find no invalidations of open relations, or we'll make syscache lookups. (In
simple tests, any necessary invalidations tend to happen at the CCI in
CommitTransactionCommand(), so the later CCI does in fact find nothing to do.
I have little confidence that should be counted upon, though.)
How might we best rearrange things to avoid these hazards?
Thanks,
nm
--
Noah Misch
EnterpriseDB http://www.enterprisedb.com
