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Commitd21690e

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Remove race condition in pg_get_expr().
Since its introduction, pg_get_expr() has intended to silentlyreturn NULL if called with an invalid relation OID, as can happenwhen scanning the catalogs concurrently with relation drops.However, there is a race condition: we check validity of the OIDat the start, but it could get dropped just afterward, leading tofailures. This is the cause of some intermittent instability we'reseeing in a proposed new test case, and presumably it's a hazard inthe field as well.We can fix this by AccessShareLock-ing the target relation for theduration of pg_get_expr(). Since we don't require any permissionson the target relation, this is semantically a bit undesirable. Butit turns out that the set_relation_column_names() subroutine alreadytakes a transient AccessShareLock on that relation, and has done sincecommit2ffa740 in 2012. Given the lack of complaints about that, itseems like there should be no harm in holding the lock a bit longer.Back-patch to all supported branches.Discussion:https://postgr.es/m/31ddcc01-a71b-4e8c-9948-01d1c47293ca@eisentraut.org
1 parent8ead39e commitd21690e

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1 file changed

+33
-35
lines changed

1 file changed

+33
-35
lines changed

‎src/backend/utils/adt/ruleutils.c

Lines changed: 33 additions & 35 deletions
Original file line numberDiff line numberDiff line change
@@ -347,8 +347,7 @@ static char *pg_get_partkeydef_worker(Oid relid, int prettyFlags,
347347
boolattrsOnly,boolmissing_ok);
348348
staticchar*pg_get_constraintdef_worker(OidconstraintId,boolfullCommand,
349349
intprettyFlags,boolmissing_ok);
350-
statictext*pg_get_expr_worker(text*expr,Oidrelid,constchar*relname,
351-
intprettyFlags);
350+
statictext*pg_get_expr_worker(text*expr,Oidrelid,intprettyFlags);
352351
staticintprint_function_arguments(StringInfobuf,HeapTupleproctup,
353352
boolprint_table_args,boolprint_defaults);
354353
staticvoidprint_function_rettype(StringInfobuf,HeapTupleproctup);
@@ -2575,36 +2574,28 @@ decompile_column_index_array(Datum column_index_array, Oid relId,
25752574
* the one specified by the second parameter. This is sufficient for
25762575
* partial indexes, column default expressions, etc. We also support
25772576
* Var-free expressions, for which the OID can be InvalidOid.
2577+
*
2578+
* If the OID is nonzero but not actually valid, don't throw an error,
2579+
* just return NULL. This is a bit questionable, but it's what we've
2580+
* done historically, and it can help avoid unwanted failures when
2581+
* examining catalog entries for just-deleted relations.
25782582
* ----------
25792583
*/
25802584
Datum
25812585
pg_get_expr(PG_FUNCTION_ARGS)
25822586
{
25832587
text*expr=PG_GETARG_TEXT_PP(0);
25842588
Oidrelid=PG_GETARG_OID(1);
2589+
text*result;
25852590
intprettyFlags;
2586-
char*relname;
25872591

25882592
prettyFlags=PRETTYFLAG_INDENT;
25892593

2590-
if (OidIsValid(relid))
2591-
{
2592-
/* Get the name for the relation */
2593-
relname=get_rel_name(relid);
2594-
2595-
/*
2596-
* If the OID isn't actually valid, don't throw an error, just return
2597-
* NULL. This is a bit questionable, but it's what we've done
2598-
* historically, and it can help avoid unwanted failures when
2599-
* examining catalog entries for just-deleted relations.
2600-
*/
2601-
if (relname==NULL)
2602-
PG_RETURN_NULL();
2603-
}
2594+
result=pg_get_expr_worker(expr,relid,prettyFlags);
2595+
if (result)
2596+
PG_RETURN_TEXT_P(result);
26042597
else
2605-
relname=NULL;
2606-
2607-
PG_RETURN_TEXT_P(pg_get_expr_worker(expr,relid,relname,prettyFlags));
2598+
PG_RETURN_NULL();
26082599
}
26092600

26102601
Datum
@@ -2613,31 +2604,25 @@ pg_get_expr_ext(PG_FUNCTION_ARGS)
26132604
text*expr=PG_GETARG_TEXT_PP(0);
26142605
Oidrelid=PG_GETARG_OID(1);
26152606
boolpretty=PG_GETARG_BOOL(2);
2607+
text*result;
26162608
intprettyFlags;
2617-
char*relname;
26182609

26192610
prettyFlags=pretty ? (PRETTYFLAG_PAREN |PRETTYFLAG_INDENT |PRETTYFLAG_SCHEMA) :PRETTYFLAG_INDENT;
26202611

2621-
if (OidIsValid(relid))
2622-
{
2623-
/* Get the name for the relation */
2624-
relname=get_rel_name(relid);
2625-
/* See notes above */
2626-
if (relname==NULL)
2627-
PG_RETURN_NULL();
2628-
}
2612+
result=pg_get_expr_worker(expr,relid,prettyFlags);
2613+
if (result)
2614+
PG_RETURN_TEXT_P(result);
26292615
else
2630-
relname=NULL;
2631-
2632-
PG_RETURN_TEXT_P(pg_get_expr_worker(expr,relid,relname,prettyFlags));
2616+
PG_RETURN_NULL();
26332617
}
26342618

26352619
statictext*
2636-
pg_get_expr_worker(text*expr,Oidrelid,constchar*relname,intprettyFlags)
2620+
pg_get_expr_worker(text*expr,Oidrelid,intprettyFlags)
26372621
{
26382622
Node*node;
26392623
List*context;
26402624
char*exprstr;
2625+
Relationrel=NULL;
26412626
char*str;
26422627

26432628
/* Convert input TEXT object to C string */
@@ -2648,16 +2633,29 @@ pg_get_expr_worker(text *expr, Oid relid, const char *relname, int prettyFlags)
26482633

26492634
pfree(exprstr);
26502635

2651-
/* Prepare deparse context if needed */
2636+
/*
2637+
* Prepare deparse context if needed. If we are deparsing with a relid,
2638+
* we need to transiently open and lock the rel, to make sure it won't go
2639+
* away underneath us. (set_relation_column_names would lock it anyway,
2640+
* so this isn't really introducing any new behavior.)
2641+
*/
26522642
if (OidIsValid(relid))
2653-
context=deparse_context_for(relname,relid);
2643+
{
2644+
rel=try_relation_open(relid,AccessShareLock);
2645+
if (rel==NULL)
2646+
returnNULL;
2647+
context=deparse_context_for(RelationGetRelationName(rel),relid);
2648+
}
26542649
else
26552650
context=NIL;
26562651

26572652
/* Deparse */
26582653
str=deparse_expression_pretty(node,context, false, false,
26592654
prettyFlags,0);
26602655

2656+
if (rel!=NULL)
2657+
relation_close(rel,AccessShareLock);
2658+
26612659
returnstring_to_text(str);
26622660
}
26632661

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