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Commitceb224b

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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 parent8c5da20 commitceb224b

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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
@@ -344,8 +344,7 @@ static char *pg_get_partkeydef_worker(Oid relid, int prettyFlags,
344344
boolattrsOnly,boolmissing_ok);
345345
staticchar*pg_get_constraintdef_worker(OidconstraintId,boolfullCommand,
346346
intprettyFlags,boolmissing_ok);
347-
statictext*pg_get_expr_worker(text*expr,Oidrelid,constchar*relname,
348-
intprettyFlags);
347+
statictext*pg_get_expr_worker(text*expr,Oidrelid,intprettyFlags);
349348
staticintprint_function_arguments(StringInfobuf,HeapTupleproctup,
350349
boolprint_table_args,boolprint_defaults);
351350
staticvoidprint_function_rettype(StringInfobuf,HeapTupleproctup);
@@ -2384,36 +2383,28 @@ decompile_column_index_array(Datum column_index_array, Oid relId,
23842383
* the one specified by the second parameter. This is sufficient for
23852384
* partial indexes, column default expressions, etc. We also support
23862385
* Var-free expressions, for which the OID can be InvalidOid.
2386+
*
2387+
* If the OID is nonzero but not actually valid, don't throw an error,
2388+
* just return NULL. This is a bit questionable, but it's what we've
2389+
* done historically, and it can help avoid unwanted failures when
2390+
* examining catalog entries for just-deleted relations.
23872391
* ----------
23882392
*/
23892393
Datum
23902394
pg_get_expr(PG_FUNCTION_ARGS)
23912395
{
23922396
text*expr=PG_GETARG_TEXT_PP(0);
23932397
Oidrelid=PG_GETARG_OID(1);
2398+
text*result;
23942399
intprettyFlags;
2395-
char*relname;
23962400

23972401
prettyFlags=PRETTYFLAG_INDENT;
23982402

2399-
if (OidIsValid(relid))
2400-
{
2401-
/* Get the name for the relation */
2402-
relname=get_rel_name(relid);
2403-
2404-
/*
2405-
* If the OID isn't actually valid, don't throw an error, just return
2406-
* NULL. This is a bit questionable, but it's what we've done
2407-
* historically, and it can help avoid unwanted failures when
2408-
* examining catalog entries for just-deleted relations.
2409-
*/
2410-
if (relname==NULL)
2411-
PG_RETURN_NULL();
2412-
}
2403+
result=pg_get_expr_worker(expr,relid,prettyFlags);
2404+
if (result)
2405+
PG_RETURN_TEXT_P(result);
24132406
else
2414-
relname=NULL;
2415-
2416-
PG_RETURN_TEXT_P(pg_get_expr_worker(expr,relid,relname,prettyFlags));
2407+
PG_RETURN_NULL();
24172408
}
24182409

24192410
Datum
@@ -2422,31 +2413,25 @@ pg_get_expr_ext(PG_FUNCTION_ARGS)
24222413
text*expr=PG_GETARG_TEXT_PP(0);
24232414
Oidrelid=PG_GETARG_OID(1);
24242415
boolpretty=PG_GETARG_BOOL(2);
2416+
text*result;
24252417
intprettyFlags;
2426-
char*relname;
24272418

24282419
prettyFlags=pretty ? (PRETTYFLAG_PAREN |PRETTYFLAG_INDENT |PRETTYFLAG_SCHEMA) :PRETTYFLAG_INDENT;
24292420

2430-
if (OidIsValid(relid))
2431-
{
2432-
/* Get the name for the relation */
2433-
relname=get_rel_name(relid);
2434-
/* See notes above */
2435-
if (relname==NULL)
2436-
PG_RETURN_NULL();
2437-
}
2421+
result=pg_get_expr_worker(expr,relid,prettyFlags);
2422+
if (result)
2423+
PG_RETURN_TEXT_P(result);
24382424
else
2439-
relname=NULL;
2440-
2441-
PG_RETURN_TEXT_P(pg_get_expr_worker(expr,relid,relname,prettyFlags));
2425+
PG_RETURN_NULL();
24422426
}
24432427

24442428
statictext*
2445-
pg_get_expr_worker(text*expr,Oidrelid,constchar*relname,intprettyFlags)
2429+
pg_get_expr_worker(text*expr,Oidrelid,intprettyFlags)
24462430
{
24472431
Node*node;
24482432
List*context;
24492433
char*exprstr;
2434+
Relationrel=NULL;
24502435
char*str;
24512436

24522437
/* Convert input TEXT object to C string */
@@ -2457,16 +2442,29 @@ pg_get_expr_worker(text *expr, Oid relid, const char *relname, int prettyFlags)
24572442

24582443
pfree(exprstr);
24592444

2460-
/* Prepare deparse context if needed */
2445+
/*
2446+
* Prepare deparse context if needed. If we are deparsing with a relid,
2447+
* we need to transiently open and lock the rel, to make sure it won't go
2448+
* away underneath us. (set_relation_column_names would lock it anyway,
2449+
* so this isn't really introducing any new behavior.)
2450+
*/
24612451
if (OidIsValid(relid))
2462-
context=deparse_context_for(relname,relid);
2452+
{
2453+
rel=try_relation_open(relid,AccessShareLock);
2454+
if (rel==NULL)
2455+
returnNULL;
2456+
context=deparse_context_for(RelationGetRelationName(rel),relid);
2457+
}
24632458
else
24642459
context=NIL;
24652460

24662461
/* Deparse */
24672462
str=deparse_expression_pretty(node,context, false, false,
24682463
prettyFlags,0);
24692464

2465+
if (rel!=NULL)
2466+
relation_close(rel,AccessShareLock);
2467+
24702468
returnstring_to_text(str);
24712469
}
24722470

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