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Fix memory corruptions around pg_dist_transaction accessors after a Citus downgrade is followed by an upgrade (#8121) 

DESCRIPTION: Fixes potential memory corruptions that could happen when
accessing pg_dist_transaction after a Citus downgrade is followed by a
Citus upgrade.

In case of Citus downgrade and further upgrade an undefined behavior may
be encountered. The reason is that Citus hardcoded the number of columns
in the extension's tables, but in case of downgrade and following update
some of these tables can have more columns, and some of them can be
marked as dropped.

This PR fixes all such tables using the approach introduced in #7950,
which solved the problem for the pg_dist_partition table.

See #7515 for a more thorough explanation.

Co-authored-by: Karina Litskevich <litskevichkarina@gmail.com>
pull/8124/head^2
Karina 2025-08-18 14:22:28 +03:00 committed by GitHub
parent 8d929d3bf8
commit badaa21cb1
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GPG Key ID: B5690EEEBB952194
2 changed files with 41 additions and 15 deletions
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54
src/backend/distributed/transaction/transaction_recovery.c
View File

@ -88,30 +88,33 @@ recover_prepared_transactions(PG_FUNCTION_ARGS)
void
LogTransactionRecord(int32 groupId, char *transactionName, FullTransactionId outerXid)
{
Datum values[Natts_pg_dist_transaction];
bool isNulls[Natts_pg_dist_transaction];
/* form new transaction tuple */
memset(values, 0, sizeof(values));
memset(isNulls, false, sizeof(isNulls));
values[Anum_pg_dist_transaction_groupid - 1] = Int32GetDatum(groupId);
values[Anum_pg_dist_transaction_gid - 1] = CStringGetTextDatum(transactionName);
values[Anum_pg_dist_transaction_outerxid - 1] = FullTransactionIdGetDatum(outerXid);
/* open transaction relation and insert new tuple */
/* open transaction relation */
Relation pgDistTransaction = table_open(DistTransactionRelationId(),
RowExclusiveLock);
TupleDesc tupleDescriptor = RelationGetDescr(pgDistTransaction);
/* form new transaction tuple */
Datum *values = (Datum *) palloc0(tupleDescriptor->natts * sizeof(Datum));
bool *isNulls = (bool *) palloc0(tupleDescriptor->natts * sizeof(bool));
values[Anum_pg_dist_transaction_groupid - 1] = Int32GetDatum(groupId);
values[Anum_pg_dist_transaction_gid - 1] = CStringGetTextDatum(transactionName);
values[GetOuterXidAttrIndexInPgDistTransaction(tupleDescriptor)] =
FullTransactionIdGetDatum(outerXid);
HeapTuple heapTuple = heap_form_tuple(tupleDescriptor, values, isNulls);
/* insert new tuple */
CATALOG_INSERT_WITH_SNAPSHOT(pgDistTransaction, heapTuple);
CommandCounterIncrement();
/* close relation and invalidate previous cache entry */
table_close(pgDistTransaction, NoLock);
pfree(values);
pfree(isNulls);
}
@ -309,9 +312,10 @@ RecoverWorkerTransactions(WorkerNode *workerNode, MultiConnection *connection)
SearchSysCacheExistsAttName(DistTransactionRelationId(), "outer_xid"))
{
/* Check if the transaction is created by an outer transaction from a non-main database */
outerXidDatum = heap_getattr(heapTuple,
Anum_pg_dist_transaction_outerxid,
tupleDescriptor, &outerXidIsNull);
outerXidDatum =
heap_getattr(heapTuple,
GetOuterXidAttrIndexInPgDistTransaction(tupleDescriptor) + 1,
tupleDescriptor, &outerXidIsNull);
}
else
{
@ -665,3 +669,23 @@ DeleteWorkerTransactions(WorkerNode *workerNode)
systable_endscan(scanDescriptor);
table_close(pgDistTransaction, NoLock);
}
/*
* GetOuterXidAttrIndexInPgDistTransaction returns attrnum for outer_xid attr.
*
* outer_xid attr was added to table pg_dist_transaction using alter operation after
* the version where Citus started supporting downgrades, and it's only column that we've
* introduced to pg_dist_transaction since then.
*
* And in case of a downgrade + upgrade, tupleDesc->natts becomes greater than
* Natts_pg_dist_transaction and when this happens, then we know that attrnum outer_xid is
* not Anum_pg_dist_transaction_outerxid anymore but tupleDesc->natts - 1.
*/
int
GetOuterXidAttrIndexInPgDistTransaction(TupleDesc tupleDesc)
{
return TupleDescSize(tupleDesc) == Natts_pg_dist_transaction
? (Anum_pg_dist_transaction_outerxid - 1)
: tupleDesc->natts - 1;
}

2
src/include/distributed/pg_dist_transaction.h
View File

@ -40,5 +40,7 @@ typedef FormData_pg_dist_transaction *Form_pg_dist_transaction;
#define Anum_pg_dist_transaction_gid 2
#define Anum_pg_dist_transaction_outerxid 3
extern int GetOuterXidAttrIndexInPgDistTransaction(TupleDesc tupleDesc);
#endif /* PG_DIST_TRANSACTION_H */