Fix incorrect usage of TupleDescSize() in #7950, #8120, #8124, #8121 and #8114 (#8146)

In #7950, #8120, #8124, #8121 and #8114, TupleDescSize() was used to
check whether the tuple length is `Natts_<catalog_table_name>`. However
this was wrong because TupleDescSize() returns the size of the
tupledesc, not the length of it (i.e., number of attributes).

Actually `TupleDescSize(tupleDesc) == Natts_<catalog_table_name>` was
always returning false but this didn't cause any problems because using
`tupleDesc->natts - 1` when `tupleDesc->natts ==
Natts_<catalog_table_name>` too had the same effect as using
`Anum_<column_added_later> - 1` in that case.

So this also makes me thinking of always returning `tupleDesc->natts -
1` (or `tupleDesc->natts - 2` if it's the second to last attribute) but
being more explicit seems more useful.

Even more, in the future we should probably switch to a different
implementation if / when we think of adding more columns to those
tables. We should probably scan non-dropped attributes of the relation,
enumerate them and return the attribute number of the one that we're
looking for, but seems this is not needed right now.

(cherry picked from commit 439870f3a9)

CHANGELOG.md

@@ -36,7 +36,7 @@
 
 * Fixes potential memory corruptions that could happen when accessing
   various catalog tables after a Citus downgrade is followed by a Citus
-  upgrade (#7950, #8120, #8124, #8121, #8114)
+  upgrade (#7950, #8120, #8124, #8121, #8114, #8146)
 
 * Fixes UPDATE statements with indirection and array/jsonb subscripting with
   more than one field (#7675)

src/backend/columnar/columnar_metadata.c

@@ -2132,7 +2132,7 @@ GetHighestUsedRowNumber(uint64 storageId)
 static int
 GetFirstRowNumberAttrIndexInColumnarStripe(TupleDesc tupleDesc)
 {
-	return TupleDescSize(tupleDesc) == Natts_columnar_stripe
+	return tupleDesc->natts == Natts_columnar_stripe
 		   ? (Anum_columnar_stripe_first_row_number - 1)
 		   : tupleDesc->natts - 1;
 }

src/backend/distributed/metadata/distobject.c

@@ -801,7 +801,7 @@ DistributedSequenceList(void)
 int
 GetForceDelegationAttrIndexInPgDistObject(TupleDesc tupleDesc)
 {
-	return TupleDescSize(tupleDesc) == Natts_pg_dist_object
+	return tupleDesc->natts == Natts_pg_dist_object
 		   ? (Anum_pg_dist_object_force_delegation - 1)
 		   : tupleDesc->natts - 1;
 }

src/backend/distributed/metadata/metadata_utility.c

@@ -4486,7 +4486,7 @@ UnblockDependingBackgroundTasks(BackgroundTask *task)
 int
 GetAutoConvertedAttrIndexInPgDistPartition(TupleDesc tupleDesc)
 {
-	return TupleDescSize(tupleDesc) == Natts_pg_dist_partition
+	return tupleDesc->natts == Natts_pg_dist_partition
 		   ? (Anum_pg_dist_partition_autoconverted - 1)
 		   : tupleDesc->natts - 1;
 }
@@ -4506,7 +4506,7 @@ GetAutoConvertedAttrIndexInPgDistPartition(TupleDesc tupleDesc)
 int
 GetNodesInvolvedAttrIndexInPgDistBackgroundTask(TupleDesc tupleDesc)
 {
-	return TupleDescSize(tupleDesc) == Natts_pg_dist_background_task
+	return tupleDesc->natts == Natts_pg_dist_background_task
 		   ? (Anum_pg_dist_background_task_nodes_involved - 1)
 		   : tupleDesc->natts - 1;
 }

src/backend/distributed/transaction/transaction_recovery.c

@@ -685,7 +685,7 @@ DeleteWorkerTransactions(WorkerNode *workerNode)
 int
 GetOuterXidAttrIndexInPgDistTransaction(TupleDesc tupleDesc)
 {
-	return TupleDescSize(tupleDesc) == Natts_pg_dist_transaction
+	return tupleDesc->natts == Natts_pg_dist_transaction
 		   ? (Anum_pg_dist_transaction_outerxid - 1)
 		   : tupleDesc->natts - 1;
 }