Revert "Prevent incorrect updates of pg_index while reindexing pg_index itself."

This reverts commit 8835284e0d903156632f76eea941cf5c592717ec of 2011-04-15.
There's a better way to do it, which will follow shortly.

src/backend/catalog/index.c

@@ -1423,13 +1423,6 @@ index_build(Relation heapRelation,
 		HeapTuple	indexTuple;
 		Form_pg_index indexForm;
 
-		/*
-		 * Broken HOT chains should not get reported in system catalogs; in
-		 * particular it would be quite dangerous to try to modify the index's
-		 * pg_index entry if we are reindexing pg_index itself.
-		 */
-		Assert(!IsSystemRelation(heapRelation));
-
 		pg_index = heap_open(IndexRelationId, RowExclusiveLock);
 
 		indexTuple = SearchSysCacheCopy(INDEXRELID,
@@ -1489,13 +1482,7 @@ index_build(Relation heapRelation,
  * A side effect is to set indexInfo->ii_BrokenHotChain to true if we detect
  * any potentially broken HOT chains.  Currently, we set this if there are
  * any RECENTLY_DEAD entries in a HOT chain, without trying very hard to
- * detect whether they're really incompatible with the chain tip.  However,
+ * detect whether they're really incompatible with the chain tip.
- * we do not ever set ii_BrokenHotChain true when the relation is a system
- * catalog.  This is to avoid problematic behavior when reindexing pg_index
- * itself: we can't safely change the index's indcheckxmin field when we're
- * partway through such an operation.  It should be okay since the set of
- * indexes on a system catalog ought not change during concurrent operations,
- * so that no HOT chain in it could ever become broken.
  */
 double
 IndexBuildHeapScan(Relation heapRelation,
@@ -1505,7 +1492,6 @@ IndexBuildHeapScan(Relation heapRelation,
 				   IndexBuildCallback callback,
 				   void *callback_state)
 {
-	bool		is_system_catalog;
 	HeapScanDesc scan;
 	HeapTuple	heapTuple;
 	Datum		values[INDEX_MAX_KEYS];
@@ -1525,9 +1511,6 @@ IndexBuildHeapScan(Relation heapRelation,
 	 */
 	Assert(OidIsValid(indexRelation->rd_rel->relam));
 
-	/* Remember if it's a system catalog */
-	is_system_catalog = IsSystemRelation(heapRelation);
-
 	/*
 	 * Need an EState for evaluation of index expressions and partial-index
 	 * predicates.	Also a slot to hold the current tuple.
@@ -1666,7 +1649,6 @@ IndexBuildHeapScan(Relation heapRelation,
 					{
 						indexIt = false;
 						/* mark the index as unsafe for old snapshots */
-						if (!is_system_catalog)
 						indexInfo->ii_BrokenHotChain = true;
 					}
 					else if (indexInfo->ii_BrokenHotChain)
@@ -1756,7 +1738,6 @@ IndexBuildHeapScan(Relation heapRelation,
 					{
 						indexIt = false;
 						/* mark the index as unsafe for old snapshots */
-						if (!is_system_catalog)
 						indexInfo->ii_BrokenHotChain = true;
 					}
 					else if (indexInfo->ii_BrokenHotChain)
@@ -2371,14 +2352,7 @@ reindex_index(Oid indexId)
 	 * We can also reset indcheckxmin, because we have now done a
 	 * non-concurrent index build, *except* in the case where index_build
 	 * found some still-broken HOT chains.
-	 *
-	 * When reindexing a system catalog, don't do any of this --- it would be
-	 * particularly risky to try to modify pg_index while we are reindexing
-	 * pg_index itself.  We don't support CREATE INDEX CONCURRENTLY on system
-	 * catalogs anyway, and they should never have indcheckxmin set either.
 	 */
-	if (!IsSystemRelation(heapRelation))
-	{
 	pg_index = heap_open(IndexRelationId, RowExclusiveLock);
 
 	indexTuple = SearchSysCacheCopy(INDEXRELID,
@@ -2399,7 +2373,6 @@ reindex_index(Oid indexId)
 		CatalogUpdateIndexes(pg_index, indexTuple);
 	}
 	heap_close(pg_index, RowExclusiveLock);
-	}
 
 	/* Close rels, but keep locks */
 	index_close(iRel, NoLock);