Fix potential assertion failure when reindexing a pg_class index.

When reindexing individual indexes on pg_class it was possible to
either trigger an assertion failure:
TRAP: FailedAssertion("!(!ReindexIsProcessingIndex(((index)->rd_id)))

That's because reindex_index() called SetReindexProcessing() - which
enables an asserts ensuring no index insertions happen into the index
- before calling RelationSetNewRelfilenode(). That not correct for
indexes on pg_class, because RelationSetNewRelfilenode() updates the
relevant pg_class row, which needs to update the indexes.

The are two reasons this wasn't noticed earlier. Firstly the bug
doesn't trigger when reindexing all of pg_class, as reindex_relation
has code "hiding" all yet-to-be-reindexed indexes. Secondly, the bug
only triggers when the the update to pg_class doesn't turn out to be a
HOT update - otherwise there's no index insertion to trigger the
bug. Most of the time there's enough space, making this bug hard to
trigger.

To fix, move RelationSetNewRelfilenode() to before the
SetReindexProcessing() (and, together with some other code, to outside
of the PG_TRY()).

To make sure the error checking intended by SetReindexProcessing() is
more robust, modify CatalogIndexInsert() to check
ReindexIsProcessingIndex() even when the update is a HOT update.

Also add a few regression tests for REINDEXing of system catalogs.

The last two improvements would have prevented some of the issues
fixed in 5c1560606dc4c from being introduced in the first place.

Reported-By: Michael Paquier
Diagnosed-By: Tom Lane and Andres Freund
Author: Andres Freund
Reviewed-By: Tom Lane
Discussion: https://postgr.es/m/20190418011430.GA19133@paquier.xyz
Backpatch: 9.4-, the bug is present in all branches

contrib/test_decoding/expected/rewrite.out

@@ -103,6 +103,10 @@ COMMIT;
  -- repeated rewrites in different transactions
 VACUUM FULL pg_class;
 VACUUM FULL pg_class;
+-- reindexing of important relations / indexes
+REINDEX TABLE pg_class;
+REINDEX INDEX pg_class_oid_index;
+REINDEX INDEX pg_class_tblspc_relfilenode_index;
 INSERT INTO replication_example(somedata, testcolumn1) VALUES (5, 3);
 BEGIN;
 INSERT INTO replication_example(somedata, testcolumn1) VALUES (6, 4);

contrib/test_decoding/sql/rewrite.sql

@@ -74,6 +74,11 @@ COMMIT;
 VACUUM FULL pg_class;
 VACUUM FULL pg_class;
 
+-- reindexing of important relations / indexes
+REINDEX TABLE pg_class;
+REINDEX INDEX pg_class_oid_index;
+REINDEX INDEX pg_class_tblspc_relfilenode_index;
+
 INSERT INTO replication_example(somedata, testcolumn1) VALUES (5, 3);
 
 BEGIN;

src/backend/catalog/index.c

@@ -3290,29 +3290,35 @@ reindex_index(Oid indexId, bool skip_constraint_checks, char persistence,
 	 */
 	TransferPredicateLocksToHeapRelation(iRel);
 
+	/* Fetch info needed for index_build */
+	indexInfo = BuildIndexInfo(iRel);
+
+	/* If requested, skip checking uniqueness/exclusion constraints */
+	if (skip_constraint_checks)
+	{
+		if (indexInfo->ii_Unique || indexInfo->ii_ExclusionOps != NULL)
+			skipped_constraint = true;
+		indexInfo->ii_Unique = false;
+		indexInfo->ii_ExclusionOps = NULL;
+		indexInfo->ii_ExclusionProcs = NULL;
+		indexInfo->ii_ExclusionStrats = NULL;
+	}
+
+	/*
+	 * Build a new physical relation for the index. Need to do that before
+	 * "officially" starting the reindexing with SetReindexProcessing -
+	 * otherwise the necessary pg_class changes cannot be made with
+	 * encountering assertions.
+	 */
+	RelationSetNewRelfilenode(iRel, persistence, InvalidTransactionId,
+							  InvalidMultiXactId);
+
+	/* ensure SetReindexProcessing state isn't leaked */
 	PG_TRY();
 	{
 		/* Suppress use of the target index while rebuilding it */
 		SetReindexProcessing(heapId, indexId);
 
-		/* Fetch info needed for index_build */
-		indexInfo = BuildIndexInfo(iRel);
-
-		/* If requested, skip checking uniqueness/exclusion constraints */
-		if (skip_constraint_checks)
-		{
-			if (indexInfo->ii_Unique || indexInfo->ii_ExclusionOps != NULL)
-				skipped_constraint = true;
-			indexInfo->ii_Unique = false;
-			indexInfo->ii_ExclusionOps = NULL;
-			indexInfo->ii_ExclusionProcs = NULL;
-			indexInfo->ii_ExclusionStrats = NULL;
-		}
-
-		/* We'll build a new physical relation for the index */
-		RelationSetNewRelfilenode(iRel, persistence, InvalidTransactionId,
-								  InvalidMultiXactId);
-
 		/* Initialize the index and rebuild */
 		/* Note: we do not need to re-establish pkey setting */
 		index_build(heapRelation, iRel, indexInfo, false, true);

src/backend/catalog/indexing.c

@@ -80,9 +80,15 @@ CatalogIndexInsert(CatalogIndexState indstate, HeapTuple heapTuple)
 	Datum		values[INDEX_MAX_KEYS];
 	bool		isnull[INDEX_MAX_KEYS];
 
-	/* HOT update does not require index inserts */
+	/*
+	 * HOT update does not require index inserts. But with asserts enabled we
+	 * want to check that it'd be legal to currently insert into the
+	 * table/index.
+	 */
+#ifndef USE_ASSERT_CHECKING
 	if (HeapTupleIsHeapOnly(heapTuple))
 		return;
+#endif
 
 	/*
 	 * Get information from the state structure.  Fall out if nothing to do.
@@ -104,8 +110,10 @@ CatalogIndexInsert(CatalogIndexState indstate, HeapTuple heapTuple)
 	for (i = 0; i < numIndexes; i++)
 	{
 		IndexInfo  *indexInfo;
+		Relation	index;
 
 		indexInfo = indexInfoArray[i];
+		index = relationDescs[i];
 
 		/* If the index is marked as read-only, ignore it */
 		if (!indexInfo->ii_ReadyForInserts)
@@ -118,7 +126,16 @@ CatalogIndexInsert(CatalogIndexState indstate, HeapTuple heapTuple)
 		Assert(indexInfo->ii_Expressions == NIL);
 		Assert(indexInfo->ii_Predicate == NIL);
 		Assert(indexInfo->ii_ExclusionOps == NULL);
-		Assert(relationDescs[i]->rd_index->indimmediate);
+		Assert(index->rd_index->indimmediate);
+
+		/* see earlier check above */
+#ifdef USE_ASSERT_CHECKING
+		if (HeapTupleIsHeapOnly(heapTuple))
+		{
+			Assert(!ReindexIsProcessingIndex(RelationGetRelid(index)));
+			continue;
+		}
+#endif /* USE_ASSERT_CHECKING */
 
 		/*
 		 * FormIndexDatum fills in its values and isnull parameters with the

src/test/regress/expected/create_index.out

@@ -2921,6 +2921,24 @@ REINDEX (VERBOSE) TABLE reindex_verbose;
 INFO:  index "reindex_verbose_pkey" was reindexed
 DROP TABLE reindex_verbose;
 --
+-- check that system tables can be reindexed
+--
+-- whole tables
+REINDEX TABLE pg_class; -- mapped, non-shared, critical
+REINDEX TABLE pg_index; -- non-mapped, non-shared, critical
+REINDEX TABLE pg_operator; -- non-mapped, non-shared, critical
+REINDEX TABLE pg_database; -- mapped, shared, critical
+REINDEX TABLE pg_shdescription; -- mapped, shared non-critical
+-- Check that individual system indexes can be reindexed. That's a bit
+-- different from the entire-table case because reindex_relation
+-- treats e.g. pg_class special.
+REINDEX INDEX pg_class_oid_index; -- mapped, non-shared, critical
+REINDEX INDEX pg_class_relname_nsp_index; -- mapped, non-shared, non-critical
+REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
+REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
+REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
+REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+--
 -- REINDEX SCHEMA
 --
 REINDEX SCHEMA schema_to_reindex; -- failure, schema does not exist

src/test/regress/sql/create_index.sql

@@ -989,6 +989,27 @@ CREATE TABLE reindex_verbose(id integer primary key);
 REINDEX (VERBOSE) TABLE reindex_verbose;
 DROP TABLE reindex_verbose;
 
+--
+-- check that system tables can be reindexed
+--
+
+-- whole tables
+REINDEX TABLE pg_class; -- mapped, non-shared, critical
+REINDEX TABLE pg_index; -- non-mapped, non-shared, critical
+REINDEX TABLE pg_operator; -- non-mapped, non-shared, critical
+REINDEX TABLE pg_database; -- mapped, shared, critical
+REINDEX TABLE pg_shdescription; -- mapped, shared non-critical
+
+-- Check that individual system indexes can be reindexed. That's a bit
+-- different from the entire-table case because reindex_relation
+-- treats e.g. pg_class special.
+REINDEX INDEX pg_class_oid_index; -- mapped, non-shared, critical
+REINDEX INDEX pg_class_relname_nsp_index; -- mapped, non-shared, non-critical
+REINDEX INDEX pg_index_indexrelid_index; -- non-mapped, non-shared, critical
+REINDEX INDEX pg_index_indrelid_index; -- non-mapped, non-shared, non-critical
+REINDEX INDEX pg_database_oid_index; -- mapped, shared, critical
+REINDEX INDEX pg_shdescription_o_c_index; -- mapped, shared, non-critical
+
 --
 -- REINDEX SCHEMA
 --