amcheck: Skip unlogged relations during recovery.

contrib/amcheck failed to consider the possibility that unlogged
relations will not have any main relation fork files when running in hot
standby mode.  This led to low-level "can't happen" errors that complain
about the absence of a relfilenode file.

To fix, simply skip verification of unlogged index relations during
recovery.  In passing, add a direct check for the presence of a main
fork just before verification proper begins, so that we cleanly verify
the presence of the main relation fork file.

Author: Andrey Borodin, Peter Geoghegan
Reported-By: Andrey Borodin
Diagnosed-By: Andrey Borodin
Discussion: https://postgr.es/m/DA9B33AC-53CB-4643-96D4-7A0BBC037FA1@yandex-team.ru
Backpatch: 10-, where amcheck was introduced.

contrib/amcheck/verify_nbtree.c

@@ -35,6 +35,7 @@
 #include "lib/bloomfilter.h"
 #include "miscadmin.h"
 #include "storage/lmgr.h"
+#include "storage/smgr.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
 
@@ -128,6 +129,7 @@ PG_FUNCTION_INFO_V1(bt_index_parent_check);
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
 									bool heapallindexed, bool rootdescend);
 static inline void btree_index_checkable(Relation rel);
+static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
 								 bool rootdescend);
@@ -225,7 +227,6 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 	Oid			heapid;
 	Relation	indrel;
 	Relation	heaprel;
-	bool		heapkeyspace;
 	LOCKMODE	lockmode;
 
 	if (parentcheck)
@@ -275,10 +276,22 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 	/* Relation suitable for checking as B-Tree? */
 	btree_index_checkable(indrel);
 
+	if (btree_index_mainfork_expected(indrel))
+	{
+		bool	heapkeyspace;
+
+		RelationOpenSmgr(indrel);
+		if (!smgrexists(indrel->rd_smgr, MAIN_FORKNUM))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg("index \"%s\" lacks a main relation fork",
+							RelationGetRelationName(indrel))));
+
 		/* Check index, possibly against table it is an index on */
 		heapkeyspace = _bt_heapkeyspace(indrel);
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
 							 heapallindexed, rootdescend);
+	}
 
 	/*
 	 * Release locks early. That's ok here because nothing in the called
@@ -324,6 +337,28 @@ btree_index_checkable(Relation rel)
 				 errdetail("Index is not valid.")));
 }
 
+/*
+ * Check if B-Tree index relation should have a file for its main relation
+ * fork.  Verification uses this to skip unlogged indexes when in hot standby
+ * mode, where there is simply nothing to verify.
+ *
+ * NB: Caller should call btree_index_checkable() before calling here.
+ */
+static inline bool
+btree_index_mainfork_expected(Relation rel)
+{
+	if (rel->rd_rel->relpersistence != RELPERSISTENCE_UNLOGGED ||
+		!RecoveryInProgress())
+		return true;
+
+	ereport(NOTICE,
+			(errcode(ERRCODE_READ_ONLY_SQL_TRANSACTION),
+			 errmsg("cannot verify unlogged index \"%s\" during recovery, skipping",
+					RelationGetRelationName(rel))));
+
+	return false;
+}
+
 /*
  * Main entry point for B-Tree SQL-callable functions. Walks the B-Tree in
  * logical order, verifying invariants as it goes.  Optionally, verification