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.

1 files changed, 36 insertions, 2 deletions

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index afbcd8ce768..f88d06997f7 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -33,6 +33,7 @@
 #include "lib/bloomfilter.h"
 #include "miscadmin.h"
 #include "storage/lmgr.h"
+#include "storage/smgr.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
 
@@ -120,6 +121,7 @@ PG_FUNCTION_INFO_V1(bt_index_parent_check);
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
 						bool heapallindexed);
 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 readonly, bool heapallindexed);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
@@ -252,8 +254,18 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed)
 	/* Relation suitable for checking as B-Tree? */
 	btree_index_checkable(indrel);
 
-	/* Check index, possibly against table it is an index on */
-	bt_check_every_level(indrel, heaprel, parentcheck, heapallindexed);
+	if (btree_index_mainfork_expected(indrel))
+	{
+		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 */
+		bt_check_every_level(indrel, heaprel, parentcheck, heapallindexed);
+	}
 
 	/*
 	 * Release locks early. That's ok here because nothing in the called
@@ -300,6 +312,28 @@ btree_index_checkable(Relation rel)
 }
 
 /*
+ * 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
  * checks if the heap relation contains any tuples that are not represented in