Fix amcheck's handling of incomplete root splits in B-tree

When the root page is being split, it's normal that root page
according to the metapage is not marked BTP_ROOT. Fix bogus error in
amcheck about that case.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/abd65090-5336-42cc-b768-2bdd66738404@iki.fi
Backpatch-through: 14

contrib/amcheck/verify_nbtree.c

@@ -721,7 +721,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 							 errmsg("block %u is not leftmost in index \"%s\"",
 									current, RelationGetRelationName(state->rel))));
 
-				if (level.istruerootlevel && !P_ISROOT(opaque))
+				if (level.istruerootlevel && (!P_ISROOT(opaque) && !P_INCOMPLETE_SPLIT(opaque)))
 					ereport(ERROR,
 							(errcode(ERRCODE_INDEX_CORRUPTED),
 							 errmsg("block %u is not true root in index \"%s\"",

src/test/modules/nbtree/Makefile

@@ -1,6 +1,6 @@
 # src/test/modules/nbtree/Makefile
 
-EXTRA_INSTALL = src/test/modules/injection_points
+EXTRA_INSTALL = src/test/modules/injection_points contrib/amcheck
 
 REGRESS = nbtree_incomplete_splits
 

src/test/modules/nbtree/expected/nbtree_incomplete_splits.out

@@ -14,6 +14,7 @@
 -- splits in "incomplete" state
 set client_min_messages TO 'warning';
 create extension if not exists injection_points;
+create extension if not exists amcheck;
 reset client_min_messages;
 -- Make all injection points local to this process, for concurrency.
 SELECT injection_points_set_local();
@@ -87,6 +88,12 @@ begin
   if c <> 200 then
     raise 'unexpected count % ', c;
   end if;
+
+  -- Also check the index with amcheck. Both to test that the index is
+  -- valid, but also to test that amcheck doesn't wrongly complain
+  -- about incomplete splits.
+  perform bt_index_parent_check('nbtree_incomplete_splits_i_idx'::regclass, true, true);
+
   return true;
 end;
 $$;

src/test/modules/nbtree/sql/nbtree_incomplete_splits.sql

@@ -15,6 +15,7 @@
 -- splits in "incomplete" state
 set client_min_messages TO 'warning';
 create extension if not exists injection_points;
+create extension if not exists amcheck;
 reset client_min_messages;
 
 -- Make all injection points local to this process, for concurrency.
@@ -86,6 +87,12 @@ begin
   if c <> 200 then
     raise 'unexpected count % ', c;
   end if;
+
+  -- Also check the index with amcheck. Both to test that the index is
+  -- valid, but also to test that amcheck doesn't wrongly complain
+  -- about incomplete splits.
+  perform bt_index_parent_check('nbtree_incomplete_splits_i_idx'::regclass, true, true);
+
   return true;
 end;
 $$;