Add amcheck missing downlink tests.

Also use palloc0() for main amcheck state, and adjust a few comments.

Somehow I pushed old version of patch in commit
4eaf7eac, so commit the difference.

Peter Geoghegan

contrib/amcheck/expected/check_btree.out

--- minimal test, basically just verifying that amcheck
 CREATE TABLE bttest_a(id int8);
 CREATE TABLE bttest_b(id int8);
 CREATE TABLE bttest_multi(id int8, data int8);
+CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
+-- Stabalize tests
+ALTER TABLE bttest_a SET (autovacuum_enabled = false);
+ALTER TABLE bttest_b SET (autovacuum_enabled = false);
+ALTER TABLE bttest_multi SET (autovacuum_enabled = false);
+ALTER TABLE delete_test_table SET (autovacuum_enabled = false);
 INSERT INTO bttest_a SELECT * FROM generate_series(1, 100000);
 INSERT INTO bttest_b SELECT * FROM generate_series(100000, 1, -1);
 INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
@@ -120,9 +125,25 @@ SELECT bt_index_parent_check('bttest_multi_idx', true);
  
 (1 row)
 
+--
+-- Test for multilevel page deletion/downlink present checks
+--
+INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,80000) i;
+ALTER TABLE delete_test_table ADD PRIMARY KEY (a,b,c,d);
+DELETE FROM delete_test_table WHERE a > 40000;
+VACUUM delete_test_table;
+DELETE FROM delete_test_table WHERE a > 10;
+VACUUM delete_test_table;
+SELECT bt_index_parent_check('delete_test_table_pkey', true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
+DROP TABLE delete_test_table;
 DROP OWNED BY bttest_role; -- permissions
 DROP ROLE bttest_role;

contrib/amcheck/sql/check_btree.sql

--- minimal test, basically just verifying that amcheck
 CREATE TABLE bttest_a(id int8);
 CREATE TABLE bttest_b(id int8);
 CREATE TABLE bttest_multi(id int8, data int8);
+CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
+
+-- Stabalize tests
+ALTER TABLE bttest_a SET (autovacuum_enabled = false);
+ALTER TABLE bttest_b SET (autovacuum_enabled = false);
+ALTER TABLE bttest_multi SET (autovacuum_enabled = false);
+ALTER TABLE delete_test_table SET (autovacuum_enabled = false);
 
 INSERT INTO bttest_a SELECT * FROM generate_series(1, 100000);
 INSERT INTO bttest_b SELECT * FROM generate_series(100000, 1, -1);
@@ -71,9 +77,21 @@ TRUNCATE bttest_multi;
 INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
 SELECT bt_index_parent_check('bttest_multi_idx', true);
 
+--
+-- Test for multilevel page deletion/downlink present checks
+--
+INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,80000) i;
+ALTER TABLE delete_test_table ADD PRIMARY KEY (a,b,c,d);
+DELETE FROM delete_test_table WHERE a > 40000;
+VACUUM delete_test_table;
+DELETE FROM delete_test_table WHERE a > 10;
+VACUUM delete_test_table;
+SELECT bt_index_parent_check('delete_test_table_pkey', true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
+DROP TABLE delete_test_table;
 DROP OWNED BY bttest_role; -- permissions
 DROP ROLE bttest_role;

contrib/amcheck/verify_nbtree.c

@@ -340,7 +340,7 @@ bt_check_every_level(Relation rel, Relation heaprel, bool readonly,
 	/*
 	 * Initialize state for entire verification operation
 	 */
-	state = palloc(sizeof(BtreeCheckState));
+	state = palloc0(sizeof(BtreeCheckState));
 	state->rel = rel;
 	state->heaprel = heaprel;
 	state->readonly = readonly;
@@ -772,13 +772,15 @@ nextpage:
  * - That tuples report that they have the expected number of attributes.
  *	 INCLUDE index pivot tuples should not contain non-key attributes.
  *
- * Furthermore, when state passed shows ShareLock held, and target page is
- * internal page, function also checks:
+ * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect downlinks lower bound.
  *
+ * - That downlink to block was encountered in parent where that's expected.
+ *   (Limited to heapallindexed readonly callers.)
+ *
  * This is also where heapallindexed callers use their Bloom filter to
- * fingerprint IndexTuples.
+ * fingerprint IndexTuples for later IndexBuildHeapScan() verification.
  *
  * Note:  Memory allocated in this routine is expected to be released by caller
  * resetting state->targetcontext.
@@ -1074,7 +1076,7 @@ bt_target_page_check(BtreeCheckState *state)
 	/*
 	 * * Check if page has a downlink in parent *
 	 *
-	 * This can only be checked in readonly + heapallindexed case.
+	 * This can only be checked in heapallindexed + readonly case.
 	 */
 	if (state->heapallindexed && state->readonly)
 		bt_downlink_missing_check(state);
@@ -1561,7 +1563,7 @@ bt_downlink_missing_check(BtreeCheckState *state)
 	 * infinity items.  Besides, bt_downlink_check() is unwilling to descend
 	 * multiple levels.  (The similar bt_downlink_check() P_ISDELETED() check
 	 * within bt_check_level_from_leftmost() won't reach the page either, since
-	 * the leaf's live siblings should have their sibling links updating to
+	 * the leaf's live siblings should have their sibling links updated to
 	 * bypass the deletion target page when it is marked fully dead.)
 	 *
 	 * If this error is raised, it might be due to a previous multi-level page