Add a test for half-dead pages in B-tree indexes

To increase our test coverage in general, and because I will use this
in the next commit to test a bug we currently have in amcheck.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/33e39552-6a2a-46f3-8b34-3f9f8004451f@garret.ru

src/backend/access/nbtree/nbtpage.c

@@ -33,6 +33,7 @@
 #include "storage/indexfsm.h"
 #include "storage/predicate.h"
 #include "storage/procarray.h"
+#include "utils/injection_point.h"
 #include "utils/memdebug.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
@@ -2003,6 +2004,10 @@ _bt_pagedel(Relation rel, Buffer leafbuf, BTVacState *vstate)
 				return;
 			}
 		}
+		else
+		{
+			INJECTION_POINT("nbtree-finish-half-dead-page-vacuum", NULL);
+		}
 
 		/*
 		 * Then unlink it from its siblings.  Each call to
@@ -2349,6 +2354,8 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 
 	_bt_unlockbuf(rel, leafbuf);
 
+	INJECTION_POINT("nbtree-leave-page-half-dead", NULL);
+
 	/*
 	 * Check here, as calling loops will have locks held, preventing
 	 * interrupts from being processed.

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

@@ -0,0 +1,71 @@
+--
+-- Test half-dead pages in B-tree indexes.
+--
+-- Half-dead pages is an intermediate state while vacuum is deleting a
+-- page. You can encounter them if you query concurrently with vacuum,
+-- or if vacuum is interrupted while it's deleting a page. A B-tree
+-- with half-dead pages is a valid state, but they rarely observed by
+-- other backends in practice because, so it's good to have some
+-- targeted tests to exercise them.
+--
+-- This uses injection points to interrupt some page deletions
+set client_min_messages TO 'warning';
+create extension if not exists injection_points;
+reset client_min_messages;
+-- Make all injection points local to this process, for concurrency.
+SELECT injection_points_set_local();
+ injection_points_set_local 
+----------------------------
+ 
+(1 row)
+
+-- Use the index for all the queries
+set enable_seqscan=off;
+-- Print a NOTICE whenever a half-dead page is deleted
+SELECT injection_points_attach('nbtree-finish-half-dead-page-vacuum', 'notice');
+ injection_points_attach 
+-------------------------
+ 
+(1 row)
+
+create table nbtree_half_dead_pages(id bigint) with (autovacuum_enabled = off);
+insert into nbtree_half_dead_pages SELECT g from generate_series(1, 150000) g;
+create index nbtree_half_dead_pages_id_idx on nbtree_half_dead_pages using btree (id);
+delete from nbtree_half_dead_pages where id > 100000 and id < 120000;
+-- Run VACUUM and interrupt it so that it leaves behind a half-dead page
+SELECT injection_points_attach('nbtree-leave-page-half-dead', 'error');
+ injection_points_attach 
+-------------------------
+ 
+(1 row)
+
+vacuum nbtree_half_dead_pages;
+ERROR:  error triggered for injection point nbtree-leave-page-half-dead
+CONTEXT:  while vacuuming index "nbtree_half_dead_pages_id_idx" of relation "public.nbtree_half_dead_pages"
+SELECT injection_points_detach('nbtree-leave-page-half-dead');
+ injection_points_detach 
+-------------------------
+ 
+(1 row)
+
+select * from nbtree_half_dead_pages where id > 99998 and id < 120002;
+   id   
+--------
+  99999
+ 100000
+ 120000
+ 120001
+(4 rows)
+
+-- Finish the deletion and re-check
+vacuum nbtree_half_dead_pages;
+NOTICE:  notice triggered for injection point nbtree-finish-half-dead-page-vacuum
+select * from nbtree_half_dead_pages where id > 99998 and id < 120002;
+   id   
+--------
+  99999
+ 100000
+ 120000
+ 120001
+(4 rows)
+

src/test/modules/nbtree/meson.build

@@ -10,6 +10,7 @@ tests += {
   'bd': meson.current_build_dir(),
   'regress': {
     'sql': [
+      'nbtree_half_dead_pages',
       'nbtree_incomplete_splits',
     ],
   },

src/test/modules/nbtree/sql/nbtree_half_dead_pages.sql

@@ -0,0 +1,43 @@
+--
+-- Test half-dead pages in B-tree indexes.
+--
+-- Half-dead pages is an intermediate state while vacuum is deleting a
+-- page. You can encounter them if you query concurrently with vacuum,
+-- or if vacuum is interrupted while it's deleting a page. A B-tree
+-- with half-dead pages is a valid state, but they rarely observed by
+-- other backends in practice because, so it's good to have some
+-- targeted tests to exercise them.
+--
+
+-- This uses injection points to interrupt some page deletions
+set client_min_messages TO 'warning';
+create extension if not exists injection_points;
+reset client_min_messages;
+
+-- Make all injection points local to this process, for concurrency.
+SELECT injection_points_set_local();
+
+-- Use the index for all the queries
+set enable_seqscan=off;
+
+-- Print a NOTICE whenever a half-dead page is deleted
+SELECT injection_points_attach('nbtree-finish-half-dead-page-vacuum', 'notice');
+
+create table nbtree_half_dead_pages(id bigint) with (autovacuum_enabled = off);
+
+insert into nbtree_half_dead_pages SELECT g from generate_series(1, 150000) g;
+
+create index nbtree_half_dead_pages_id_idx on nbtree_half_dead_pages using btree (id);
+
+delete from nbtree_half_dead_pages where id > 100000 and id < 120000;
+
+-- Run VACUUM and interrupt it so that it leaves behind a half-dead page
+SELECT injection_points_attach('nbtree-leave-page-half-dead', 'error');
+vacuum nbtree_half_dead_pages;
+SELECT injection_points_detach('nbtree-leave-page-half-dead');
+
+select * from nbtree_half_dead_pages where id > 99998 and id < 120002;
+
+-- Finish the deletion and re-check
+vacuum nbtree_half_dead_pages;
+select * from nbtree_half_dead_pages where id > 99998 and id < 120002;