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

3 files changed, 115 insertions, 0 deletions

diff --git a/src/test/modules/nbtree/expected/nbtree_half_dead_pages.out b/src/test/modules/nbtree/expected/nbtree_half_dead_pages.out
new file mode 100644
index 00000000000..8fd472f8df2
--- /dev/null
+++ b/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)
+
diff --git a/src/test/modules/nbtree/meson.build b/src/test/modules/nbtree/meson.build
index efebf30a16f..1b9a34d37ca 100644
--- a/src/test/modules/nbtree/meson.build
+++ b/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',
     ],
   },
diff --git a/src/test/modules/nbtree/sql/nbtree_half_dead_pages.sql b/src/test/modules/nbtree/sql/nbtree_half_dead_pages.sql
new file mode 100644
index 00000000000..d4b9a3f824d
--- /dev/null
+++ b/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;