Add test for HeapBitmapScan's broken skip_fetch optimization

In the previous commit HeapBitmapScan's skip_fetch optimization was removed,
due to being broken in not easily fixable ways. Add a test that verifies we
don't re-introduce this bug if somebody tries to re-add the feature.

Only add the test to master for now, it's possible it's not entirely
stable. That seems sufficient, as we're not going to re-introduce the feature
on the backbranches. I did verify that the test passes on all branches. If the
test turns out to be unproblematic, we can backpatch it later, should we feel
a need to do so.

Discussion: https://postgr.es/m/CAEze2Wg3gXXZTr6_rwC+s4-o2ZVFB5F985uUSgJTsECx6AmGcQ@mail.gmail.com

src/test/isolation/expected/index-only-bitmapscan.out

@@ -0,0 +1,48 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s2_vacuum s2_mod s1_explain s1_begin s1_prepare s1_fetch_1 s2_vacuum s1_fetch_all s1_commit
+step s2_vacuum: 
+    VACUUM (TRUNCATE false) ios_bitmap;
+
+step s2_mod: 
+  DELETE FROM ios_bitmap WHERE a > 1;
+
+step s1_explain: 
+    EXPlAIN (COSTS OFF) DECLARE foo NO SCROLL CURSOR FOR SELECT row_number() OVER () FROM ios_bitmap WHERE a > 0 or b > 0;
+
+QUERY PLAN                                         
+---------------------------------------------------
+WindowAgg                                          
+  Window: w1 AS (ROWS UNBOUNDED PRECEDING)         
+  ->  Bitmap Heap Scan on ios_bitmap               
+        Recheck Cond: ((a > 0) OR (b > 0))         
+        ->  BitmapOr                               
+              ->  Bitmap Index Scan on ios_bitmap_a
+                    Index Cond: (a > 0)            
+              ->  Bitmap Index Scan on ios_bitmap_b
+                    Index Cond: (b > 0)            
+(9 rows)
+
+step s1_begin: BEGIN;
+step s1_prepare: 
+    DECLARE foo NO SCROLL CURSOR FOR SELECT row_number() OVER () FROM ios_bitmap WHERE a > 0 or b > 0;
+
+step s1_fetch_1: 
+    FETCH FROM foo;
+
+row_number
+----------
+         1
+(1 row)
+
+step s2_vacuum: 
+    VACUUM (TRUNCATE false) ios_bitmap;
+
+step s1_fetch_all: 
+    FETCH ALL FROM foo;
+
+row_number
+----------
+(0 rows)
+
+step s1_commit: COMMIT;

src/test/isolation/isolation_schedule

@@ -17,6 +17,7 @@ test: partial-index
 test: two-ids
 test: multiple-row-versions
 test: index-only-scan
+test: index-only-bitmapscan
 test: predicate-lock-hot-tuple
 test: update-conflict-out
 test: deadlock-simple

src/test/isolation/specs/index-only-bitmapscan.spec

@@ -0,0 +1,108 @@
+# In the past we supported index-only bitmap heapscans. However the
+# implementation was unsound, see
+# https://postgr.es/m/873c33c5-ef9e-41f6-80b2-2f5e11869f1c%40garret.ru
+#
+# This test reliably triggered the problem before we removed the
+# optimization. We keep the test around to make it less likely for a similar
+# problem to be re-introduced.
+
+setup
+{
+    -- by using a low fillfactor and a wide tuple we can get multiple blocks
+    -- with just few rows
+    CREATE TABLE ios_bitmap (a int NOT NULL, b int not null, pad char(1024) default '')
+    WITH (AUTOVACUUM_ENABLED = false, FILLFACTOR = 10);
+
+    INSERT INTO ios_bitmap SELECT g.i, g.i FROM generate_series(1, 10) g(i);
+
+    CREATE INDEX ios_bitmap_a ON ios_bitmap(a);
+    CREATE INDEX ios_bitmap_b ON ios_bitmap(b);
+}
+
+teardown
+{
+    DROP TABLE ios_bitmap;
+}
+
+
+session s1
+
+setup
+{
+    SET enable_seqscan = false;
+}
+
+step s1_begin { BEGIN; }
+step s1_commit { COMMIT; }
+
+
+# The test query uses an or between two indexes to ensure make it more likely
+# to use a bitmap index scan
+#
+# The row_number() hack is a way to have something returned (isolationtester
+# doesn't display empty rows) while still allowing for the index-only scan
+# optimization in bitmap heap scans, which requires an empty targetlist.
+step s1_prepare
+{
+    DECLARE foo NO SCROLL CURSOR FOR SELECT row_number() OVER () FROM ios_bitmap WHERE a > 0 or b > 0;
+}
+
+step s1_explain
+{
+    EXPlAIN (COSTS OFF) DECLARE foo NO SCROLL CURSOR FOR SELECT row_number() OVER () FROM ios_bitmap WHERE a > 0 or b > 0;
+}
+
+step s1_fetch_1
+{
+    FETCH FROM foo;
+}
+
+step s1_fetch_all
+{
+    FETCH ALL FROM foo;
+}
+
+
+session s2
+
+# Don't delete row 1 so we have a row for the cursor to "rest" on.
+step s2_mod
+{
+  DELETE FROM ios_bitmap WHERE a > 1;
+}
+
+# Disable truncation, as otherwise we'll just wait for a timeout while trying
+# to acquire the lock
+step s2_vacuum
+{
+    VACUUM (TRUNCATE false) ios_bitmap;
+}
+
+permutation
+  # Vacuum first, to ensure VM exists, otherwise the bitmapscan will consider
+  # VM to be size 0, due to caching. Can't do that in setup because
+  s2_vacuum
+
+  # Delete nearly all rows, to make issue visible
+  s2_mod
+
+  # Verify that the appropriate plan is chosen
+  s1_explain
+
+  # Create a cursor
+  s1_begin
+  s1_prepare
+
+  # Fetch one row from the cursor, that ensures the index scan portion is done
+  # before the vacuum in the next step
+  s1_fetch_1
+
+  # With the bug this vacuum would have marked pages as all-visible that the
+  # scan in the next step then would have considered all-visible, despite all
+  # rows from those pages having been removed.
+  s2_vacuum
+
+  # If this returns any rows, the bug is present
+  s1_fetch_all
+
+  s1_commit