Make bms_prev_member work correctly with a 64 bit bitmapword

5c067521 erroneously had coded bms_prev_member assuming that a bitmapword
would always hold 32 bits and started it's search on what it thought was the
highest 8-bits of the word.  This was not the case if bitmapwords were 64
bits.

In passing add a test to exercise this function a little. Previously there was
no coverage at all.

David Rowly

src/backend/nodes/bitmapset.c

@@ -1167,7 +1167,7 @@ bms_prev_member(const Bitmapset *a, int prevbit)
 		if (w != 0)
 		{
 			int			result;
-			int			shift = 24;
+			int			shift = BITS_PER_BITMAPWORD - 8;
 			result = wordnum * BITS_PER_BITMAPWORD;
 
 			while ((w >> shift) == 0)

src/test/regress/expected/partition_prune.out

@@ -1747,6 +1747,29 @@ explain (analyze, costs off, summary off, timing off) execute ab_q3 (2, 2);
          Filter: ((b >= $1) AND (b <= $2) AND (a < $0))
 (10 rows)
 
+-- Test a backwards Append scan
+create table list_part (a int) partition by list (a);
+create table list_part1 partition of list_part for values in (1);
+create table list_part2 partition of list_part for values in (2);
+create table list_part3 partition of list_part for values in (3);
+create table list_part4 partition of list_part for values in (4);
+insert into list_part select generate_series(1,4);
+begin;
+-- Don't select an actual value out of the table as the order of the Append's
+-- subnodes may not be stable.
+declare cur SCROLL CURSOR for select 1 from list_part where a > (select 1) and a < (select 4);
+-- move beyond the final row
+move 3 from cur;
+-- Ensure we get two rows.
+fetch backward all from cur;
+ ?column? 
+----------
+        1
+        1
+(2 rows)
+
+commit;
+drop table list_part;
 -- Parallel append
 -- Suppress the number of loops each parallel node runs for.  This is because
 -- more than one worker may run the same parallel node if timing conditions

src/test/regress/sql/partition_prune.sql

@@ -359,6 +359,31 @@ execute ab_q3 (1, 8);
 
 explain (analyze, costs off, summary off, timing off) execute ab_q3 (2, 2);
 
+-- Test a backwards Append scan
+create table list_part (a int) partition by list (a);
+create table list_part1 partition of list_part for values in (1);
+create table list_part2 partition of list_part for values in (2);
+create table list_part3 partition of list_part for values in (3);
+create table list_part4 partition of list_part for values in (4);
+
+insert into list_part select generate_series(1,4);
+
+begin;
+
+-- Don't select an actual value out of the table as the order of the Append's
+-- subnodes may not be stable.
+declare cur SCROLL CURSOR for select 1 from list_part where a > (select 1) and a < (select 4);
+
+-- move beyond the final row
+move 3 from cur;
+
+-- Ensure we get two rows.
+fetch backward all from cur;
+
+commit;
+
+drop table list_part;
+
 -- Parallel append
 
 -- Suppress the number of loops each parallel node runs for.  This is because