Fix incorrect computations of length of null bitmap in pageinspect.

Instead of using our standard macro for this calculation, this code
did it itself ... and got it wrong, leading to incorrect display of
the null bitmap in some cases.  Noted and fixed by Maksim Milyutin.

In passing, remove a uselessly duplicative error check.

Errors were introduced in commit d6061f83; back-patch to 9.6
where that came in.

Maksim Milyutin, reviewed by Andrey Borodin

Discussion: https://postgr.es/m/ec295792-a69f-350f-6287-25a20e8f31d5@gmail.com

contrib/pageinspect/expected/page.out

@@ -92,3 +92,20 @@ create table test_part1 partition of test_partitioned for values from ( 1 ) to (
 select get_raw_page('test_part1', 0); -- get farther and error about empty table
 ERROR:  block number 0 is out of range for relation "test_part1"
 drop table test_partitioned;
+-- check null bitmap alignment for table whose number of attributes is multiple of 8
+create table test8 (f1 int, f2 int, f3 int, f4 int, f5 int, f6 int, f7 int, f8 int);
+insert into test8(f1, f8) values (x'f1'::int, 0);
+select t_bits, t_data from heap_page_items(get_raw_page('test8', 0));
+  t_bits  |       t_data       
+----------+--------------------
+ 10000001 | \xf100000000000000
+(1 row)
+
+select tuple_data_split('test8'::regclass, t_data, t_infomask, t_infomask2, t_bits)
+    from heap_page_items(get_raw_page('test8', 0));
+                      tuple_data_split                       
+-------------------------------------------------------------
+ {"\\xf1000000",NULL,NULL,NULL,NULL,NULL,NULL,"\\x00000000"}
+(1 row)
+
+drop table test8;

contrib/pageinspect/heapfuncs.c

@@ -234,7 +234,7 @@ heap_page_items(PG_FUNCTION_ARGS)
 					int			bits_len;
 
 					bits_len =
-						((tuphdr->t_infomask2 & HEAP_NATTS_MASK) / 8 + 1) * 8;
+						BITMAPLEN(HeapTupleHeaderGetNatts(tuphdr)) * BITS_PER_BYTE;
 					values[11] = CStringGetTextDatum(
 													 bits_to_text(tuphdr->t_bits, bits_len));
 				}
@@ -436,24 +436,19 @@ tuple_data_split(PG_FUNCTION_ARGS)
 		int			bits_str_len;
 		int			bits_len;
 
-		bits_len = (t_infomask2 & HEAP_NATTS_MASK) / 8 + 1;
+		bits_len = BITMAPLEN(t_infomask2 & HEAP_NATTS_MASK) * BITS_PER_BYTE;
 		if (!t_bits_str)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("argument of t_bits is null, but it is expected to be null and %d character long",
-							bits_len * 8)));
+							bits_len)));
 
 		bits_str_len = strlen(t_bits_str);
-		if ((bits_str_len % 8) != 0)
-			ereport(ERROR,
-					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("length of t_bits is not a multiple of eight")));
-
-		if (bits_len * 8 != bits_str_len)
+		if (bits_len != bits_str_len)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("unexpected length of t_bits %u, expected %d",
-							bits_str_len, bits_len * 8)));
+							bits_str_len, bits_len)));
 
 		/* do the conversion */
 		t_bits = text_to_bits(t_bits_str, bits_str_len);

contrib/pageinspect/sql/page.sql

@@ -41,3 +41,11 @@ select get_raw_page('test_partitioned', 0); -- error about partitioned table
 create table test_part1 partition of test_partitioned for values from ( 1 ) to (100);
 select get_raw_page('test_part1', 0); -- get farther and error about empty table
 drop table test_partitioned;
+
+-- check null bitmap alignment for table whose number of attributes is multiple of 8
+create table test8 (f1 int, f2 int, f3 int, f4 int, f5 int, f6 int, f7 int, f8 int);
+insert into test8(f1, f8) values (x'f1'::int, 0);
+select t_bits, t_data from heap_page_items(get_raw_page('test8', 0));
+select tuple_data_split('test8'::regclass, t_data, t_infomask, t_infomask2, t_bits)
+    from heap_page_items(get_raw_page('test8', 0));
+drop table test8;