To: PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
From: Maksim Milyutin <milyutinma@gmail.com>
Subject: [HACKERS] wrong t_bits alignment in pageinspect
Message-ID: <ec295792-a69f-350f-6287-25a20e8f31d5@gmail.com>
Date: Fri, 15 Dec 2017 16:53:41 +0300
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Hi!


I found out the problem in exposing values of t_bits field from 
heap_page_items function.

When the number of attributes in table is multiple of eight, t_bits 
column shows double number of bits in which data fields are included.

For example:

# create table tbl(f1 int, f2 int, f3 int, f4 int, f5 int, f6 int, f7 
int, f8 int);
# insert into tbl(f1, f8) values (x'f1'::int, 0);

# select t_bits, t_data from heap_page_items(get_raw_page('tbl', 0));

                    t_bits      |       t_data
            ------------------+--------------------
  1000000110001111 | \xf100000000000000

I suppose the prefix 10000001 corresponds to real value of t_bits, the 
rest part 10001111 - to the lower byte of f1 field of tbl.


Attached patch fixes this issue.


-- 
Regards,
Maksim Milyutin


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diff --git a/contrib/pageinspect/heapfuncs.c b/contrib/pageinspect/heapfuncs.c
index ca4d3f5..fe53a1a 100644
--- a/contrib/pageinspect/heapfuncs.c
+++ b/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;
+						TYPEALIGN(8, (tuphdr->t_infomask2 & HEAP_NATTS_MASK));
 					values[11] = CStringGetTextDatum(
 													 bits_to_text(tuphdr->t_bits, bits_len));
 				}

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