pageinspect: Add page_checksum function

Author: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>

contrib/pageinspect/expected/page.out

@@ -49,6 +49,12 @@ SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
      8192 |       4
 (1 row)
 
+SELECT page_checksum(get_raw_page('test1', 0), 0) IS NOT NULL AS silly_checksum_test;
+ silly_checksum_test 
+---------------------
+ t
+(1 row)
+
 SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bits)
     FROM heap_page_items(get_raw_page('test1', 0));
        tuple_data_split        

contrib/pageinspect/pageinspect--1.5--1.6.sql

@@ -75,3 +75,11 @@ CREATE FUNCTION hash_metapage_info(IN page bytea,
     OUT mapp int8[])
 AS 'MODULE_PATHNAME', 'hash_metapage_info'
 LANGUAGE C STRICT PARALLEL SAFE;
+
+--
+-- page_checksum()
+--
+CREATE FUNCTION page_checksum(IN page bytea, IN blkno int4)
+RETURNS smallint
+AS 'MODULE_PATHNAME', 'page_checksum'
+LANGUAGE C STRICT PARALLEL SAFE;

contrib/pageinspect/rawpage.c

@@ -24,6 +24,7 @@
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
+#include "storage/checksum.h"
 #include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/rel.h"
@@ -280,3 +281,39 @@ page_header(PG_FUNCTION_ARGS)
 
 	PG_RETURN_DATUM(result);
 }
+
+/*
+ * page_checksum
+ *
+ * Compute checksum of a raw page
+ */
+
+PG_FUNCTION_INFO_V1(page_checksum);
+
+Datum
+page_checksum(PG_FUNCTION_ARGS)
+{
+	bytea	   *raw_page = PG_GETARG_BYTEA_P(0);
+	uint32		blkno = PG_GETARG_INT32(1);
+	int			raw_page_size;
+	PageHeader	page;
+
+	if (!superuser())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 (errmsg("must be superuser to use raw page functions"))));
+
+	raw_page_size = VARSIZE(raw_page) - VARHDRSZ;
+
+	/*
+	 * Check that the supplied page is of the right size.
+	 */
+	if (raw_page_size != BLCKSZ)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("incorrect size of input page (%d bytes)", raw_page_size)));
+
+	page = (PageHeader) VARDATA(raw_page);
+
+	PG_RETURN_INT16(pg_checksum_page((char *)page, blkno));
+}

contrib/pageinspect/sql/page.sql

@@ -24,6 +24,8 @@ SELECT get_raw_page('test1', 0) = get_raw_page('test1', 'main', 0);
 
 SELECT pagesize, version FROM page_header(get_raw_page('test1', 0));
 
+SELECT page_checksum(get_raw_page('test1', 0), 0) IS NOT NULL AS silly_checksum_test;
+
 SELECT tuple_data_split('test1'::regclass, t_data, t_infomask, t_infomask2, t_bits)
     FROM heap_page_items(get_raw_page('test1', 0));
 

doc/src/sgml/pageinspect.sgml

@@ -73,12 +73,55 @@
 test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
     lsn    | checksum | flags  | lower | upper | special | pagesize | version | prune_xid
 -----------+----------+--------+-------+-------+---------+----------+---------+-----------
- 0/24A1B50 |        1 |      1 |   232 |   368 |    8192 |     8192 |       4 |         0
+ 0/24A1B50 |        0 |      1 |   232 |   368 |    8192 |     8192 |       4 |         0
 </screen>
       The returned columns correspond to the fields in the
       <structname>PageHeaderData</> struct.
       See <filename>src/include/storage/bufpage.h</> for details.
      </para>
+
+     <para>
+      The <structfield>checksum</structfield> field is the checksum stored in
+      the page, which might be incorrect if the page is somehow corrupted.  If
+      data checksums are not enabled for this instance, then the value stored
+      is meaningless.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term>
+     <function>page_checksum(page bytea, blkno int4) returns smallint</function>
+     <indexterm>
+      <primary>page_checksum</primary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      <function>page_checksum</function> computes the checksum for the page, as if
+      it was located at the given block.
+     </para>
+
+     <para>
+      A page image obtained with <function>get_raw_page</function> should be
+      passed as argument.  For example:
+<screen>
+test=# SELECT page_checksum(get_raw_page('pg_class', 0), 0);
+ page_checksum
+---------------
+         13443
+</screen>
+      Note that the checksum depends on the block number, so matching block
+      numbers should be passed (except when doing esoteric debugging).
+     </para>
+
+     <para>
+      The checksum computed with this function can be compared with
+      the <structfield>checksum</structfield> result field of the
+      function <function>page_header</function>.  If data checksums are
+      enabled for this instance, then the two values should be equal.
+     </para>
     </listitem>
    </varlistentry>