Add an explicit test to catch changes in checksumming calculations.

Seems like a good idea in view of 006517432 and addd034ae.

Michael Paquier, Tom Lane

Discussion: https://postgr.es/m/20200306075230.GA118430@paquier.xyz

contrib/pageinspect/sql/checksum.sql

@@ -0,0 +1,31 @@
+--
+-- Verify correct calculation of checksums
+--
+-- Postgres' checksum algorithm produces different answers on little-endian
+-- and big-endian machines.  The results of this test also vary depending
+-- on the configured block size.  This test has several different expected
+-- results files to handle the following possibilities:
+--
+--	BLCKSZ	end	file
+--	8K	LE	checksum.out
+--	8K	BE	checksum_1.out
+--
+-- In future we might provide additional expected-results files for other
+-- block sizes, but there seems little point as long as so many other
+-- test scripts also show false failures for non-default block sizes.
+--
+
+-- This is to label the results files with blocksize:
+SHOW block_size;
+
+SHOW block_size \gset
+
+-- Apply page_checksum() to some different data patterns and block numbers
+SELECT blkno,
+    page_checksum(decode(repeat('01', :block_size), 'hex'), blkno) AS checksum_01,
+    page_checksum(decode(repeat('04', :block_size), 'hex'), blkno) AS checksum_04,
+    page_checksum(decode(repeat('ff', :block_size), 'hex'), blkno) AS checksum_ff,
+    page_checksum(decode(repeat('abcd', :block_size / 2), 'hex'), blkno) AS checksum_abcd,
+    page_checksum(decode(repeat('e6d6', :block_size / 2), 'hex'), blkno) AS checksum_e6d6,
+    page_checksum(decode(repeat('4a5e', :block_size / 2), 'hex'), blkno) AS checksum_4a5e
+  FROM generate_series(0, 100, 50) AS a (blkno);