Mask out checksum in gpcheckmirrorseg.pl

Add the checksum masking and clearly separate where that masking is applied
separately from when the lp array masking is applied.

This will ensure the data in buf only updated once.
Signed-off-by: NC.J. Jameson <cjameson@pivotal.io>

gpMgmt/bin/gpcheckmirrorseg.pl

@@ -804,12 +804,12 @@ sub get_presh2_txt
 #
 #			# bits			  64 16			
 #			# bytes			   8 2
-#			my @foo = unpack("L2 S S S S S S ", $buf);
+#			my @foo = unpack("L2 S S S S S S ", $buf); # referring to PageHeaderData in bufpage.h for details
 #			
 #			shift @foo; # discard XLogRecPtr
 #			shift @foo;
 #			
-#			shift @foo; # discard pd_tli, pd_flags
+#			shift @foo; # discard pd_checksum, pd_flags
 #			shift @foo;
 #			
 #			my $pd_lower = shift @foo;
@@ -892,6 +892,10 @@ sub get_presh2_txt
 #
 #				push @pagehdr, $skippi;
 #			}
+#
+#			@pagehdr[2] = 0; # mask out the checksum
+#			my $page_header_with_checksum_masked = pack("L2 S6", @pagehdr);
+#			substr($buf, 0, length($page_header_with_checksum_masked)) = $page_header_with_checksum_masked;
 #			
 #			my $ii = 0;
 #			
@@ -900,23 +904,21 @@ sub get_presh2_txt
 #			#   heap case:  find tuples and fix hint bits
 #			while ($ii < scalar(@baz))
 #			{
-#				my $s1 = $baz[$ii];
+#				my $s1 = $baz[$ii]; # $s1 and $s2 is representing data stored in ItemIdData, see itemid.h for details
 #				my $s2 = $baz[$ii+1];
 #				
 #				$ii += 2;
 #				
 #				my $lp_off = $s1;
-#				$lp_off = $lp_off & 0x7FFF;
-#				
-#				my $lp_flags = 0;
-#				
-#				my $lp_len = $s2 >> 1;
+#				$lp_off = $lp_off & 0x7FFF; # discard bit of lp_flags information
+#
+#				my $lp_len = $s2 >> 1; # discard bit of lp_flags information
 #
 #				if ($isBtree)
 #				{
 #					# fixup s2 (part of lp_flags) -- mask out
 #					# LP_DELETE hint bit
-#					$baz[$ii-1] = 0xFFFFFE & $s2;
+#					$baz[$ii-1] = 0xFFFE & $s2;
 #				}
 #				else # heap tuple hint bit cleanup
 #				{
@@ -934,10 +936,9 @@ sub get_presh2_txt
 #
 #			if ($isBtree)
 #			{
-#				# use revised lp_flags in page hdr
-#				push @pagehdr, @baz;
-#				my $fixhdr = pack("L2 S S S S S S S$twomax", @pagehdr);
-#				substr($buf, 0, length($fixhdr)) = $fixhdr;
+#				# use revised lp_flags in the lp array
+#				my $fix_lp_array = pack("S$twomax", @baz);
+#				substr($buf, length($page_header_with_checksum_masked), length($fix_lp_array)) = $fix_lp_array;
 #			}
 #
 #		  L_endloop: