Add custom filtering rules to the TAP tests of pg_upgrade

002_pg_upgrade.pl gains support for a new environment variable called
"filter_rules", that can be used to point to a file that includes a
set of custom regular expressions that would be applied to the dumps of
the origin and target clusters when doing a cross-version test (aka when
defining olddump and oldinstall), to give the possibility to reshape
dynamically the dumps in the same way as the internals of the buildfarm
code so as the tests are able to pass in scenarios where one expects
them to even if pg_dump generates slightly-different outputs depending
on the versions involved.

This option is not used when pg_upgrade runs with the same version for
the origin and target clusters, and it is the last piece I see as
required to be able to plug-in more efficiently the TAP tests of
pg_upgrade with the buildfarm or just a CI.

Author: Anton A. Melnikov
Discussion: https://postgr.es/m/49f389ba-95ce-8a9b-09ae-f60650c0e7c7@inbox.ru

src/bin/pg_upgrade/TESTING

@@ -17,6 +17,21 @@ the creation of the dump):
 export olddump=...somewhere/dump.sql	(old version's dump)
 export oldinstall=...otherversion/	(old version's install base path)
 
+"filter_rules" is a variable that can be used to specify a file with custom
+filtering rules applied before comparing the dumps of the PostgreSQL
+instances near the end of the tests, in the shape of regular expressions
+valid for perl.  This is useful to enforce certain validation cases where
+pg_dump could create inconsistent outputs across major versions.
+For example:
+
+	# Remove all CREATE POLICY statements
+	s/^CREATE\sPOLICY.*//mgx
+	# Replace REFRESH with DROP for materialized views
+	s/^REFRESH\s(MATERIALIZED\sVIEW)/DROP $1/mgx
+
+Lines beginning with '#' and empty lines are ignored.  One rule can be
+defined per line.
+
 Finally, the tests can be done by running
 
 	make check

src/bin/pg_upgrade/t/002_pg_upgrade.pl

@@ -45,6 +45,31 @@ sub filter_dump
 	# Remove empty lines.
 	$dump_contents =~ s/^\n//mgx;
 
+	# Apply custom filtering rules, if any.
+	if (defined($ENV{filter_rules}))
+	{
+		my $filter_file = $ENV{filter_rules};
+		die "no file with custom filter rules found!" unless -e $filter_file;
+
+		open my $filter_handle, '<', $filter_file
+		  or die "could not open $filter_file";
+		while (<$filter_handle>)
+		{
+			my $filter_line = $_;
+
+			# Skip comments and empty lines
+			next if ($filter_line =~ /^#/);
+			next if ($filter_line =~ /^\s*$/);
+
+			# Apply lines with filters.
+			note "Applying custom rule $filter_line to $dump_file";
+			my $filter = "\$dump_contents =~ $filter_line";
+			## no critic (ProhibitStringyEval)
+			eval $filter;
+		}
+		close $filter_handle;
+	}
+
 	my $dump_file_filtered = "${dump_file}_filtered";
 	open(my $dh, '>', $dump_file_filtered)
 	  || die "opening $dump_file_filtered";