Add simple script to check for right recursion in Bison grammars.

We should generally use left-recursion not right-recursion to parse lists.
Bison hasn't got any built-in way to check for this type of inefficiency,
and I didn't find anything on the net in a quick search, so I wrote a
little Perl script to do it.  Add to src/tools/ so we don't have to
re-invent this wheel next time we wonder if we're doing anything stupid.

Currently, the only place that seems to need fixing is plpgsql's stmt_else
production, so the problem doesn't appear to be common enough to warrant
trying to include such a test in our standard build process.  If we did
want to do that, we'd need a way to ignore some false positives, such as
a_expr := '-' a_expr

src/tools/check_bison_recursion.pl

@@ -0,0 +1,74 @@
+#! /usr/bin/perl
+
+#################################################################
+#
+# check_bison_recursion.pl -- check for right recursion in Bison grammars
+#
+# The standard way to parse list constructs in Bison grammars is via left
+# recursion, wherein a nonterminal symbol has itself as the first symbol
+# in one of its expansion rules.  It is also possible to parse a list via
+# right recursion, wherein a nonterminal symbol has itself as the last
+# symbol of an expansion; but that's a bad way to write it because a long
+# enough list will result in parser stack overflow.  Since Bison doesn't
+# have any built-in way to warn about use of right recursion, we use this
+# script when we want to check for the problem.
+#
+# To use: run bison with the -v switch, then feed the produced y.output
+# file to this script.
+#
+# Copyright (c) 2011, PostgreSQL Global Development Group
+#
+# src/tools/check_bison_recursion.pl
+#################################################################
+
+use strict;
+use warnings;
+
+my $debug = 0;
+
+# must retain this across input lines
+my $cur_nonterminal;
+
+# We parse the input and emit warnings on the fly.
+my $in_grammar = 0;
+
+while (<>) {
+    my $rule_number;
+    my $rhs;
+
+    # We only care about the "Grammar" part of the input.
+    if (m/^Grammar$/) {
+	$in_grammar = 1;
+    } elsif (m/^Terminal/) {
+	$in_grammar = 0;
+    } elsif ($in_grammar) {
+	if (m/^\s*(\d+)\s+(\S+):\s+(.*)$/) {
+	    # first rule for nonterminal
+	    $rule_number = $1;
+	    $cur_nonterminal = $2;
+	    $rhs = $3;
+	} elsif (m/^\s*(\d+)\s+\|\s+(.*)$/) {
+	    # additional rule for nonterminal
+	    $rule_number = $1;
+	    $rhs = $2;
+	}
+    }
+
+    # Process rule if we found one
+    if (defined $rule_number) {
+	# deconstruct the RHS
+	$rhs =~ s|^/\* empty \*/$||;
+	my @rhs = split '\s', $rhs;
+	print "Rule $rule_number: $cur_nonterminal := @rhs\n" if $debug;
+	# We complain if the nonterminal appears as the last RHS element
+	# but not elsewhere, since "expr := expr + expr" is reasonable
+	my $lastrhs = pop @rhs;
+	if (defined $lastrhs &&
+	    $cur_nonterminal eq $lastrhs &&
+	    !grep { $cur_nonterminal eq $_ } @rhs) {
+	    print "Right recursion in rule $rule_number: $cur_nonterminal := $rhs\n";
+	}
+    }
+}
+
+exit 0;