Fix #1109 - allow empty if or else branches

This changesets allow empty if or else branches:

if true {
} else {
}

It works by emitting on the parser stack an AST node that doesn't
do anything (a no-op). This allows the less intrusive code
as no part of the if evaluation code has been touched.

lib/puppet/parser/ast.rb

@@ -91,6 +91,7 @@ require 'puppet/parser/ast/ifstatement'
 require 'puppet/parser/ast/leaf'
 require 'puppet/parser/ast/minus'
 require 'puppet/parser/ast/node'
+require 'puppet/parser/ast/nop'
 require 'puppet/parser/ast/not'
 require 'puppet/parser/ast/resource'
 require 'puppet/parser/ast/resource_defaults'

lib/puppet/parser/ast/nop.rb

@@ -0,0 +1,11 @@
+require 'puppet/parser/ast/branch'
+
+class Puppet::Parser::AST
+    # This class is a no-op, it doesn't produce anything
+    # when evaluated, hence it's name :-)
+    class Nop < AST::Leaf
+        def evaluate(scope)
+            # nothing to do
+        end
+    end
+end

lib/puppet/parser/grammar.ra

@@ -417,6 +417,18 @@ ifstatement:      IF expression LBRACE statements RBRACE else {
         args[:else] = val[5]
     end
 
+    result = ast AST::IfStatement, args
+}
+                | IF expression LBRACE RBRACE else {
+    args = {
+        :test => val[1],
+        :statements => ast(AST::Nop)
+    }
+
+    if val[4]
+        args[:else] = val[4]
+    end
+
     result = ast AST::IfStatement, args
 }
 
@@ -424,6 +436,9 @@ else:             # nothing
                 | ELSE LBRACE statements RBRACE {
     result = ast AST::Else, :statements => val[2]
 }
+                | ELSE LBRACE RBRACE {
+    result = ast AST::Else, :statements => ast(AST::Nop)
+}
 
 # Unlike yacc/bison, it seems racc
 # gives tons of shift/reduce warnings

spec/unit/parser/ast/nop.rb

@@ -0,0 +1,20 @@
+#!/usr/bin/env ruby
+
+require File.dirname(__FILE__) + '/../../../spec_helper'
+
+describe Puppet::Parser::AST::Nop do
+
+    before do
+        @scope = mock 'scope'
+    end
+
+    it "should do nothing on evaluation" do
+        Puppet::Parser::AST.expects(:safeevaluate).never
+        Puppet::Parser::AST::Nop.new({}).evaluate(@scope)
+    end
+
+    it "should not return anything" do
+        Puppet::Parser::AST::Nop.new({}).evaluate(@scope).should be_nil
+    end
+
+end

spec/unit/parser/parser.rb

@@ -116,4 +116,30 @@ describe Puppet::Parser do
         
     end
      
+    describe Puppet::Parser, "when parsing if statements" do
+
+        it "should not raise errors with empty if" do
+            lambda { @parser.parse("if true { }") }.should_not raise_error
+        end
+
+        it "should not raise errors with empty else" do
+            lambda { @parser.parse("if false { notice('if') } else { }") }.should_not raise_error
+        end
+
+        it "should not raise errors with empty if and else" do
+            lambda { @parser.parse("if false { } else { }") }.should_not raise_error
+        end
+
+        it "should create a nop node for empty branch" do
+            AST::Nop.expects(:new)
+            @parser.parse("if true { }")
+        end
+
+        it "should create a nop node for empty else branch" do
+            AST::Nop.expects(:new)
+            @parser.parse("if true { notice('test') } else { }")
+        end
+
+    end
+
  end

test/data/snippets/emptyifelse.pp

@@ -0,0 +1,9 @@
+
+if false {
+} else {
+  # nothing here
+}
+
+if true {
+  # still nothing
+}