(PUP-11751) Handle possible nil values in interpolation

The previous logic did not handle values set to nil; this brings
back the `nil_in_scope?` method to preserve the previous logic.

Author: tvpartytonight

lib/puppet/pops/lookup/interpolation.rb

@@ -91,15 +91,18 @@ def interpolate_method(method_key)
           else
             scope = lookup_invocation.scope
             val = nil
-            if (default_val = lookup_invocation.default_values[root_key])
+            if (default_key_exists = lookup_invocation.default_values.include?(root_key) )
               catch(:undefined_variable) { val = scope[root_key] }
             else
               val = scope[root_key]
             end
-            if val.nil? && default_val.nil?
-              nil
-            elsif val.nil?
-              lookup_invocation.report_found_in_defaults(root_key, default_val)
+            if val.nil? && !nil_in_scope?(scope, root_key)
+              if default_key_exists
+                lookup_invocation.report_found_in_defaults(root_key,
+                                                           lookup_invocation.default_values[root_key])
+              else
+                nil
+              end
             else
               lookup_invocation.report_found(root_key, val)
             end
@@ -127,6 +130,15 @@ def interpolate_method(method_key)
     interpolate_method
   end
 
+  # Because the semantics of Puppet::Parser::Scope#include? differs from Hash#include?
+  def nil_in_scope?(scope, key)
+    if scope.is_a?(Hash)
+      scope.include?(key)
+    else
+      scope.exist?(key)
+    end
+  end
+
   def get_method_and_data(data, allow_methods)
     match = data.match(/^(\w+)\((?:["]([^"]+)["]|[']([^']+)['])\)$/)
     if match

spec/unit/pops/lookup/interpolation_spec.rb

@@ -45,6 +45,18 @@ def expect_lookup(*keys)
     keys.each { |key| expect(adapter).to receive(:track).with(key) }
   end
 
+  context 'when the invocation has a default falsey values' do
+    let(:lookup_invocation) { Lookup::Invocation.new(scope, {}, {'key' => false, 'key2' => nil}, nil) }
+
+    it 'converts boolean false to a "false" string in interpolation' do
+      expect(interpolator.interpolate('%{key}', lookup_invocation, true)).to eq("false")
+    end
+
+    it 'converts nil to an empty string in interpolation' do
+      expect(interpolator.interpolate('%{key2}', lookup_invocation, true)).to eq("")
+    end
+  end
+
   context 'when interpolating nested data' do
     let(:nested_hash) { {'a' => {'aa' => "%{alias('aaa')}"}} }