Let load_module_metadata succeed on empty file

Some modules or module versions don't have a metadata.json file, but we
might still want to use the load_module_metadata function on them. The
lack of a file can still give us important information. For example, it
might tell us that the version of the module installed is "very old"
even if we can't read the version number directly. This patch adds a
parameter to let the user specify if an empty file is acceptable. To
preserve backwards compatibility it does not change the current default
behavior, which is to raise an error if metadata.json does not exist.

README.markdown

@@ -539,6 +539,15 @@ Loads the metadata.json of a target module. Can be used to determine module vers
   notify { $metadata['author']: }
   ~~~
 
+If you do not want to fail the catalog compilation if the metadata file for a module is not present:
+
+  ~~~
+  $metadata = load_module_metadata('mysql', true)
+  if empty($metadata) {
+    notify { "This module does not have a metadata.json file.": }
+  }
+  ~~~
+
 *Type*: rvalue.
 
 #### `lstrip`

lib/puppet/parser/functions/load_module_metadata.rb

@@ -2,14 +2,22 @@ module Puppet::Parser::Functions
   newfunction(:load_module_metadata, :type => :rvalue, :doc => <<-EOT
   EOT
   ) do |args|
-    raise(Puppet::ParseError, "load_module_metadata(): Wrong number of arguments, expects one") unless args.size == 1
+    raise(Puppet::ParseError, "load_module_metadata(): Wrong number of arguments, expects one or two") unless [1,2].include?(args.size)
     mod = args[0]
+    allow_empty_metadata = args[1]
     module_path = function_get_module_path([mod])
     metadata_json = File.join(module_path, 'metadata.json')
 
-    raise(Puppet::ParseError, "load_module_metadata(): No metadata.json file for module #{mod}") unless File.exists?(metadata_json)
-
-    metadata = PSON.load(File.read(metadata_json))
+    metadata_exists = File.exists?(metadata_json)
+    if metadata_exists
+      metadata = PSON.load(File.read(metadata_json))
+    else
+      if allow_empty_metadata
+        metadata = {}
+      else
+        raise(Puppet::ParseError, "load_module_metadata(): No metadata.json file for module #{mod}")
+      end
+    end
 
     return metadata
   end

spec/functions/load_module_metadata_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 describe 'load_module_metadata' do
   it { is_expected.not_to eq(nil) }
   it { is_expected.to run.with_params().and_raise_error(Puppet::ParseError, /wrong number of arguments/i) }
-  it { is_expected.to run.with_params("one", "two").and_raise_error(Puppet::ParseError, /wrong number of arguments/i) }
+  it { is_expected.to run.with_params("one", "two", "three").and_raise_error(Puppet::ParseError, /wrong number of arguments/i) }
 
   it "should json parse the file" do
      allow(scope).to receive(:function_get_module_path).with(['science']).and_return('/path/to/module/')
@@ -13,4 +13,17 @@ describe 'load_module_metadata' do
      result = subject.call(['science'])
      expect(result['name']).to eq('spencer-science')
   end
+
+  it "should fail by default if there is no metadata.json" do
+     allow(scope).to receive(:function_get_module_path).with(['science']).and_return('/path/to/module/')
+     allow(File).to receive(:exists?).with(/metadata.json/).and_return(false)
+     expect {subject.call(['science'])}.to raise_error(Puppet::ParseError)
+  end
+
+  it "should return nil if user allows empty metadata.json" do
+     allow(scope).to receive(:function_get_module_path).with(['science']).and_return('/path/to/module/')
+     allow(File).to receive(:exists?).with(/metadata.json/).and_return(false)
+     result = subject.call(['science', true])
+     expect(result).to eq({})
+  end
 end