Extract CloudFormationInterpolatingEruby class

lib/stack_master.rb

@@ -45,6 +45,7 @@ module StackMaster
   autoload :StackDefinition, 'stack_master/stack_definition'
   autoload :TemplateCompiler, 'stack_master/template_compiler'
   autoload :Identity, 'stack_master/identity'
+  autoload :CloudFormationInterpolatingEruby, 'stack_master/cloudformation_interpolating_eruby'
 
   autoload :StackDiffer, 'stack_master/stack_differ'
   autoload :Validator, 'stack_master/validator'

lib/stack_master/cloudformation_interpolating_eruby.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require 'erubis'
+
+module StackMaster
+  #  This class is a modified version of `Erubis::Eruby`. It allows using
+  # `<%= %>` ERB expressions to interpolate values into a source string. We use
+  # this capability to enrich user data scripts with data and parameters pulled
+  # from the AWS CloudFormation service. The evaluation produces an array of
+  # objects ready for use in a CloudFormation `Fn::Join` intrinsic function.
+  class CloudFormationInterpolatingEruby < Erubis::Eruby
+    include Erubis::ArrayEnhancer
+
+    # A convenience method that loads a template from a file and evaluates it.
+    #
+    # @param source_path [String] The path to the file to evaluate.
+    def self.evaluate_file(source_path, context = Erubis::Context.new)
+      template_contents = File.read(source_path)
+      eruby = new(template_contents)
+      eruby.filename = source_path
+      eruby.evaluate(context)
+    end
+
+    # @return [Array] The result of evaluating the source: an array of strings
+    #         from the source intermindled with Hash objects from the ERB
+    #         expressions. To be included in a CloudFormation template, this
+    #         value needs to be used in a CloudFormation `Fn::Join` intrinsic
+    #         function.
+    # @see Erubis::Eruby#evaluate
+    # @example
+    #   CloudFormationInterpolatingEruby.new("my_variable=<%= { 'Ref' => 'Param1' } %>;").evaluate
+    #     #=> ['my_variable=', { 'Ref' => 'Param1' }, ';']
+    def evaluate(_context = Erubis::Context.new)
+      format_lines_for_cloudformation(super)
+    end
+
+    # @see Erubis::Eruby#add_expr
+    def add_expr(src, code, indicator)
+      if indicator == '='
+        src << " #{@bufvar} << (" << code << ');'
+      else
+        super
+      end
+    end
+
+    private
+
+    # Split up long strings containing multiple lines. One string per line in the
+    # CloudFormation array makes the compiled template and diffs more readable.
+    def format_lines_for_cloudformation(source)
+      source.flat_map do |lines|
+        lines = lines.to_s if lines.is_a?(Symbol)
+        next(lines) unless lines.is_a?(String)
+
+        newlines = Array.new(lines.count("\n"), "\n")
+        newlines = lines.split("\n").map { |line| "#{line}#{newlines.pop}" }
+        newlines.insert(0, "\n") if lines.start_with?("\n")
+        newlines
+      end
+    end
+  end
+end

lib/stack_master/sparkle_formation/template_file.rb

@@ -5,19 +5,6 @@ module StackMaster
   module SparkleFormation
     TemplateFileNotFound = ::Class.new(StandardError)
 
-    class SfEruby < Erubis::Eruby
-      include Erubis::ArrayEnhancer
-
-      def add_expr(src, code, indicator)
-        case indicator
-          when '='
-            src << " #{@bufvar} << (" << code << ');'
-          else
-            super
-        end
-      end
-    end
-
     class TemplateContext < AttributeStruct
       include ::SparkleFormation::SparkleAttribute
       include ::SparkleFormation::SparkleAttribute::Aws
@@ -49,47 +36,12 @@ def render(file_name, vars = {})
       end
     end
 
-    # Splits up long strings with multiple lines in them to multiple strings
-    # in the CF array. Makes the compiled template and diffs more readable.
-    class CloudFormationLineFormatter
-      def self.format(template)
-        new(template).format
-      end
-
-      def initialize(template)
-        @template = template
-      end
-
-      def format
-        @template.flat_map do |lines|
-          lines = lines.to_s if Symbol === lines
-          if String === lines
-            newlines = []
-            lines.count("\n").times do
-              newlines << "\n"
-            end
-            newlines = lines.split("\n").map do |line|
-              "#{line}#{newlines.pop}"
-            end
-            if lines.start_with?("\n")
-              newlines.insert(0, "\n")
-            end
-            newlines
-          else
-            lines
-          end
-        end
-      end
-    end
-
     module Template
       def self.render(prefix, file_name, vars)
         file_path = File.join(::SparkleFormation.sparkle_path, prefix, file_name)
-        template = File.read(file_path)
         template_context = TemplateContext.build(vars, prefix)
-        compiled_template = SfEruby.new(template).evaluate(template_context)
-        CloudFormationLineFormatter.format(compiled_template)
-      rescue Errno::ENOENT => e
+        CloudFormationInterpolatingEruby.evaluate_file(file_path, template_context)
+      rescue Errno::ENOENT
         Kernel.raise TemplateFileNotFound, "Could not find template file at path: #{file_path}"
       end
     end

spec/stack_master/cloudformation_interpolating_eruby_spec.rb

@@ -0,0 +1,62 @@
+RSpec.describe(StackMaster::CloudFormationInterpolatingEruby) do
+  describe('#evaluate') do
+    subject(:evaluate) { described_class.new(user_data).evaluate }
+
+    context('given a simple user data script') do
+      let(:user_data) { <<~SHELL }
+        #!/bin/bash
+
+        REGION=ap-southeast-2
+        echo $REGION
+      SHELL
+
+      it 'returns an array of lines' do
+        expect(evaluate).to eq([
+          "#!/bin/bash\n",
+          "\n",
+          "REGION=ap-southeast-2\n",
+          "echo $REGION\n",
+        ])
+      end
+    end
+
+    context('given a user data script referring parameters') do
+      let(:user_data) { <<~SHELL }
+        #!/bin/bash
+        <%= { 'Ref' => 'Param1' } %> <%= { 'Ref' => 'Param2' } %>
+      SHELL
+
+      it 'includes CloudFormation objects in the array' do
+        expect(evaluate).to eq([
+          "#!/bin/bash\n",
+          { 'Ref' => 'Param1' },
+          ' ',
+          { 'Ref' => 'Param2' },
+          "\n",
+        ])
+      end
+    end
+  end
+
+  describe('.evaluate_file') do
+    subject(:evaluate_file) { described_class.evaluate_file('my/userdata.sh') }
+
+    context('given a simple user data script file') do
+      before { allow(File).to receive(:read).with('my/userdata.sh').and_return(<<~SHELL) }
+        #!/bin/bash
+
+        REGION=ap-southeast-2
+        echo $REGION
+      SHELL
+
+      it 'returns an array of lines' do
+        expect(evaluate_file).to eq([
+          "#!/bin/bash\n",
+          "\n",
+          "REGION=ap-southeast-2\n",
+          "echo $REGION\n",
+        ])
+      end
+    end
+  end
+end