Evaluate ERB syntax in FileDataSource

launchdarkly-server-sdk.gemspec

@@ -31,6 +31,8 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "timecop", "~> 0.9"
   spec.add_development_dependency "listen", "~> 3.3" # see file_data_source.rb
   spec.add_development_dependency "webrick", "~> 1.7"
+  spec.add_development_dependency "erb", "~> 2.2.3"
+
   # required by dynamodb
   spec.add_development_dependency "oga", "~> 2.2"
 

lib/ldclient-rb/file_data_source.rb

@@ -15,11 +15,21 @@ module LaunchDarkly
   rescue LoadError
   end
 
+  @@have_erb = false
+  begin
+    require 'erb'
+    @@have_erb = true
+  rescue LoadError
+  end
+
   # @private
   def self.have_listen?
     @@have_listen
   end
 
+  def self.have_erb?
+    @@have_erb
+  end
   #
   # Provides a way to use local files as a source of feature flag state. This allows using a
   # predetermined feature flag state without an actual LaunchDarkly connection.
@@ -192,7 +202,9 @@ def load_all
     end
 
     def load_file(path, all_data)
-      parsed = parse_content(IO.read(path))
+      content = IO.read(path)
+      content = ERB.new(content).result if LaunchDarkly.have_erb?
+      parsed = parse_content(content)
       (parsed[:flags] || {}).each do |key, flag|
         add_item(all_data, FEATURES, flag)
       end

spec/file_data_source_spec.rb

@@ -71,6 +71,17 @@ def []=(key, value)
 EOF
   }
 
+    let(:json_with_erb) { <<-EOF
+      {
+        "flagValues": {
+          "onePlusOne": <%= 1 + 1 %>,
+          "constantTwo": 2
+        }
+      }
+EOF
+  }
+
+
   let(:all_properties_yaml) { <<-EOF
 ---
 flags:
@@ -86,6 +97,15 @@ def []=(key, value)
 EOF
   }
 
+  let(:yaml_with_erb) { <<-EOF
+---
+flagValues:
+  onePlusOne: <%= 1 + 1 %>
+  constantTwo: 2
+EOF
+  }
+
+
   let(:unsafe_yaml) { <<-EOF
 --- !ruby/hash:BadClassWeShouldNotInstantiate
 foo: bar
@@ -132,6 +152,41 @@ def with_data_source(options)
     end
   end
 
+  it "evaulates erb in json if ERB is available" do
+    file = make_temp_file(yaml_with_erb)
+    with_data_source({ paths: [ file.path ] }) do |ds|
+      ds.start
+      onePlusOne = @store.all(LaunchDarkly::FEATURES).dig(:onePlusOne, :variations)
+      constantTwo = @store.all(LaunchDarkly::FEATURES).dig(:constantTwo, :variations)
+      expect(onePlusOne).to eq([2])
+      expect(constantTwo).to eq([2])
+    end
+  end
+
+  it "does not evaulate ERB if ERB is not available" do
+    expect(LaunchDarkly).to receive(:have_erb?).and_return(false)
+    file = make_temp_file(yaml_with_erb)
+    with_data_source({ paths: [ file.path ] }) do |ds|
+      ds.start
+      onePlusOne = @store.all(LaunchDarkly::FEATURES).dig(:onePlusOne, :variations)
+      constantTwo = @store.all(LaunchDarkly::FEATURES).dig(:constantTwo, :variations)
+      expect(onePlusOne).to eq(['<%= 1 + 1 %>'])
+      expect(constantTwo).to eq([2])
+    end
+  end
+
+  it "evaulates erb in json if ERB is available" do
+    file = make_temp_file(json_with_erb)
+    with_data_source({ paths: [ file.path ] }) do |ds|
+      ds.start
+      onePlusOne = @store.all(LaunchDarkly::FEATURES).dig(:onePlusOne, :variations)
+      constantTwo = @store.all(LaunchDarkly::FEATURES).dig(:constantTwo, :variations)
+      expect(onePlusOne).to eq([2])
+      expect(constantTwo).to eq([2])
+    end
+  end
+
+
   it "loads flags on start - from JSON" do
     file = make_temp_file(all_properties_json)
     with_data_source({ paths: [ file.path ] }) do |ds|