Pin the Heroku build's NodeJS version to what the app requires

CHANGELOG.md

@@ -6,6 +6,11 @@ main
   natively since `5.0`, and every Rails version this gem supports is `>= 5.2`.
   To upgrade, remove `rails_12factor` from your project's `Gemfile`
 * Replace `EmberCli::App#yarn_enabled?` with `EmberCli::App#yarn?`
+* Pin the NodeJS version in the `package.json` that `rails generate
+  ember:heroku` writes, from the `engines.node` the Ember applications
+  declare. Applications that declare different versions are left unpinned
+* Add `EmberCli::App#node_engine`, reading `engines.node` from the Ember
+  application's `package.json`
 
 0.13.2
 ------

lib/ember_cli/app.rb

@@ -1,3 +1,5 @@
+require "json"
+
 require "html_page/renderer"
 require "ember_cli/path_set"
 require "ember_cli/shell"
@@ -99,6 +101,16 @@ def bower?
       paths.bower_json.exist?
     end
 
+    def node_engine
+      package_json = paths.package_json
+
+      if package_json.exist?
+        JSON.parse(package_json.read).dig("engines", "node")
+      end
+    rescue JSON::ParserError
+      nil
+    end
+
     def to_rack
       deploy.to_rack
     end

lib/ember_cli/path_set.rb

@@ -35,6 +35,10 @@ def gemfile
       @gemfile ||= root.join("Gemfile")
     end
 
+    def package_json
+      @package_json ||= root.join("package.json")
+    end
+
     def bower_json
       root.join("bower.json")
     end

lib/generators/ember/heroku/heroku_generator.rb

@@ -16,6 +16,26 @@ def identify_as_yarn_project
 
     private
 
+    def node_engine
+      return @node_engine if defined?(@node_engine)
+
+      declared = apps.map(&:node_engine).compact.uniq
+
+      @node_engine =
+        if declared.size > 1
+          say_status(
+            :conflict,
+            "Ember applications declare different `engines.node` " \
+            "(#{declared.join(", ")}); pin one in package.json by hand",
+            :red,
+          )
+
+          nil
+        else
+          declared.first
+        end
+    end
+
     def cache_directories
       all_cached_directories.map do |cached_directory|
         cached_directory.relative_path_from(Rails.root).to_s

lib/generators/ember/heroku/templates/package.json.erb

@@ -4,5 +4,10 @@
     "bower": "*"
   },
   <% end %>
+  <% if node_engine %>
+  "engines": {
+    "node": <%= node_engine.to_json %>
+  },
+  <% end %>
   "cacheDirectories": <%= cache_directories.to_json %>
 }

spec/generators/ember/heroku/heroku_generator_spec.rb

@@ -82,6 +82,54 @@ def gemfile_contents
       end
     end
 
+    describe "engines" do
+      it "pins the NodeJS version the Ember applications declare" do
+        setup_destination
+        configure_applications(node_engine: ">= 20.19.0")
+
+        run_generator
+
+        expect(package_json.fetch("engines")).to eq("node" => ">= 20.19.0")
+      end
+
+      it "omits engines when no Ember application declares one" do
+        setup_destination
+        configure_applications(node_engine: nil)
+
+        run_generator
+
+        expect(package_json.keys).not_to include("engines")
+      end
+
+      it "omits engines when the Ember applications disagree" do
+        setup_destination
+        configure_applications(
+          { node_engine: ">= 20.19.0" },
+          { node_engine: ">= 22.0.0" },
+        )
+
+        run_generator
+
+        expect(package_json.keys).not_to include("engines")
+      end
+
+      def configure_applications(*attributes)
+        apps = attributes.map do |app_attributes|
+          instance_double(
+            EmberCli::App,
+            {
+              bower?: false,
+              cached_directories: [],
+              yarn?: false,
+            }.merge(app_attributes),
+          )
+        end
+
+        allow(EmberCli).to receive(:apps).
+          and_return(apps.map.with_index { |app, i| ["app-#{i}", app] }.to_h)
+      end
+    end
+
     def depend_on_bower(bower_enabled)
       allow_any_instance_of(EmberCli::App).
         to receive(:bower?).and_return(bower_enabled)

spec/lib/ember_cli/app_spec.rb

@@ -93,6 +93,42 @@
     end
   end
 
+  describe "#node_engine" do
+    it "reads engines.node from the application's package.json" do
+      stub_package_json('{"engines":{"node":">= 20.19.0"}}')
+      app = EmberCli::App.new("with node engine")
+
+      expect(app.node_engine).to eq ">= 20.19.0"
+    end
+
+    it "returns nil when the package.json declares no engines" do
+      stub_package_json('{"name":"frontend"}')
+      app = EmberCli::App.new("without engines")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    it "returns nil when the package.json is absent" do
+      stub_paths(package_json: double("Pathname", exist?: false))
+      app = EmberCli::App.new("without package json")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    it "returns nil when the package.json is malformed" do
+      stub_package_json("{")
+      app = EmberCli::App.new("with malformed package json")
+
+      expect(app.node_engine).to be_nil
+    end
+
+    def stub_package_json(contents)
+      stub_paths(
+        package_json: double("Pathname", exist?: true, read: contents),
+      )
+    end
+  end
+
   describe "#compile" do
     it "exits with exit status of 0" do
       passed = EmberCli["my-app"].compile