Warn when using default node or yarn versions (#1401)

* Warn when using default node or yarn versions

Many developers don't realize they're using a default version. When it changes, like https://devcenter.heroku.com/changelog-items/2710, they're surprised to find that even a correctly committed `package.json` with a node engine declaration does not install the version they're expecting. This is because the Ruby build pack does not duplicate the Node build pack's version detection logic. 

The Ruby build pack installs a version of node for historical reasons. It pre-dates multi-build packs from when applications simply needed **a** version of node for execjs to function along with the Rails asset pipeline (sprockets). As the ecosystem has matured, our recommendations also change. We recommend not relying on this default behavior.

It's worth noting that the CNB behavior (not yet available on Heroku) defers installing node to the nodes-engine build pack which is the ideal behavior so that there are not two independent paths for node support.

* Add changelog notes

---------

Co-authored-by: Richard Schneeman <rschneeman@rschnee-ltmgmqp.internal.salesforce.com>

Author: schneems

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Main (unreleased)
 
+- Warn when relying on default Node.js or Yarn versions (https://github.com/heroku/heroku-buildpack-ruby/pull/1401)
+- Warn when default Node.js or Yarn versions change (https://github.com/heroku/heroku-buildpack-ruby/pull/1401)
+
 ## v261 (2023/11/02)
 
 - JRuby 9.4.5.0 is now available

lib/language_pack/helpers/nodebin.rb

@@ -1,19 +1,20 @@
 require 'json'
 
 class LanguagePack::Helpers::Nodebin
+  NODE_VERSION = "20.9.0"
+  YARN_VERSION = "1.22.19"
+
   def self.hardcoded_node_lts
-    version = "20.9.0"
     {
-      "number" => version,
-      "url"    => "https://heroku-nodebin.s3.us-east-1.amazonaws.com/node/release/linux-x64/node-v#{version}-linux-x64.tar.gz"
+      "number" => NODE_VERSION,
+      "url"    => "https://heroku-nodebin.s3.us-east-1.amazonaws.com/node/release/linux-x64/node-v#{NODE_VERSION}-linux-x64.tar.gz"
     }
   end
 
   def self.hardcoded_yarn
-    version = "1.22.19"
     {
-      "number" => version,
-      "url"    => "https://heroku-nodebin.s3.us-east-1.amazonaws.com/yarn/release/yarn-v#{version}.tar.gz"
+      "number" => YARN_VERSION,
+      "url"    => "https://heroku-nodebin.s3.us-east-1.amazonaws.com/yarn/release/yarn-v#{YARN_VERSION}.tar.gz"
     }
   end
 

lib/language_pack/ruby.rb

@@ -1018,6 +1018,26 @@ def add_node_js_binary
     if Pathname(build_path).join("package.json").exist? ||
          bundler.has_gem?('execjs') ||
          bundler.has_gem?('webpacker')
+
+      version = @node_installer.version
+      old_version = @metadata.fetch("default_node_version") { version }
+
+      if version != version
+        warn(<<~WARNING, inline: true)
+          Default version of Node.js changed (#{old_version} to #{version})
+        WARNING
+      end
+
+      warn(<<~WARNING, inline: true)
+        Installing a default version (#{version}) of Node.js.
+        This version is not pinned and can change over time, causing unexpected failures.
+
+        Heroku recommends placing the `heroku/nodejs` buildpack in front of
+        `heroku/ruby` to install a specific version of node:
+
+        https://devcenter.heroku.com/articles/ruby-support#node-js-support
+      WARNING
+
       [@node_installer.binary_path]
     else
       []
@@ -1028,6 +1048,26 @@ def add_yarn_binary
     return [] if yarn_preinstalled?
 
     if Pathname(build_path).join("yarn.lock").exist? || bundler.has_gem?('webpacker')
+
+      version = @yarn_installer.version
+      old_version = @metadata.fetch("default_yarn_version") { version }
+
+      if version != version
+        warn(<<~WARNING, inline: true)
+          Default version of Yarn changed (#{old_version} to #{version})
+        WARNING
+      end
+
+      warn(<<~WARNING, inline: true)
+        Installing a default version (#{version}) of Yarn
+        This version is not pinned and can change over time, causing unexpected failures.
+
+        Heroku recommends placing the `heroku/nodejs` buildpack in front of
+        `heroku/ruby` to install a specific version of node:
+
+        https://devcenter.heroku.com/articles/ruby-support#node-js-support
+      WARNING
+
       [@yarn_installer.name]
     else
       []

spec/hatchet/node_spec.rb

@@ -16,6 +16,9 @@
       expect(app.output).to include("bin/node is the node directory")
       expect(app.output).to_not include(".heroku/node/bin/node is the node directory")
 
+      expect(app.output).to include("Installing a default version (#{LanguagePack::Helpers::Nodebin::YARN_VERSION}) of Yarn")
+      expect(app.output).to include("Installing a default version (#{LanguagePack::Helpers::Nodebin::NODE_VERSION}) of Node.js")
+
       expect(app.run("which node")).to match("/app/bin/node")     # We put node in bin/node
       expect(app.run("which yarn")).to match("/app/vendor/yarn-") # We put yarn in /app/vendor/yarn-
     end