Add spec for yarn2 modules with zero install

spec/ci/yarn2_spec.rb

@@ -1,6 +1,8 @@
 require_relative '../spec_helper'
 
 describe "yarn2 examples" do
+  # Example uses the pnp linker and pushes all dependencies in .yarn/cache.
+  # No packages should be downloaded and buildcache is not used in this mode.
   it "pnp zero-install with should deploy successfully and not use the cache" do
     app = Hatchet::Runner.new("spec/fixtures/repos/yarn2-pnp-zero-install")
     app.deploy do |app|
@@ -14,6 +16,8 @@
     end
   end
 
+  # Example uses the pnp linker but does not push dependencies in .yarn/cache.
+  # All packages will either be restored from buildcache or downloaded.
   it "pnp nonzero-install should deploy successfully and reuse the cache" do
     app = Hatchet::Runner.new("spec/fixtures/repos/yarn2-pnp-nonzero-install")
     app.deploy do |app|
@@ -28,6 +32,25 @@
     end
   end
 
+  # Example uses the node-modules linker (e.g. node standard module resolution)
+  # and pushes all dependencies in .yarn/cache. No packages should be downloaded
+  # and buildcache is not used in this mode.
+  it "node_modules zero install should deploy successfully and reuse the cache" do
+    app = Hatchet::Runner.new("spec/fixtures/repos/yarn2-modules-zero-install")
+    app.deploy do |app|
+      expect(successful_body(app).strip).to eq("Hello from yarn2-modules-zero-install")
+
+      run!('git commit --allow-empty -m "deploy again to test cache reuse"')
+      app.push!
+
+      expect(app.output).to include("yarn cache is checked into source control and cannot be cached")
+      expect(successful_body(app).strip).to eq("Hello from yarn2-modules-zero-install")
+    end
+  end
+
+  # Example uses the node-modules linker (e.g. node standard module resolution)
+  # and does not push dependencies in .yarn/cache. All packages will either be 
+  # restored from the buildcache or downloaded.
   it "node_modules nonzero install should deploy successfully and reuse the cache" do
     app = Hatchet::Runner.new("spec/fixtures/repos/yarn2-modules-nonzero-install")
     app.deploy do |app|

spec/fixtures/repos/yarn2-modules-zero-install/.gitignore

@@ -0,0 +1,8 @@
+node_modules/
+.yarn/*
+!.yarn/cache
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

spec/fixtures/repos/yarn2-modules-zero-install/.yarnrc.yml

@@ -0,0 +1,2 @@
+yarnPath: .yarn/releases/yarn-3.1.1.cjs
+nodeLinker: node-modules

spec/fixtures/repos/yarn2-modules-zero-install/index.js

@@ -0,0 +1,11 @@
+const express = require("express");
+const app = express();
+const port = process.env["PORT"] || 3000;
+
+app.get("/", (_req,res) => {
+  res.send("Hello from yarn2-modules-zero-install");
+});
+
+app.listen(port, () => {
+  console.log(`yarn2-modules-zero-install listening on ${port}`);
+});

spec/fixtures/repos/yarn2-modules-zero-install/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "yarn2-modules-zero-install",
+  "version": "1.0.0",
+  "main": "index.js",
+  "license": "MIT",
+  "scripts": {
+    "start": "yarn node index.js"
+  },
+  "dependencies": {
+    "express": "^4.17.2"
+  },
+  "packageManager": "yarn@3.1.1"
+}