Remap a mounted Vite build's asset URLs onto the mount point

CHANGELOG.md

@@ -4,6 +4,9 @@ main
 * Raise `EmberCli::BuildError` when `ember build` or a dependency
   installation fails, instead of exiting the process from inside the gem —
   `rake ember:compile` and `rake ember:install` still exit nonzero
+* Remap the root-relative asset URLs in a Vite build's `index.html` onto the
+  mount point when `mount_ember_app` serves the application from a path other
+  than `/`, so non-root mounts work without a matching `rootURL`
 
 0.13.1
 ------

README.md

@@ -591,7 +591,15 @@ Rails.application.routes.draw do
 end
 ```
 
-Then set each Ember application's `rootURL` to the mount point:
+The `index.html` of a Vite build refers to its assets with root-relative URLs.
+When the application is served from the build output, `mount_ember_app` remaps
+those references onto the mount point, where the assets are served — references
+that already carry the mount point, from a `rootURL` configured to match it,
+are left alone. Development-server-backed applications are unaffected: their
+asset URLs point at the development server absolutely.
+
+For classic (Broccoli-based) applications, set each Ember application's
+`rootURL` to the mount point:
 
 ```javascript
 // frontend/config/environment.js

app/helpers/ember_rails_helper.rb

@@ -8,6 +8,12 @@ def render_ember_app(name, &block)
 
     head, body = markup_capturer.capture
 
-    render html: EmberCli[name].index_html(head: head, body: body).html_safe
+    index_html = EmberCli[name].index_html(
+      head: head,
+      body: body,
+      mount_point: params[:ember_mount_point],
+    )
+
+    render html: index_html.html_safe
   end
 end

lib/ember_cli/app.rb

@@ -63,11 +63,11 @@ def build
       end
     end
 
-    def index_html(head:, body:)
+    def index_html(head:, body:, mount_point: nil)
       html = HtmlPage::Renderer.new(
         head: head,
         body: body,
-        content: deploy.index_html,
+        content: remap_to_mount_point(deploy.index_html, mount_point),
       )
 
       html.render
@@ -131,6 +131,25 @@ def test?
       env.to_s == "test"
     end
 
+    # The `index.html` of a Vite build refers to its assets with root-relative
+    # URLs. When the application is mounted somewhere other than `/`, remap
+    # them onto the mount point, where `mount_ember_assets` serves them.
+    # References that already carry the mount point — a `rootURL` configured
+    # to match it — are left alone.
+    def remap_to_mount_point(html, mount_point)
+      prefix = mount_point.to_s.chomp("/")
+
+      if prefix.empty? || !paths.vite?
+        return html
+      end
+
+      already_mounted = Regexp.escape(prefix.delete_prefix("/"))
+
+      html.gsub(%r{(\s)(src|href)=(["'])/(?!/)(?!#{already_mounted}/)}i) do
+        "#{$1}#{$2}=#{$3}#{prefix}/"
+      end
+    end
+
     def deploy
       deploy_strategy.new(self)
     end

lib/ember_cli/route_helpers.rb

@@ -6,7 +6,7 @@ module Routing
     class Mapper
       def mount_ember_app(app_name, to:, **options)
         routing_options = options.deep_merge(
-          defaults: { ember_app: app_name },
+          defaults: { ember_app: app_name, ember_mount_point: to },
         )
 
         routing_options.reverse_merge!(

spec/features/user_views_ember_app_spec.rb

@@ -35,6 +35,17 @@
     end
   end
 
+  scenario "remaps the document's asset URLs onto the mount point", js: false do
+    unless EmberCli["my-app"].paths.vite?
+      skip "Root-relative asset URLs are only remapped for Vite builds"
+    end
+
+    visit include_index_path
+
+    expect(page).to have_css(%{script[src^="/no-block/"]}, visible: false)
+    expect(page).to have_no_css(%{script[src^="/@embroider"]}, visible: false)
+  end
+
   scenario "is redirected with trailing slash", js: false do
     expect(include_index_path).to eq("/no-block")
 

spec/lib/ember_cli/app_spec.rb

@@ -100,6 +100,50 @@
     end
   end
 
+  describe "#index_html" do
+    it "remaps a Vite build's root-relative URLs onto the mount point" do
+      app = build_app("frontend", vite: true, environment: "test")
+      stub_deployed_index_html(app, <<~HTML)
+        <html><head>
+        <script type="module" src="/@embroider/virtual/vendor.js"></script>
+        <link rel="stylesheet" href="/assets/app.css">
+        <script type="module" src="https://cdn.example.com/analytics.js"></script>
+        <script src="//cdn.example.com/protocol-relative.js"></script>
+        <link rel="stylesheet" href="/admin/already-mounted.css">
+        </head><body></body></html>
+      HTML
+
+      index_html = app.index_html(head: "", body: "", mount_point: "/admin")
+
+      expect(index_html).to include(%{src="/admin/@embroider/virtual/vendor.js"})
+      expect(index_html).to include(%{href="/admin/assets/app.css"})
+      expect(index_html).to include(%{src="https://cdn.example.com/analytics.js"})
+      expect(index_html).to include(%{src="//cdn.example.com/protocol-relative.js"})
+      expect(index_html).to include(%{href="/admin/already-mounted.css"})
+      expect(index_html).not_to include(%{/admin/admin/})
+    end
+
+    it "leaves the document alone when mounted at the root" do
+      app = build_app("frontend", vite: true, environment: "test")
+      content = %{<html><head><script src="/assets/app.js"></script></head><body></body></html>}
+      stub_deployed_index_html(app, content)
+
+      index_html = app.index_html(head: "", body: "", mount_point: "/")
+
+      expect(index_html).to include(%{src="/assets/app.js"})
+    end
+
+    it "leaves a classic build alone" do
+      app = build_app("frontend", vite: false, environment: "test")
+      content = %{<html><head><script src="/assets/app.js"></script></head><body></body></html>}
+      stub_deployed_index_html(app, content)
+
+      index_html = app.index_html(head: "", body: "", mount_point: "/admin")
+
+      expect(index_html).to include(%{src="/assets/app.js"})
+    end
+  end
+
   describe "#root_path" do
     it "delegates to PathSet" do
       root_path = Pathname.new(".")
@@ -197,6 +241,10 @@ def build_app(name, vite:, environment:, **options)
     EmberCli::App.new(name, **options)
   end
 
+  def stub_deployed_index_html(app, html)
+    allow(app).to receive(:deploy).and_return(double(index_html: html))
+  end
+
   def stub_paths(method_to_value)
     allow_any_instance_of(EmberCli::PathSet).
       to receive(method_to_value.keys.first).