adding interpolation to redirects

README.md

@@ -82,6 +82,25 @@ With custom redirects, you can move pages to new routes but still preserve the o
 }
 ```
 
+##### Interpolating Env Var Values
+It's common to want to be able to test the frontend against various backends. The `url` key supports environment variable substitution using `${ENV_VAR_NAME}`. For instance, if there was a staging and production Heroku app for your API, you could setup the config above like the following:
+
+```json
+{
+  "redirects": {
+    "/old/gone/": {
+      "url": "${NEW_SITE_DOMAIN}/new/here/"
+    }
+  }
+}
+```
+
+Then using the [config vars](https://devcenter.heroku.com/articles/config-vars), you can point the frontend app to the appropriate backend. To match the original proxy setup:
+
+```bash
+$ heroku config:set NEW_SITE_DOMAIN="https://example.herokapp.com"
+```
+
 #### Custom Error Pages
 You can replace the default nginx 404 and 500 error pages by defining the path to one in your config.
 

scripts/config/lib/nginx_config.rb

@@ -30,11 +30,18 @@ class NginxConfig
       uri.path = ""
       json["proxies"][loc]["host"] = uri.to_s
     end
+
     json["clean_urls"] ||= DEFAULT[:clean_urls]
     json["https_only"] ||= DEFAULT[:https_only]
+
     json["routes"] ||= {}
     json["routes"] = NginxConfigUtil.parse_routes(json["routes"])
+
     json["redirects"] ||= {}
+    json["redirects"].each do |loc, hash|
+      json["redirects"][loc].merge!("url" => NginxConfigUtil.interpolate(hash["url"], ENV))
+    end
+
     json["error_page"] ||= nil
     json["debug"] ||= ENV['STATIC_DEBUG']
     json.each do |key, value|

spec/fixtures/redirects/public_html/interpolation.html

+hello interpolation

spec/fixtures/redirects/static.json

@@ -3,6 +3,10 @@
     "/old/gone": {
       "url": "/",
       "status": 302
+    },
+    "/old/interpolation": {
+      "url": "${INTERPOLATED_URL}",
+      "status": 302
     }
   }
 }

spec/simple_spec.rb

@@ -89,6 +89,12 @@ RSpec.describe "Simple" do
       expect(response.code).to eq("302")
       expect(response["location"]).to eq("http://#{AppRunner::HOST_IP}/")
     end
+
+    it "should redirect using interpolated urls" do
+      response = app.get("/old/interpolation")
+      expect(response.code).to eq("302")
+      expect(response["location"]).to eq("http://#{AppRunner::HOST_IP}/interpolation.html")
+    end
   end
 
   describe "https only" do

spec/support/app_runner.rb

@@ -23,14 +23,18 @@ class AppRunner
     @run       = false
     @debug     = debug
     env.merge!("STATIC_DEBUG" => true) if @debug
+
+    # support interpolation specs
+    env.merge!("INTERPOLATED_URL" => "/interpolation.html")
+
     @container = Docker::Container.create(
-      'Image'      => BuildpackBuilder::TAG,
-      'Cmd'        => ["bash", "-c", "cp -rf /src/* /app/ && /app/bin/boot"],
+      "Image"      => BuildpackBuilder::TAG,
+      "Cmd"        => ["bash", "-c", "cp -rf /src/* /app/ && /app/bin/boot"],
       # Env format is [KEY1=VAL1 KEY2=VAL2]
-      'Env'        => env.to_a.map {|i| i.join("=") },
-      'HostConfig' => {
-        'Binds' => ["#{fixtures_path(fixture)}:/src"],
-        'PortBindings' => {
+      "Env"        => env.to_a.map {|i| i.join("=") },
+      "HostConfig" => {
+        "Binds" => ["#{fixtures_path(fixture)}:/src"],
+        "PortBindings" => {
           "#{CONTAINER_PORT}/tcp" => [{
             "HostIp"   => HOST_IP,
             "HostPort" => HOST_PORT,