implement configurable http-reuse feature

jobs/haproxy/spec

@@ -704,7 +704,15 @@ properties:
   ha_proxy.backend_config:
     description: |
       Raw HAProxy config that will be added to the default HTTP + routed HTTP backend definitions, provided either as a multiline text blob or as an array of lines.
-
+  ha_proxy.backend_config_targeted:
+    description: |
+      A map of existing backends which enables more fine-grained configuration of the backend.
+      example:
+        backend_config_targeted:
+          http-routers-http1: |
+            http-reuse aggressive
+          http-routers-http2: |
+            http-reuse safe
   ha_proxy.custom_http_error_files:
     description: |
       A map of status codes to errorfile contents

jobs/haproxy/templates/haproxy.config.erb

@@ -869,6 +869,13 @@ backend <%= backend[:name] %>
   <%- if properties.ha_proxy.backend_config -%>
     <%= format_indented_multiline_config(p("ha_proxy.backend_config")) %>
   <%- end -%>
+  <%- backend_configs = p('ha_proxy.backend_config_targeted', {})  -%>
+  <%- backend_configs.keys.each do |backend_name| -%>
+  <%- if backend[:name] == backend_name -%>
+    <%= format_indented_multiline_config(backend_configs[backend_name]) -%>
+    <%- end -%>
+  <%- end -%>
+
     <%- p('ha_proxy.custom_http_error_files', {}).keys.each do |status_code| -%>
         errorfile <%= status_code %> /var/vcap/jobs/haproxy/errorfiles/custom<%=status_code%>.http
     <%- end -%>

spec/haproxy/templates/haproxy_config/backend_http_spec.rb

@@ -40,6 +40,55 @@
     end
   end
 
+  context 'when ha_proxy.backend_config_targeted is h1' do
+    let(:properties) do
+      {
+        'backend_config_targeted' => {
+          'http-routers-http1' => 'http-reuse aggressive'
+        }
+      }
+    end
+
+    it 'includes the config' do
+      expect(backend_http1).to include('http-reuse aggressive')
+    end
+  end
+
+  context 'when ha_proxy.backend_config_targeted is h2' do
+    let(:properties) do
+      {
+        'enable_http2' => true,
+        'backend_ssl' => 'verify',
+        'backend_config_targeted' => {
+          'http-routers-http2' => 'http-reuse safe'
+        }
+      }
+    end
+
+    it 'includes the config' do
+      expect(backend_http2).to include('http-reuse safe')
+    end
+  end
+
+  context 'when ha_proxy.backend_config_targeted is h1 and h2' do
+    let(:properties) do
+      {
+        'enable_http2' => true,
+        'backend_ssl' => 'verify',
+        'disable_backend_http2_websockets' => true,
+        'backend_config_targeted' => {
+          'http-routers-http1' => 'http-reuse aggressive',
+          'http-routers-http2' => 'http-reuse safe'
+        }
+      }
+    end
+
+    it 'includes the config' do
+      expect(backend_http1).to include('http-reuse aggressive')
+      expect(backend_http2).to include('http-reuse safe')
+    end
+  end
+
   context 'when ha_proxy.custom_http_error_files is provided' do
     let(:properties) do
       {