Check if memcached is running during memcache spec, and fail if not r…

…unning (#591)

* Check if memcached is running during memcache spec, and fail if not running

I was trying to run the test suite locally, and it failed here because I
didn't have memcached running (yet):

  1) Rack::MiniProfiler::MemcacheStore page struct storage can store a PageStruct and retrieve it
     Failure/Error: @client.set("#{@Prefix}#{page_struct[:id]}", Marshal::dump(page_struct), @expires_in_seconds)

     Dalli::RingError:
       No server available
     # ./lib/mini_profiler/storage/memcache_store.rb:22:in `save'
     # ./spec/lib/storage/memcache_store_spec.rb:17:in `block (4 levels) in <top (required)>'

This adds a `alive?` method that the spec can check, and give more
specific information to the user. I've also included a
docker-compose.yml to give a one-liner to start memcached.

* Don't bother with a docker-compose at this point. Suggest using docker directly

lib/mini_profiler/storage/memcache_store.rb

@@ -18,6 +18,15 @@ def initialize(args = nil)
         @expires_in_seconds = args[:expires_in] || EXPIRES_IN_SECONDS
       end
 
+      def alive?
+        begin
+          @client.alive!
+          true
+        rescue Dalli::RingError
+          false
+        end
+      end
+
       def save(page_struct)
         @client.set("#{@prefix}#{page_struct[:id]}", Marshal::dump(page_struct), @expires_in_seconds)
       end

spec/lib/storage/memcache_store_spec.rb

@@ -6,6 +6,9 @@
 
     before do
       @store = Rack::MiniProfiler::MemcacheStore.new
+      unless @store.alive?
+        fail 'Memcached does not appear to be running on localhost:11211. Use your favorite package manager to install and run it, use Docker with: `docker run -it --rm memcached`'
+      end
     end
 
     describe 'storage' do