Add flamegraph path to response headers (#601)

* Add flamegraph path to response headers

The motivation for this is to enable easy access to the flamegraph
when using the async-flamegraph action. Currently, if have a CURL
request that you are wanting to profile, you have to add the
`pp=async-flamegraph` query parameter to the request, then copy the
first ID from the `X-MiniProfiler-Ids` header, then add that ID to the
`/mini-profiler-resources/flamegraph?id=` path in the browser.
This assumes that you know that the current request profile id is the
first one in the `X-MiniProfiler-Ids` header (personally I was not
confident of this until I read the source code) and that you know the
path to the flamegraph endpoint.

I originally added the protocol, host, and port to the path, but I
didn't know if that was guaranteed to be correct in all cases, so I
removed it.

* Add note of X-MiniProfiler-Flamegraph-Path header to README

README.md

@@ -180,7 +180,7 @@ To generate [flamegraphs](http://samsaffron.com/archive/2013/03/19/flame-graphs-
 
 Then, to view the flamegraph as a direct HTML response from your request, just visit any page in your app with `?pp=flamegraph` appended to the URL, or add the header `X-Rack-Mini-Profiler` to the request with the value `flamegraph`.
 
-Conversely, if you want your regular response instead (which is specially useful for JSON and/or XHR requests), just append the `?pp=async-flamegraph` parameter to your request/fetch URL; the request will then return as normal, and the flamegraph data will be stored for later *async* viewing, both for this request and for all subsequent requests made by this page (based on the `REFERER` header). For viewing these async flamegraphs, use the 'flamegraph' link that will appear inside the MiniProfiler UI for these requests.
+Conversely, if you want your regular response instead (which is specially useful for JSON and/or XHR requests), just append the `?pp=async-flamegraph` parameter to your request/fetch URL; the request will then return as normal, and the flamegraph data will be stored for later *async* viewing, both for this request and for all subsequent requests made by this page (based on the `REFERER` header). For viewing these async flamegraphs, use the 'flamegraph' link that will appear inside the MiniProfiler UI for these requests or path returned in the `X-MiniProfiler-Flamegraph-Path` header.
 
 Note: Mini Profiler will not record SQL timings for a request if it asks for a flamegraph. The rationale behind this is to keep
 Mini Profiler's methods that are responsible for generating the timings data out of the flamegraph.
@@ -466,7 +466,7 @@ If you are using Heroku Redis, you may need to add the following to your `config
 
 ```ruby
 if Rails.env.production?
-  Rack::MiniProfiler.config.storage_options = { 
+  Rack::MiniProfiler.config.storage_options = {
     url: ENV["REDIS_URL"],
     ssl_params: { verify_mode: OpenSSL::SSL::VERIFY_NONE }
   }

lib/mini_profiler.rb

@@ -436,6 +436,7 @@ def inject_profiler(env, status, headers, body)
       # inject header
       if headers.is_a? Hash
         headers['X-MiniProfiler-Ids'] = ids_comma_separated(env)
+        headers['X-MiniProfiler-Flamegraph-Path'] = flamegraph_path(env) if current.page_struct[:has_flamegraph]
       end
 
       if current.inject_js && content_type =~ /text\/html/
@@ -605,6 +606,10 @@ def ids_comma_separated(env)
       ids(env).join(",")
     end
 
+    def flamegraph_path(env)
+      @config.base_url_path + 'flamegraph?id=' + current.page_struct[:id]
+    end
+
     # cancels automatic injection of profile script for the current page
     def cancel_auto_inject(env)
       current.inject_js = false

spec/integration/mini_profiler_spec.rb

@@ -177,7 +177,9 @@ def app
       get '/html?pp=async-flamegraph'
       expect(last_response).to be_ok
       id = last_response.headers['X-MiniProfiler-Ids'].split(",")[0]
-      get "/mini-profiler-resources/flamegraph?id=#{id}"
+      flamegraph_path = last_response.headers['X-MiniProfiler-Flamegraph-Path']
+      expect(flamegraph_path).to eq("/mini-profiler-resources/flamegraph?id=#{id}")
+      get flamegraph_path
       expect(last_response).to be_ok
       expect(last_response.body).to include("var graph = {")