From 57f18fa6ac8dd1ab9f273530e9aea01052e0ca0a Mon Sep 17 00:00:00 2001
From: Pedro Lambert <pedrolambert@gmail.com>
Date: Thu, 2 Nov 2017 18:47:36 -0400
Subject: [PATCH] [WIP] Propagate `sampling_priority`

---
 lib/ddtrace/contrib/faraday/middleware.rb | 10 ++++++----
 lib/ddtrace/contrib/http/patcher.rb       |  3 +++
 lib/ddtrace/span.rb                       |  2 ++
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/lib/ddtrace/contrib/faraday/middleware.rb b/lib/ddtrace/contrib/faraday/middleware.rb
index a5bc3545b56..9d7ff64eeef 100644
--- a/lib/ddtrace/contrib/faraday/middleware.rb
+++ b/lib/ddtrace/contrib/faraday/middleware.rb
@@ -8,6 +8,8 @@ module Contrib
     module Faraday
       # Middleware implements a faraday-middleware for ddtrace instrumentation
       class Middleware < ::Faraday::Middleware
+        include Ext::DistributedTracing
+
         DEFAULT_ERROR_HANDLER = lambda do |env|
           Ext::HTTP::ERROR_RANGE.cover?(env[:status])
         end
@@ -54,10 +56,10 @@ def handle_response(span, env)
         end
 
         def propagate!(span, env)
-          env[:request_headers].merge!(
-            Ext::DistributedTracing::HTTP_HEADER_TRACE_ID => span.trace_id.to_s,
-            Ext::DistributedTracing::HTTP_HEADER_PARENT_ID => span.span_id.to_s
-          )
+          env[:request_headers][HTTP_HEADER_TRACE_ID] = span.trace_id.to_s
+          env[:request_headers][HTTP_HEADER_PARENT_ID] = span.span_id.to_s
+          return unless span.sampling_priority
+          env[:request_headers][HTTP_HEADER_SAMPLING_PRIORITY] = span.sampling_priority.to_s
         end
 
         def dd_pin
diff --git a/lib/ddtrace/contrib/http/patcher.rb b/lib/ddtrace/contrib/http/patcher.rb
index 09d6dc3ed71..6dfed3aba2f 100644
--- a/lib/ddtrace/contrib/http/patcher.rb
+++ b/lib/ddtrace/contrib/http/patcher.rb
@@ -132,6 +132,9 @@ def request(req, body = nil, &block) # :yield: +response+
                   unless Datadog::Contrib::HTTP.should_skip_distributed_tracing?(pin)
                     req.add_field(Datadog::Ext::DistributedTracing::HTTP_HEADER_TRACE_ID, span.trace_id)
                     req.add_field(Datadog::Ext::DistributedTracing::HTTP_HEADER_PARENT_ID, span.span_id)
+                    if span.sampling_priority
+                      req.add_field(Datadog::Ext::DistributedTracing::HTTP_HEADER_SAMPLING_PRIORITY, span.sampling_priority)
+                    end
                   end
                 rescue StandardError => e
                   Datadog::Tracer.log.error("error preparing span for http request: #{e}")
diff --git a/lib/ddtrace/span.rb b/lib/ddtrace/span.rb
index 8c4cdb07483..fecd6010d9f 100644
--- a/lib/ddtrace/span.rb
+++ b/lib/ddtrace/span.rb
@@ -46,6 +46,7 @@ def initialize(tracer, name, options = {})
       @span_id = Datadog::Utils.next_id
       @parent_id = options.fetch(:parent_id, 0)
       @trace_id = options.fetch(:trace_id, Datadog::Utils.next_id)
+      @sampling_priority = options[:sampling_priority]
 
       @context = options.fetch(:context, nil)
 
@@ -160,6 +161,7 @@ def parent=(parent)
         @parent_id = parent.span_id
         @service ||= parent.service
         @sampled = parent.sampled
+        @sampling_priority = parent.sampling_priority
       end
     end