[CLIENT] Surface deprecation headers from Elasticsearch

Author: picandocodigo

elasticsearch-transport/lib/elasticsearch/transport/transport/base.rb

@@ -355,6 +355,8 @@ def perform_request(method, path, params={}, body=nil, headers=nil, opts={}, &bl
 
           __trace  method, path, params, connection.connection.headers, body, url, response, nil, 'N/A', duration if tracer
 
+          warnings(response.headers['warning']) if response.headers&.[]('warning')
+
           Response.new response.status, json || response.body, response.headers
         ensure
           @last_request_at = Time.now
@@ -428,6 +430,10 @@ def user_agent_header(client)
             "elasticsearch-ruby/#{VERSION} (#{meta.join('; ')})"
           end
         end
+
+        def warnings(warning)
+          warn("warning: #{warning}")
+        end
       end
     end
   end

elasticsearch-transport/spec/elasticsearch/transport/client_spec.rb

@@ -1147,6 +1147,32 @@
       end
     end
 
+    context 'when Elasticsearch response includes a warning header' do
+      let(:client) do
+        Elasticsearch::Transport::Client.new(hosts: hosts)
+      end
+
+      let(:warning) { 'Elasticsearch warning: "deprecation warning"' }
+
+      it 'prints a warning' do
+        allow_any_instance_of(Elasticsearch::Transport::Transport::Response).to receive(:headers) do
+          { 'warning' => warning }
+        end
+
+        begin
+          stderr      = $stderr
+          fake_stderr = StringIO.new
+          $stderr     = fake_stderr
+
+          client.perform_request('GET', '/')
+          fake_stderr.rewind
+          expect(fake_stderr.string).to eq("warning: #{warning}\n")
+        ensure
+          $stderr = stderr
+        end
+      end
+    end
+
     context 'when a header is set on an endpoint request' do
       let(:client) { described_class.new(host: hosts) }
       let(:headers) { { 'user-agent' => 'my ruby app' } }