calculate http error_timeout based upon capacity option

lib/semian/net_http.rb

@@ -75,10 +75,19 @@ def reset_exceptions
     def raw_semian_options
       @raw_semian_options ||= begin
         @raw_semian_options = Semian::NetHTTP.retrieve_semian_configuration(address, port)
-        @raw_semian_options = @raw_semian_options.dup unless @raw_semian_options.nil?
+        unless @raw_semian_options.nil?
+          @raw_semian_options = @raw_semian_options.dup
+          calculate_error_timeout if @raw_semian_options[:capacity] && !@raw_semian_options[:error_timeout]
+        end
       end
     end
 
+    def calculate_error_timeout
+      capacity = @raw_semian_options[:capacity]
+      error_timeout = (read_timeout * capacity) / (1 - capacity)
+      @raw_semian_options.merge!({ error_timeout: error_timeout})
+    end
+
     def resource_exceptions
       Semian::NetHTTP.exceptions
     end

test/net_http_test.rb

@@ -353,6 +353,29 @@ def test_persistent_state_after_server_restart
     end
   end
 
+  def test_capacity_option_calculates_the_error_timeout_based_upon_read_timeout
+    options_with_capacity = DEFAULT_SEMIAN_OPTIONS.dup.merge!({ capacity: 0.75 })
+    options_with_capacity.delete(:error_timeout)
+    semian_configuration_proc = proc do |host, port|
+      options_with_capacity.merge(name: "#{host}_#{port}")
+    end
+    with_semian_configuration(semian_configuration_proc) do
+      http = Net::HTTP.new(HOSTNAME, TOXIC_PORT)
+      assert_equal 180, http.raw_semian_options[:error_timeout]
+    end
+  end
+
+  def test_capacity_option_defaults_to_error_timeout_when_both_are_provided
+    options_with_capacity = DEFAULT_SEMIAN_OPTIONS.dup.merge!({ capacity: 0.75 })
+    semian_configuration_proc = proc do |host, port|
+      options_with_capacity.merge(name: "#{host}_#{port}")
+    end
+    with_semian_configuration(semian_configuration_proc) do
+      http = Net::HTTP.new(HOSTNAME, TOXIC_PORT)
+      assert_equal 10, http.raw_semian_options[:error_timeout]
+    end
+  end
+
   private
 
   def with_semian_configuration(options = DEFAULT_SEMIAN_CONFIGURATION)