add ignore_network_errors_at_startup

lib/fluent/plugin/out_forward.rb

@@ -70,6 +70,9 @@ class ConnectionClosedError < Error; end
     desc 'Enable client-side DNS round robin.'
     config_param :dns_round_robin, :bool, default: false # heartbeat_type 'udp' is not available for this
 
+    desc 'Ignore DNS resolution and errors at startup time.'
+    config_param :ignore_network_errors_at_startup, :bool, default: false
+
     desc 'Compress buffered data.'
     config_param :compress, :enum, list: [:text, :gzip], default: :text
 
@@ -148,7 +151,15 @@ def configure(conf)
         if @heartbeat_type == :none
           @nodes << NoneHeartbeatNode.new(self, server, failure: failure)
         else
-          @nodes << Node.new(self, server, failure: failure)
+          node = Node.new(self, server, failure: failure)
+          begin
+            node.validate_host_resolution!
+          rescue => e
+            raise unless @ignore_network_errors_at_startup
+            log.warn "failed to resolve node name when configured", server: (server.name || server.host), error: e
+            node.disable!
+          end
+          @nodes << node
         end
       end
 
@@ -453,7 +464,7 @@ def initialize(sender, server, failure:)
 
         @resolved_host = nil
         @resolved_time = 0
-        resolved_host  # check dns
+        @resolved_once = false
       end
 
       attr_accessor :usock
@@ -462,6 +473,10 @@ def initialize(sender, server, failure:)
       attr_reader :sockaddr  # used by on_heartbeat
       attr_reader :failure, :available # for test
 
+      def validate_host_resolution!
+        resolved_host
+      end
+
       def available?
         @available
       end
@@ -550,9 +565,20 @@ def send_data(tag, chunk)
 
       # FORWARD_TCP_HEARTBEAT_DATA = FORWARD_HEADER + ''.to_msgpack + [].to_msgpack
       def send_heartbeat
+        begin
+          dest_addr = resolved_host
+          @resolved_once = true
+        rescue ::SocketError => e
+          if !@resolved_once && @sender.ignore_network_errors_at_startup
+            @log.warn "failed to resolve node name in heartbeating", server: @name || @host, error: e
+            return
+          end
+          raise
+        end
+
         case @sender.heartbeat_type
         when :tcp
-          @sender.create_transfer_socket(resolved_host, port) do |sock|
+          @sender.create_transfer_socket(dest_addr, port) do |sock|
             ## don't send any data to not cause a compatibility problem
             # sock.write FORWARD_TCP_HEARTBEAT_DATA
 

test/plugin/test_out_forward.rb

@@ -135,6 +135,24 @@ def read_ack_from_sock(sock, unpacker)
     end
   end
 
+  test 'configure with ignore_network_errors_at_startup' do
+    normal_conf = config_element('match', '**', {}, [
+        config_element('server', '', {'name' => 'test', 'host' => 'unexisting.yaaaaaaaaaaaaaay.host.example.com'})
+      ])
+    assert_raise SocketError do
+      create_driver(normal_conf)
+    end
+
+    conf = config_element('match', '**', {'ignore_network_errors_at_startup' => 'true'}, [
+        config_element('server', '', {'name' => 'test', 'host' => 'unexisting.yaaaaaaaaaaaaaay.host.example.com'})
+      ])
+    @d = d = create_driver(conf)
+    expected_log = "failed to resolve node name when configured"
+    expected_detail = 'server="test" error_class=SocketError'
+    logs = d.logs
+    assert{ logs.any?{|log| log.include?(expected_log) && log.include?(expected_detail) } }
+  end
+
   test 'compress_default_value' do
     @d = d = create_driver
     assert_equal :text, d.instance.compress