add #after_start method to detect finishing of calling #start methods

This is to make a detection that #start method completely finished.
Without this method, flush/enqueue threads will run without consideration even if Output#start takes
long time to finish. Output#start may need long time to finish if it tries network connection or
service detection over internet.

lib/fluent/plugin/base.rb

@@ -24,11 +24,11 @@ class Base
       include Configurable
       include SystemConfig::Mixin
 
-      State = Struct.new(:configure, :start, :stop, :before_shutdown, :shutdown, :after_shutdown, :close, :terminate)
+      State = Struct.new(:configure, :start, :after_start, :stop, :before_shutdown, :shutdown, :after_shutdown, :close, :terminate)
 
       def initialize
         super
-        @_state = State.new(false, false, false, false, false, false, false, false)
+        @_state = State.new(false, false, false, false, false, false, false, false, false)
       end
 
       def has_router?
@@ -37,7 +37,7 @@ def has_router?
 
       def configure(conf)
         super
-        @_state ||= State.new(false, false, false, false, false, false, false, false)
+        @_state ||= State.new(false, false, false, false, false, false, false, false, false)
         @_state.configure = true
         self
       end
@@ -47,6 +47,11 @@ def start
         self
       end
 
+      def after_start
+        @_state.after_start = true
+        self
+      end
+
       def stop
         @_state.stop = true
         self
@@ -85,6 +90,10 @@ def started?
         @_state.start
       end
 
+      def after_started?
+        @_state.after_start
+      end
+
       def stopped?
         @_state.stop
       end

lib/fluent/plugin/output.rb

@@ -384,6 +384,11 @@ def start
         @secondary.start if @secondary
       end
 
+      def after_start
+        super
+        @secondary.after_start if @secondary
+      end
+
       def stop
         @secondary.stop if @secondary
         @buffer.stop if @buffering && @buffer
@@ -922,6 +927,11 @@ def enqueue_thread_run
           interval = @buffer_config.flush_thread_interval
         end
 
+        while !self.after_started? && !self.stopped?
+          sleep interval
+        end
+        log.debug "enqueue_thread actually running"
+
         begin
           while @output_flush_threads_running
             now_int = Time.now.to_i
@@ -969,6 +979,11 @@ def flush_thread_run(state)
         clock_id = Process::CLOCK_MONOTONIC rescue Process::CLOCK_MONOTONIC_RAW
         state.next_time = Process.clock_gettime(clock_id) + flush_thread_interval
 
+        while !self.after_started? && !self.stopped?
+          sleep flush_thread_interval
+        end
+        log.debug "flush_thread actually running"
+
         begin
           # This thread don't use `thread_current_running?` because this thread should run in `before_shutdown` phase
           while @output_flush_threads_running

lib/fluent/root_agent.rb

@@ -137,6 +137,9 @@ def start
       lifecycle(desc: true) do |i| # instance
         i.start unless i.started?
       end
+      lifecycle(desc: true) do |i|
+        i.after_start unless i.after_started?
+      end
     end
 
     def flush!