Be sure to detach one-shot timer watcher

lib/fluent/plugin_helper/timer.rb

@@ -66,6 +66,7 @@ class TimerWatcher < Coolio::TimerWatcher
         def initialize(title, interval, repeat, log, checker, &callback)
           @title = title
           @callback = callback
+          @repeat = repeat
           @log = log
           @checker = checker
           super(interval, repeat)
@@ -76,8 +77,12 @@ def on_timer
         rescue => e
           @log.error "Unexpected error raised. Stopping the timer.", title: @title, error: e
           @log.error_backtrace
-          self.detach
+          detach
           @log.error "Timer detached.", title: @title
+        ensure
+          if attached?
+            detach unless @repeat
+          end
         end
       end
     end

test/plugin_helper/test_timer.rb

@@ -97,4 +97,35 @@ class Dummy < Fluent::Plugin::TestBase
 
     d1.shutdown; d1.close; d1.terminate
   end
+
+  test 'can run at once' do
+    d1 = Dummy.new
+    d1.configure(config_element())
+    assert !d1.timer_running?
+    d1.start
+    assert d1.timer_running?
+
+    waiting_assertion = true
+    waiting_timer = true
+    counter = 0
+    d1.timer_execute(:test, 1, repeat: false) do
+      sleep(0.1) while waiting_assertion
+      counter += 1
+      waiting_timer = false
+    end
+
+    watchers = d1._event_loop.watchers.reject {|w| w.is_a?(Fluent::PluginHelper::EventLoop::DefaultWatcher) }
+    assert_equal(1, watchers.size)
+    assert(watchers.first.attached?)
+
+    waiting_assertion = false
+    sleep(0.1) while waiting_timer
+
+    assert_equal(1, counter)
+    assert_false(watchers.first.attached?)
+    watchers = d1._event_loop.watchers.reject {|w| w.is_a?(Fluent::PluginHelper::EventLoop::DefaultWatcher) }
+    assert_equal(0, watchers.size)
+
+    d1.shutdown; d1.close; d1.terminate
+  end
 end