Try to fix journal rotation

lib/fluent/plugin/in_systemd.rb

@@ -19,9 +19,6 @@ class SystemdInput < Input
       def configure(conf)
         super
         @pos_writer = PosWriter.new(@pos_file)
-        @journal = Systemd::Journal.new(path: @path)
-        @journal.filter(*@filters)
-        seek
       end
 
       def start
@@ -37,6 +34,12 @@ def shutdown
 
       private
 
+      def init_journal
+        @journal = Systemd::Journal.new(path: @path)
+        @journal.filter(*@filters)
+        seek
+      end
+
       def seek
         seek_to(@pos_writer.cursor || read_from)
       rescue Systemd::JournalError
@@ -59,6 +62,7 @@ def read_from
       end
 
       def run
+        init_journal
         Thread.current.abort_on_exception = true
         watch do |entry|
           begin
@@ -67,6 +71,7 @@ def run
             log.error("Exception emitting record: #{e}")
           end
         end
+        @pos_writer.sync
       end
 
       def formatted(entry)

lib/fluent/plugin/systemd/pos_writer.rb

@@ -27,6 +27,10 @@ def shutdown
           write_pos
         end
 
+        def sync
+          write_pos
+        end
+
         def update(c)
           return unless @path
           @lock.synchronize { @cursor = c }

test/plugin/systemd/test_pos_writer.rb

@@ -32,6 +32,17 @@ def test_writing_the_cursor_when_file_does_not_exist_yet
     FileUtils.rm_rf dir
   end
 
+  def test_syncing_the_cursor_when_file_does_not_exist_yet
+    dir = Dir.mktmpdir("posdir")
+    path = "#{dir}/foo.pos"
+    pos_writer = Fluent::Plugin::SystemdInput::PosWriter.new(path)
+    pos_writer.start
+    pos_writer.update("this is the cursor")
+    pos_writer.sync
+    assert_equal File.read(path), "this is the cursor"
+    FileUtils.rm_rf dir
+  end
+
   def test_file_permission_when_file_does_not_exist_yet
     dir = Dir.mktmpdir("posdir")
     path = "#{dir}/foo.pos"