out_file: add warn message for symlink_path setting

lib/fluent/plugin/out_file.rb

@@ -172,6 +172,14 @@ def configure(conf)
           log.warn "symlink_path is unavailable on Windows platform. disabled."
           @symlink_path = nil
         else
+          placeholder_validators(:symlink_path, @symlink_path).reject{ |v| v.type == :time }.each do |v|
+            begin
+              v.validate!
+            rescue Fluent::ConfigError => e
+              log.warn "#{e}. This means multiple chunks are competing for a single symlink_path, so some logs may not be taken from the symlink."
+            end
+          end
+
           @buffer.extend SymlinkBufferMixin
           @buffer.symlink_path = @symlink_path
           @buffer.output_plugin_for_symlink = self

test/plugin/test_out_file.rb

@@ -130,7 +130,7 @@ def create_driver(conf = CONFIG, opts = {})
           'path' => "#{TMP_DIR}/${tag}/${type}/conf_test.%Y%m%d.%H%M.log",
           'add_path_suffix' => 'false',
           'append' => "true",
-          'symlink_path' => "#{TMP_DIR}/conf_test.current.log",
+          'symlink_path' => "#{TMP_DIR}/${tag}/conf_test.current.log",
           'compress' => 'gzip',
           'recompress' => 'true',
         }, [
@@ -183,6 +183,26 @@ def create_driver(conf = CONFIG, opts = {})
         Fluent::Test::Driver::Output.new(Fluent::Plugin::NullOutput).configure(conf)
       end
     end
+
+    test 'warning for symlink_path not including correct placeholders corresponding to chunk keys' do
+      omit "Windows doesn't support symlink" if Fluent.windows?
+      conf = config_element('match', '**', {
+          'path' => "#{TMP_DIR}/${tag}/${key1}/${key2}/conf_test.%Y%m%d.%H%M.log",
+          'symlink_path' => "#{TMP_DIR}/conf_test.current.log",
+        }, [
+          config_element('buffer', 'time,tag,key1,key2', {
+              '@type' => 'file',
+              'timekey' => '1d',
+              'path' => "#{TMP_DIR}/buf_conf_test",
+          }),
+      ])
+      assert_nothing_raised do
+        d = create_driver(conf)
+        assert do
+          d.logs.count { |log| log.include?("multiple chunks are competing for a single symlink_path") } == 2
+        end
+      end
+    end
   end
 
   sub_test_case 'fully configured output' do