Support negative index for tag when extract placeholders

For example,

tag is `fluentd.test.output`

`${tag[-1]}` is extracted to `output`
`${tag[-2]}` is extracted to `test`

It is useful to extract last element of variable length tag.

lib/fluent/plugin/output.rb

@@ -38,7 +38,7 @@ class Output < Base
 
       CHUNK_KEY_PATTERN = /^[-_.@a-zA-Z0-9]+$/
       CHUNK_KEY_PLACEHOLDER_PATTERN = /\$\{[-_.@$a-zA-Z0-9]+\}/
-      CHUNK_TAG_PLACEHOLDER_PATTERN = /\$\{(tag(?:\[\d+\])?)\}/
+      CHUNK_TAG_PLACEHOLDER_PATTERN = /\$\{(tag(?:\[-?\d+\])?)\}/
       CHUNK_ID_PLACEHOLDER_PATTERN = /\$\{chunk_id\}/
 
       CHUNKING_FIELD_WARN_NUM = 4
@@ -672,7 +672,7 @@ def get_placeholders_tag(str)
         str.scan(CHUNK_TAG_PLACEHOLDER_PATTERN).map(&:first).each do |ph|
           if ph == "tag"
             parts << -1
-          elsif ph =~ /^tag\[(\d+)\]$/
+          elsif ph =~ /^tag\[(-?\d+)\]$/
             parts << $1.to_i
           end
         end
@@ -708,15 +708,17 @@ def extract_placeholders(str, chunk)
             @output_time_formatter_cache[str] ||= Fluent::Timezone.formatter(@timekey_zone, str)
             rvalue = @output_time_formatter_cache[str].call(metadata.timekey)
           end
-          # ${tag}, ${tag[0]}, ${tag[1]}, ...
+          # ${tag}, ${tag[0]}, ${tag[1]}, ... , ${tag[-2]}, ${tag[-1]}
           if @chunk_key_tag
             if str.include?('${tag}')
               rvalue = rvalue.gsub('${tag}', metadata.tag)
             end
             if str =~ CHUNK_TAG_PLACEHOLDER_PATTERN
               hash = {}
-              metadata.tag.split('.').each_with_index do |part, i|
+              tag_parts = metadata.tag.split('.')
+              tag_parts.each_with_index do |part, i|
                 hash["${tag[#{i}]}"] = part
+                hash["${tag[#{i-tag_parts.size}]}"] = part
               end
               rvalue = rvalue.gsub(CHUNK_TAG_PLACEHOLDER_PATTERN, hash)
             end

test/plugin/test_output.rb

@@ -331,14 +331,32 @@ def waiting(seconds)
       assert_equal "/mypath/2016/04/11/20-30/fluentd.test.output/test/output/value1/value2/tail", @i.extract_placeholders(tmpl, c)
     end
 
+    data(:new_api => :chunk,
+         :old_api => :metadata)
+    test '#extract_placeholders can extract negative index with tag' do |api|
+      @i.configure(config_element('ROOT', '', {}, [config_element('buffer', 'time,tag,key1,key2', {'timekey' => 60*30, 'timekey_zone' => "+0900"})]))
+      assert @i.chunk_key_time
+      assert @i.chunk_key_tag
+      assert_equal ['key1','key2'], @i.chunk_keys
+      tmpl = "/mypath/%Y/%m/%d/%H-%M/${tag}/${tag[-1]}/${tag[-2]}/${key1}/${key2}/tail"
+      t = event_time('2016-04-11 20:30:00 +0900')
+      v = {key1: "value1", key2: "value2"}
+      c = if api == :chunk
+            create_chunk(timekey: t, tag: 'fluentd.test.output', variables: v)
+          else
+            create_metadata(timekey: t, tag: 'fluentd.test.output', variables: v)
+          end
+      assert_equal "/mypath/2016/04/11/20-30/fluentd.test.output/output/test/value1/value2/tail", @i.extract_placeholders(tmpl, c)
+    end
+
     data(:new_api => :chunk,
          :old_api => :metadata)
     test '#extract_placeholders removes out-of-range tag part and unknown variable placeholders' do |api|
       @i.configure(config_element('ROOT', '', {}, [config_element('buffer', 'time,tag,key1,key2', {'timekey' => 60*30, 'timekey_zone' => "+0900"})]))
       assert @i.chunk_key_time
       assert @i.chunk_key_tag
       assert_equal ['key1','key2'], @i.chunk_keys
-      tmpl = "/mypath/%Y/%m/%d/%H-%M/${tag}/${tag[3]}/${tag[4]}/${key3}/${key4}/tail"
+      tmpl = "/mypath/%Y/%m/%d/%H-%M/${tag}/${tag[3]}/${tag[-4]}/${key3}/${key4}/tail"
       t = event_time('2016-04-11 20:30:00 +0900')
       v = {key1: "value1", key2: "value2"}
       c = if api == :chunk
@@ -483,6 +501,9 @@ def waiting(seconds)
         assert_nothing_raised do
           @i.placeholder_validate!(:path, "/my/path/${tag}/file.${tag[2]}.log")
         end
+        assert_nothing_raised do
+          @i.placeholder_validate!(:path, "/my/path/${tag}/file.${tag[-1]}.log")
+        end
       end
 
       test 'raises configuration error for a template when variable key placeholders exist but chunk keys are missing' do