Ensure that the spaces between frontmatter are kept

This allows the source-position data to be accurate

Changelog: fixed

lib/banzai/filter/front_matter_filter.rb

@@ -9,7 +9,10 @@ module Banzai
         html.sub(Gitlab::FrontMatter::PATTERN) do |_match|
           lang = $~[:lang].presence || lang_mapping[$~[:delim]]
 
-          ["```#{lang}:frontmatter", $~[:front_matter].strip!, "```", "\n"].join("\n")
+          before = $~[:before]
+          before = "\n#{before}" if $~[:encoding].presence
+
+          "#{before}```#{lang}:frontmatter\n#{$~[:front_matter]}```\n"
         end
       end
     end

lib/gitlab/front_matter.rb

@@ -11,12 +11,12 @@ module Gitlab
     DELIM = Regexp.union(DELIM_LANG.keys)
 
     PATTERN = %r{
-      \A(?:[^\r\n]*coding:[^\r\n]*\R)?        # optional encoding line
-      \s*
+      \A(?<encoding>[^\r\n]*coding:[^\r\n]*\R)? # optional encoding line
+      (?<before>\s*)
       ^(?<delim>#{DELIM})[ \t]*(?<lang>\S*)\R # opening front matter marker (optional language specifier)
       (?<front_matter>.*?)                    # front matter block content (not greedy)
       ^(\k<delim> | \.{3})                    # closing front matter marker
-      \s*
+      [^\S\r\n]*(\R|\z)
     }mx.freeze
   end
 end

lib/gitlab/wiki_pages/front_matter_parser.rb

@@ -109,7 +109,7 @@ module Gitlab
       end
 
       def parse_front_matter_block
-        wiki_content.match(Gitlab::FrontMatter::PATTERN) { |m| Block.new(*m.captures) } || Block.new
+        wiki_content.match(Gitlab::FrontMatter::PATTERN) { |m| Block.new(m[:delim], m[:lang], m[:front_matter]) } || Block.new
       end
 
       def strip_front_matter_block

spec/lib/banzai/filter/front_matter_filter_spec.rb

@@ -105,6 +105,56 @@ RSpec.describe Banzai::Filter::FrontMatterFilter do
     end
   end
 
+  context 'source position mapping' do
+    it 'keeps spaces before and after' do
+      content = <<~MD
+
+
+        ---
+
+        foo: :foo_symbol
+
+        ---  
+
+
+        # Header
+      MD
+
+      output = filter(content)
+
+      expect(output).to eq <<~MD
+
+
+        ```yaml:frontmatter
+
+        foo: :foo_symbol
+
+        ```
+
+
+        # Header
+      MD
+    end
+
+    it 'keeps an empty line in place of the encoding' do
+      content = <<~MD
+        # encoding: UTF-8
+        ---
+        foo: :foo_symbol
+        ---
+      MD
+
+      output = filter(content)
+
+      expect(output).to eq <<~MD
+
+        ```yaml:frontmatter
+        foo: :foo_symbol
+        ```
+      MD
+    end
+  end
+
   context 'on content without front matter' do
     it 'returns the content unmodified' do
       content = <<~MD
@@ -119,7 +169,7 @@ RSpec.describe Banzai::Filter::FrontMatterFilter do
 
   context 'on front matter without content' do
     it 'converts YAML front matter to a fenced code block' do
-      content = <<~MD
+      content = <<~MD.rstrip
         ---
         foo: :foo_symbol
         bar: :bar_symbol
@@ -134,7 +184,6 @@ RSpec.describe Banzai::Filter::FrontMatterFilter do
           foo: :foo_symbol
           bar: :bar_symbol
           ```
-
         MD
       end
     end