Set the encoding in c'tor and explain why it's fine

lib/gitlab/ci/trace/stream.rb

@@ -14,7 +14,14 @@ module Gitlab
 
         def initialize
           @stream = yield
-          @stream.binmode if @stream
+          if @stream
+            @stream.binmode
+            # Ci::Ansi2html::Converter would read from @stream directly,
+            # using @stream.each_line to be specific. It's safe to set
+            # the encoding here because IO#seek(bytes) and IO#read(bytes)
+            # are not characters based, so encoding doesn't matter to them.
+            @stream.set_encoding(Encoding.default_external)
+          end
         end
 
         def valid?
@@ -56,14 +63,12 @@ module Gitlab
         end
 
         def html_with_state(state = nil)
-          set_encoding_for_ansi2html
           ::Ci::Ansi2html.convert(stream, state)
         end
 
         def html(last_lines: nil)
           text = raw(last_lines: last_lines)
           stream = StringIO.new(text)
-          set_encoding_for_ansi2html(stream)
           ::Ci::Ansi2html.convert(stream).html
         end
 
@@ -117,11 +122,6 @@ module Gitlab
 
           chunks.join.lines.last(last_lines).join
         end
-
-        def set_encoding_for_ansi2html(stream = @stream)
-          # Ci::Ansi2html::Converter would read from @stream directly
-          stream.set_encoding(Encoding.default_external)
-        end
       end
     end
   end

spec/lib/gitlab/ci/trace/stream_spec.rb

@@ -64,7 +64,7 @@ describe Gitlab::Ci::Trace::Stream do
 
         result = stream.html
 
-        expect(result.lines.first).to eq("ヾ(´༎ຶД༎ຶ`)ﾉ<br><span class=\"term-fg-green\">許功蓋</span><br>")
+        expect(result).to eq("ヾ(´༎ຶД༎ຶ`)ﾉ<br><span class=\"term-fg-green\">許功蓋</span><br>")
         expect(result.encoding).to eq(Encoding.default_external)
       end
     end