Avoind unnecesary `force_encoding` operations

They're costly. This will also avoid some edge cases where
charlock_holmes assigns a weird encoding to a perfectly valid UTF-8
string.

lib/gitlab/encoding_helper.rb

@@ -14,9 +14,9 @@ module Gitlab
     ENCODING_CONFIDENCE_THRESHOLD = 50
 
     def encode!(message)
-      return nil unless message.respond_to? :force_encoding
+      return nil unless message.respond_to?(:force_encoding)
+      return message if message.encoding == Encoding::UTF_8 && message.valid_encoding?
 
-      # if message is utf-8 encoding, just return it
       message.force_encoding("UTF-8")
       return message if message.valid_encoding?
 
@@ -50,6 +50,9 @@ module Gitlab
     end
 
     def encode_utf8(message)
+      return nil unless message.is_a?(String)
+      return message if message.encoding == Encoding::UTF_8 && message.valid_encoding?
+
       detect = CharlockHolmes::EncodingDetector.detect(message)
       if detect && detect[:encoding]
         begin

spec/lib/gitlab/encoding_helper_spec.rb

@@ -6,6 +6,9 @@ describe Gitlab::EncodingHelper do
 
   describe '#encode!' do
     [
+      ["nil", nil, nil],
+      ["empty string", "".encode("ASCII-8BIT"), "".encode("UTF-8")],
+      ["invalid utf-8 encoded string", "my bad string\xE5".force_encoding("UTF-8"), "my bad string"],
       [
         'leaves ascii only string as is',
         'ascii only string',
@@ -81,6 +84,9 @@ describe Gitlab::EncodingHelper do
 
   describe '#encode_utf8' do
     [
+      ["nil", nil, nil],
+      ["empty string", "".encode("ASCII-8BIT"), "".encode("UTF-8")],
+      ["invalid utf-8 encoded string", "my bad string\xE5".force_encoding("UTF-8"), "my bad stringå"],
       [
         "encodes valid utf8 encoded string to utf8",
         "λ, λ, λ".encode("UTF-8"),
@@ -95,12 +101,18 @@ describe Gitlab::EncodingHelper do
         "encodes valid ISO-8859-1 encoded string to utf8",
         "Rüby ist eine Programmiersprache. Wir verlängern den text damit ICU die Sprache erkennen kann.".encode("ISO-8859-1", "UTF-8"),
         "Rüby ist eine Programmiersprache. Wir verlängern den text damit ICU die Sprache erkennen kann.".encode("UTF-8")
+      ],
+      [
+        # Test case from https://gitlab.com/gitlab-org/gitlab-ce/issues/39227
+        "Equifax branch name",
+        "refs/heads/Equifax".encode("UTF-8"),
+        "refs/heads/Equifax".encode("UTF-8")
       ]
     ].each do |description, test_string, xpect|
       it description do
-        r = ext_class.encode_utf8(test_string.force_encoding('UTF-8'))
+        r = ext_class.encode_utf8(test_string)
         expect(r).to eq(xpect)
-        expect(r.encoding.name).to eq('UTF-8')
+        expect(r.encoding.name).to eq('UTF-8') if xpect
       end
     end