Allow resolving conflicts with non-ASCII chars

We wanted to check that the text could be encoded as JSON, because
conflict resolutions are passed back and forth in that format, so the
file itself must be UTF-8. However, all strings from the repository come
back without an encoding from Rugged, making them ASCII_8BIT.

We force to UTF-8, and reject if it's invalid. This still leaves the
problem of a file that 'looks like' UTF-8 (contains valid UTF-8 byte
sequences), but isn't. However:

1. If the conflicts contain the problem bytes, the user will see that
   the file isn't displayed correctly.
2. If the problem bytes are outside of the conflict area, then we will
   write back the same bytes when we resolve the conflicts, even though
   we though the encoding was UTF-8.

changelogs/unreleased/allow-resolving-conflicts-in-utf-8.yml

+---
+title: Fix conflict resolution when files contain valid UTF-8 characters
+merge_request:
+author:

lib/gitlab/conflict/parser.rb

@@ -15,11 +15,9 @@ module Gitlab
         raise UnmergeableFile if text.blank? # Typically a binary file
         raise UnmergeableFile if text.length > 200.kilobytes
 
-        begin
-          text.to_json
-        rescue Encoding::UndefinedConversionError
-          raise UnsupportedEncoding
-        end
+        text.force_encoding('UTF-8')
+
+        raise UnsupportedEncoding unless text.valid_encoding?
 
         line_obj_index = 0
         line_old = 1

spec/lib/gitlab/conflict/parser_spec.rb

@@ -120,44 +120,62 @@ CONFLICT
     end
 
     context 'when the file contents include conflict delimiters' do
-      it 'raises UnexpectedDelimiter when there is a non-start delimiter first' do
+      context 'when there is a non-start delimiter first' do
+        it 'raises UnexpectedDelimiter when there is a middle delimiter first' do
           expect { parse_text('=======') }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
+        it 'raises UnexpectedDelimiter when there is an end delimiter first' do
           expect { parse_text('>>>>>>> README.md') }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
+        it 'does not raise when there is an end delimiter for a different path first' do
           expect { parse_text('>>>>>>> some-other-path.md') }.
             not_to raise_error
         end
+      end
 
-      it 'raises UnexpectedDelimiter when a start delimiter is followed by a non-middle delimiter' do
-        start_text = "<<<<<<< README.md\n"
-        end_text = "\n=======\n>>>>>>> README.md"
+      context 'when a start delimiter is followed by a non-middle delimiter' do
+        let(:start_text) { "<<<<<<< README.md\n" }
+        let(:end_text) { "\n=======\n>>>>>>> README.md" }
 
+        it 'raises UnexpectedDelimiter when it is followed by an end delimiter' do
           expect { parse_text(start_text + '>>>>>>> README.md' + end_text) }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
+        it 'raises UnexpectedDelimiter when it is followed by another start delimiter' do
           expect { parse_text(start_text + start_text + end_text) }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
+        it 'does not raise when it is followed by a start delimiter for a different path' do
           expect { parse_text(start_text + '>>>>>>> some-other-path.md' + end_text) }.
             not_to raise_error
         end
+      end
 
-      it 'raises UnexpectedDelimiter when a middle delimiter is followed by a non-end delimiter' do
-        start_text = "<<<<<<< README.md\n=======\n"
-        end_text = "\n>>>>>>> README.md"
+      context 'when a middle delimiter is followed by a non-end delimiter' do
+        let(:start_text) { "<<<<<<< README.md\n=======\n" }
+        let(:end_text) { "\n>>>>>>> README.md" }
 
+        it 'raises UnexpectedDelimiter when it is followed by another middle delimiter' do
           expect { parse_text(start_text + '=======' + end_text) }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
+        it 'raises UnexpectedDelimiter when it is followed by a start delimiter' do
           expect { parse_text(start_text + start_text + end_text) }.
             to raise_error(Gitlab::Conflict::Parser::UnexpectedDelimiter)
+        end
 
-        expect { parse_text(start_text + '>>>>>>> some-other-path.md' + end_text) }.
+        it 'does not raise when it is followed by a start delimiter for another path' do
+          expect { parse_text(start_text + '<<<<<<< some-other-path.md' + end_text) }.
             not_to raise_error
         end
+      end
 
       it 'raises MissingEndDelimiter when there is no end delimiter at the end' do
         start_text = "<<<<<<< README.md\n=======\n"
@@ -184,10 +202,21 @@ CONFLICT
           to raise_error(Gitlab::Conflict::Parser::UnmergeableFile)
       end
 
-      it 'raises UnsupportedEncoding when the file contains non-UTF-8 characters' do
+      # All text from Rugged has an encoding of ASCII_8BIT, so force that in
+      # these strings.
+      context 'when the file contains UTF-8 characters' do
+        it 'does not raise' do
+          expect { parse_text("Espa\xC3\xB1a".force_encoding(Encoding::ASCII_8BIT)) }.
+            not_to raise_error
+        end
+      end
+
+      context 'when the file contains non-UTF-8 characters' do
+        it 'raises UnsupportedEncoding' do
           expect { parse_text("a\xC4\xFC".force_encoding(Encoding::ASCII_8BIT)) }.
             to raise_error(Gitlab::Conflict::Parser::UnsupportedEncoding)
         end
       end
     end
+  end
 end