Fix expanding a collapsed diff when converting a symlink to a regular file

In this case comparing old_path and new_path is not enough because there
are two entires that match.

app/controllers/concerns/diff_for_path.rb

@@ -3,7 +3,7 @@ module DiffForPath
 
   def render_diff_for_path(diffs)
     diff_file = diffs.diff_files.find do |diff|
-      diff.old_path == params[:old_path] && diff.new_path == params[:new_path]
+      diff.file_identifier == params[:file_identifier]
     end
 
     return render_404 unless diff_file

app/views/projects/diffs/_content.html.haml

@@ -9,7 +9,7 @@
     - if !project.repository.diffable?(blob)
       .nothing-here-block This diff was suppressed by a .gitattributes entry.
     - elsif diff_file.collapsed?
-      - url = url_for(params.merge(action: :diff_for_path, old_path: diff_file.old_path, new_path: diff_file.new_path))
+      - url = url_for(params.merge(action: :diff_for_path, old_path: diff_file.old_path, new_path: diff_file.new_path, file_identifier: diff_file.file_identifier))
       .nothing-here-block.diff-collapsed{data: { diff_for_path: url } }
         This diff is collapsed.
         %a.click-to-expand

changelogs/unreleased/adam-fix-collapsed-diff-symlink-file-conversion.yml

+---
+title: Fix expanding a collapsed diff when converting a symlink to a regular file
+merge_request: 6953
+author:

lib/gitlab/diff/file.rb

@@ -126,7 +126,7 @@ module Gitlab
         repository.blob_at(commit.id, file_path)
       end
 
-      def cache_key
+      def file_identifier
         "#{file_path}-#{new_file}-#{deleted_file}-#{renamed_file}"
       end
     end

lib/gitlab/diff/file_collection/merge_request_diff.rb

@@ -39,7 +39,7 @@ module Gitlab
         # hashes that represent serialized diff lines.
         #
         def cache_highlight!(diff_file)
-          item_key = diff_file.cache_key
+          item_key = diff_file.file_identifier
 
           if highlight_cache[item_key]
             highlight_diff_file_from_cache!(diff_file, highlight_cache[item_key])

spec/features/expand_collapse_diffs_spec.rb

@@ -182,6 +182,20 @@ feature 'Expand and collapse diffs', js: true, feature: true do
         end
       end
     end
+
+    context 'expanding a diff when symlink was converted to a regular file' do
+      let(:branch) { 'symlink-expand-diff' }
+
+      it 'shows the content of the regular file' do
+        expect(page).to have_content('This diff is collapsed')
+        expect(page).to have_no_content('No longer a symlink')
+
+        find('.click-to-expand').click
+        wait_for_ajax
+
+        expect(page).to have_content('No longer a symlink')
+      end
+    end
   end
 
   context 'visiting a commit without collapsed diffs' do

spec/support/test_env.rb

@@ -23,6 +23,7 @@ module TestEnv
     'binary-encoding'                    => '7b1cf43',
     'gitattributes'                      => '5a62481',
     'expand-collapse-diffs'              => '4842455',
+    'symlink-expand-diff'                => '81e6355',
     'expand-collapse-files'              => '025db92',
     'expand-collapse-lines'              => '238e82d',
     'video'                              => '8879059',