Don't allow resolving invalid conflicts

app/controllers/projects/merge_requests_controller.rb

@@ -132,15 +132,13 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def conflicts
-    return render_404 unless @merge_request.cannot_be_merged?
-
     respond_to do |format|
       format.html { define_discussion_vars }
 
       format.json do
-        begin
-          render json: Gitlab::Conflict::FileCollection.new(@merge_request)
-        rescue Gitlab::Conflict::Parser::ParserError, Gitlab::Conflict::FileCollection::ConflictSideMissing
+        if @merge_request.can_resolve_conflicts_in_ui?
+          render json: @merge_request.conflicts
+        else
           render json: {
             message: 'Unable to resolve conflicts in the web interface for this merge request',
             type: 'error'
@@ -452,7 +450,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       noteable_id: @merge_request.id
     }
 
-    @use_legacy_diff_notes = !@merge_request.support_new_diff_notes?
+    @use_legacy_diff_notes = !@merge_request.has_complete_diff_refs?
     @grouped_diff_discussions = @merge_request.notes.inc_author_project_award_emoji.grouped_diff_discussions
 
     Banzai::NoteRenderer.render(

app/models/diff_note.rb

@@ -75,7 +75,7 @@ class DiffNote < Note
   private
 
   def supported?
-    !self.for_merge_request? || self.noteable.support_new_diff_notes?
+    !self.for_merge_request? || self.noteable.has_complete_diff_refs?
   end
 
   def noteable_diff_refs

app/models/merge_request.rb

@@ -204,7 +204,7 @@ class MergeRequest < ActiveRecord::Base
 
   def diff_start_commit
     if persisted?
-      merge_request_diff.start_commit || target_branch_head
+      merge_request_diff.start_commit
     else
       target_branch_head
     end
@@ -212,7 +212,7 @@ class MergeRequest < ActiveRecord::Base
 
   def diff_head_commit
     if persisted?
-      merge_request_diff.head_commit || source_branch_head
+      merge_request_diff.head_commit
     else
       source_branch_head
     end
@@ -682,12 +682,12 @@ class MergeRequest < ActiveRecord::Base
     merge_commit
   end
 
-  def support_new_diff_notes?
+  def has_complete_diff_refs?
     diff_sha_refs && diff_sha_refs.complete?
   end
 
   def update_diff_notes_positions(old_diff_refs:, new_diff_refs:)
-    return unless support_new_diff_notes?
+    return unless has_complete_diff_refs?
     return if new_diff_refs == old_diff_refs
 
     active_diff_notes = self.notes.diff_notes.select do |note|
@@ -715,4 +715,19 @@ class MergeRequest < ActiveRecord::Base
   def keep_around_commit
     project.repository.keep_around(self.merge_commit_sha)
   end
+
+  def conflicts
+    @conflicts ||= Gitlab::Conflict::FileCollection.new(self)
+  end
+
+  def can_resolve_conflicts_in_ui?
+    return false unless cannot_be_merged?
+    return false unless has_complete_diff_refs?
+
+    begin
+      conflicts.files.each(&:lines)
+    rescue Gitlab::Conflict::Parser::ParserError, Gitlab::Conflict::FileCollection::ConflictSideMissing
+      false
+    end
+  end
 end

app/views/projects/merge_requests/widget/open/_conflicts.html.haml

@@ -4,9 +4,9 @@
 
 %p
   Please
-  = link_to conflicts_namespace_project_merge_request_path(@project.namespace, @project, @merge_request) do
-    resolve these conflicts
-  or
+  - if @merge_request.can_resolve_conflicts_in_ui?
+    = link_to "resolve these conflicts", conflicts_namespace_project_merge_request_path(@project.namespace, @project, @merge_request)
+    or
   - if @merge_request.can_be_merged_via_command_line_by?(current_user)
     #{link_to "merge this request manually", "#modal_merge_info", class: "how_to_merge_link vlink", "data-toggle" => "modal"}.
   - else

lib/gitlab/conflict/file.rb

@@ -69,9 +69,9 @@ module Gitlab
 
         lines.each do |line|
           if line.type == 'old'
-            line.rich_text = their_highlight[line.old_line - 1].html_safe
+            line.rich_text = their_highlight[line.old_line - 1].try(:html_safe)
           else
-            line.rich_text = our_highlight[line.new_line - 1].html_safe
+            line.rich_text = our_highlight[line.new_line - 1].try(:html_safe)
           end
         end
       end