Some fixes and refactors for code related to forks.

app/helpers/commits_helper.rb

@@ -124,6 +124,8 @@ module CommitsHelper
   end
 
   def revert_commit_link(show_modal_condition, continue_to_path)
+    return unless current_user
+
     if show_modal_condition
       link_to('Revert', '#modal-revert-commit',
         'data-target' => '#modal-revert-commit',

app/models/repository.rb

@@ -628,8 +628,6 @@ class Repository
     args          = [commit.id, source_sha]
     args          << { mainline: 1 } if commit.merge_commit?
 
-    # Temporary branch exists and contains the revert commit
-    return true if (base_branch != target_branch) && find_branch(target_branch)
     return false unless diff_exists?(source_sha, commit.id)
 
     revert_index = rugged.revert_commit(*args)

app/services/commits/revert_service.rb

@@ -14,9 +14,7 @@ module Commits
       if commit
         success
       else
-        error("Sorry, we cannot revert this #{params[:revert_type_title]} automatically.
-              It may have already been reverted, or a more recent commit may
-              have updated some of its content.")
+        custom_error
       end
     rescue Repository::CommitError, Gitlab::Git::Repository::InvalidBlobName, GitHooksService::PreReceiveError, ValidationError => ex
       error(ex.message)
@@ -24,7 +22,11 @@ module Commits
 
     def commit
       if @create_merge_request
-        repository.revert(current_user, @commit, @target_branch, @commit.revert_branch_name)
+        # Temporary branch exists and contains the revert commit
+        return true if repository.find_branch(@commit.revert_branch_name)
+        return false unless create_target_branch
+
+        repository.revert(current_user, @commit, @commit.revert_branch_name)
       else
         repository.revert(current_user, @commit, @target_branch)
       end
@@ -32,6 +34,25 @@ module Commits
 
     private
 
+    def custom_error
+      if @branch_error_msg
+        error("There was an error creating the source branch: #{@branch_error_msg}")
+      else
+        error("Sorry, we cannot revert this #{params[:revert_type_title]} automatically.
+              It may have already been reverted, or a more recent commit may
+              have updated some of its content.")
+      end
+    end
+
+    def create_target_branch
+      result = CreateBranchService.new(@project, current_user)
+                                  .execute(@commit.revert_branch_name, @target_branch, source_project: @source_project)
+
+      @branch_error_msg = result[:message]
+
+      result[:status] != :error
+    end
+
     def raise_error(message)
       raise ValidationError.new(message)
     end