Default target branch to patch-n when editing file in protected branch

app/controllers/projects/blob_controller.rb

@@ -162,12 +162,20 @@ class Projects::BlobController < Projects::ApplicationController
   end
 
   def sanitized_new_branch_name
-    @new_branch ||= sanitize(strip_tags(params[:new_branch]))
+    sanitize(strip_tags(params[:new_branch]))
   end
 
   def editor_variables
     @current_branch = @ref
-    @new_branch = params[:new_branch].present? ? sanitized_new_branch_name : @ref
+
+    @new_branch =
+      if params[:new_branch].present?
+        sanitized_new_branch_name
+      elsif ::Gitlab::GitAccess.new(current_user, @project).can_push_to_branch?(@ref)
+        @ref
+      else
+        @repository.next_patch_branch
+      end
 
     @file_path =
       if action_name.to_s == 'create'

app/helpers/tree_helper.rb

@@ -48,10 +48,17 @@ module TreeHelper
 
   def allowed_tree_edit?(project = nil, ref = nil)
     project ||= @project
-    ref ||= @ref
-    return false unless project.repository.branch_names.include?(ref)
+    can?(current_user, :push_code, project)
+  end
 
-    ::Gitlab::GitAccess.new(current_user, project).can_push_to_branch?(ref)
+  def tree_edit_branch(project = @project, ref = @ref)
+    if allowed_tree_edit?
+      if can_push_branch?(project, ref)
+        ref
+      else
+        project.repository.next_patch_branch
+      end
+    end
   end
 
   def can_delete_or_replace?(blob)

app/models/repository.rb

@@ -329,6 +329,17 @@ class Repository
     commit(sha)
   end
 
+  def next_patch_branch
+    patch_branch_ids = self.branch_names.map do |n|
+      result = n.match(/\Apatch-([0-9]+)\z/)
+      result[1].to_i if result
+    end.compact
+
+    highest_patch_branch_id = patch_branch_ids.max || 0
+
+    "patch-#{highest_patch_branch_id + 1}"
+  end
+
   # Remove archives older than 2 hours
   def branches_sorted_by(value)
     case value

app/views/shared/_new_commit_form.html.haml

@@ -4,7 +4,7 @@
   .form-group.branch
     = label_tag 'new_branch', 'Target branch', class: 'control-label'
     .col-sm-10
-      = text_field_tag 'new_branch', @new_branch || @ref, required: true, class: "form-control js-new-branch"
+      = text_field_tag 'new_branch', @new_branch || tree_edit_branch, required: true, class: "form-control js-new-branch"
 
   .form-group.js-create-merge-request-form-group
     .col-sm-offset-2.col-sm-10