Improve multiple branch push performance by memoizing permission checking

If you attempt to push thousands of branches at once, the 60-second timeout
will occur because GitAccess checking does a lot of work to check if the
user has permission to push to a branch. This changes does two things:

1. Instead of making 1 DB query per branch push, use a memoized list of protected branches to check
2. Memoize what permissions the user has to perform on this project

On a test of 10,000 branch pushes, this prevents gitlab-shell from hitting the 60-second
timeout.

Closes #17225

CHANGELOG

@@ -4,6 +4,7 @@ v 8.8.0 (unreleased)
   - Assign labels and milestone to target project when moving issue. !3934 (Long Nguyen)
   - Project#open_branches has been cleaned up and no longer loads entire records into memory.
   - Escape HTML in commit titles in system note messages
+  - Improve multiple branch push performance by memoizing permission checking
   - Log to application.log when an admin starts and stops impersonating a user
   - Updated gitlab_git to 10.1.0
   - GitAccess#protected_tag? no longer loads all tags just to check if a single one exists

app/models/project.rb

@@ -767,7 +767,7 @@ class Project < ActiveRecord::Base
 
   # Check if current branch name is marked as protected in the system
   def protected_branch?(branch_name)
-    protected_branches.where(name: branch_name).any?
+    protected_branch_names.include?(branch_name)
   end
 
   def developers_can_push_to_protected_branch?(branch_name)

lib/gitlab/git_access.rb

@@ -122,6 +122,11 @@ module Gitlab
       build_status_object(true)
     end
 
+    def can_user_do_action?(action)
+      @permission_cache ||= {}
+      @permission_cache[action] ||= user.can?(action, project)
+    end
+
     def change_access_check(change)
       oldrev, newrev, ref = change.split(' ')
 
@@ -135,7 +140,7 @@ module Gitlab
           :push_code
         end
 
-      unless user.can?(action, project)
+      unless can_user_do_action?(action)
         status =
           case action
           when :force_push_code_to_protected_branches