Merge branch 'feature/precalculate-authorized-projects' into 'master'

Precalculate authorized projects in database

## What does this MR do?
It caches user's authorized projects in database instead of using multiple unions, which should simplify and speed-up things since this operation (getting authorized projects) is used a lot.

## Are there points in the code the reviewer needs to double check?
Did we miss a scenario where we need to refresh the list of projects?

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#23150 

See merge request !6839

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/gitlab/database.rb b/lib/gitlab/database.rb
index 55b8f888d53..2d5c9232425 100644
--- a/lib/gitlab/database.rb
+++ b/lib/gitlab/database.rb
@@ -35,6 +35,13 @@ module Gitlab
       order
     end
 
+    def self.serialized_transaction
+      opts = {}
+      opts[:isolation] = :serializable unless Rails.env.test? && connection.transaction_open?
+
+      connection.transaction(opts) { yield }
+    end
+
     def self.random
       Gitlab::Database.postgresql? ? "RANDOM()" : "RAND()"
     end