Merge branch 'master' into 22643-manual-job-page

* master: (200 commits)
  Fix issue boards scroll config.
  Added multi editor setting on the profile preferences page
  fix missing import of timeWeek which would cause errors in prometheus graphs with deployments
  Remove downcase from special path helper
  Rename db:seed_fu-{pg,mysql} to gitlab:setup-{pg,mysql}
  Fix error when viewing diffs without blobs
  Moves prettier to dev dependency
  Eager load event target authors whenever possible
  Do not run ee_compat_check on security branches
  Include integration tests in CE/EE testing documentation
  41054-Disallow creation of new Kubernetes integrations
  Resolve "Resizable file list and commit panel"
  Make tooltip placement bottom by default as per design guidelines
  Fix groups list icon, timestamp alignment and row height
  Avoid leaving a push event empty if payload cannot be created
  Move git operations for UpdateRemoteMirrorService into Gitlab::Git
  Move delete_remote_branches from Gitlab::Shell to Gitlab::Git::Repository
  Move push_remote_branches from Gitlab::Shell to Gitlab::Git::Repository
  Update Kubernetes service documentation
  fix issue #37843
  ...

3 files changed, 150 insertions, 0 deletions

diff --git a/doc/development/README.md b/doc/development/README.md
index b624aa37c70..12cca9f84b7 100644
--- a/doc/development/README.md
+++ b/doc/development/README.md
@@ -27,6 +27,7 @@ comments: false
 
 ## Backend guides
 
+- [GitLab utilities](utilities.md)
 - [API styleguide](api_styleguide.md) Use this styleguide if you are
   contributing to the API.
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers
diff --git a/doc/development/utilities.md b/doc/development/utilities.md
new file mode 100644
index 00000000000..951c3ef85ce
--- /dev/null
+++ b/doc/development/utilities.md
@@ -0,0 +1,92 @@
+# GitLab utilities
+
+We developed a number of utilities to ease development.
+
+## [`MergeHash`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/utils/merge_hash.rb)
+
+* Deep merges an array of hashes:
+
+    ``` ruby
+    Gitlab::Utils::MergeHash.merge(
+      [{ hello: ["world"] },
+       { hello: "Everyone" },
+       { hello: { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] } },
+        "Goodbye", "Hallo"]
+    )
+    ```
+
+    Gives:
+
+    ``` ruby
+    [
+      {
+        hello:
+          [
+            "world",
+            "Everyone",
+            { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] }
+          ]
+      },
+      "Goodbye"
+    ]
+    ```
+
+* Extracts all keys and values from a hash into an array:
+
+    ``` ruby
+    Gitlab::Utils::MergeHash.crush(
+      { hello: "world", this: { crushes: ["an entire", "hash"] } }
+    )
+    ```
+
+    Gives:
+
+    ``` ruby
+    [:hello, "world", :this, :crushes, "an entire", "hash"]
+    ```
+
+## [`StrongMemoize`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/utils/strong_memoize.rb)
+
+* Memoize the value even if it is `nil` or `false`.
+
+    We often do `@value ||= compute`, however this doesn't work well if
+    `compute` might eventually give `nil` and we don't want to compute again.
+    Instead we could use `defined?` to check if the value is set or not.
+    However it's tedious to write such pattern, and `StrongMemoize` would
+    help us use such pattern.
+
+    Instead of writing patterns like this:
+
+    ``` ruby
+    class Find
+      def result
+        return @result if defined?(@result)
+
+        @result = search
+      end
+    end
+    ```
+
+    We could write it like:
+
+    ``` ruby
+    class Find
+      include Gitlab::Utils::StrongMemoize
+
+      def result
+        strong_memoize(:result) do
+          search
+        end
+      end
+    end
+    ```
+
+* Clear memoization
+
+    ``` ruby
+    class Find
+      include Gitlab::Utils::StrongMemoize
+    end
+
+    Find.new.clear_memoization(:result)
+    ```
diff --git a/doc/development/what_requires_downtime.md b/doc/development/what_requires_downtime.md
index 05e0a64af18..9d0c62ecc35 100644
--- a/doc/development/what_requires_downtime.md
+++ b/doc/development/what_requires_downtime.md
@@ -195,6 +195,63 @@ end
 
 And that's it, we're done!
 
+## Changing Column Types For Large Tables
+
+While `change_column_type_concurrently` can be used for changing the type of a
+column without downtime it doesn't work very well for large tables. Because all
+of the work happens in sequence the migration can take a very long time to
+complete, preventing a deployment from proceeding.
+`change_column_type_concurrently` can also produce a lot of pressure on the
+database due to it rapidly updating many rows in sequence.
+
+To reduce database pressure you should instead use
+`change_column_type_using_background_migration` when migrating a column in a
+large table (e.g. `issues`). This method works similar to
+`change_column_type_concurrently` but uses background migration to spread the
+work / load over a longer time period, without slowing down deployments.
+
+Usage of this method is fairly simple:
+
+```ruby
+class ExampleMigration < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  class Issue < ActiveRecord::Base
+    self.table_name = 'issues'
+
+    include EachBatch
+
+    def self.to_migrate
+      where('closed_at IS NOT NULL')
+    end
+  end
+
+  def up
+    change_column_type_using_background_migration(
+      Issue.to_migrate,
+      :closed_at,
+      :datetime_with_timezone
+    )
+  end
+
+  def down
+    change_column_type_using_background_migration(
+      Issue.to_migrate,
+      :closed_at,
+      :datetime
+    )
+  end
+end
+```
+
+This would change the type of `issues.closed_at` to `timestamp with time zone`.
+
+Keep in mind that the relation passed to
+`change_column_type_using_background_migration` _must_ include `EachBatch`,
+otherwise it will raise a `TypeError`.
+
 ## Adding Indexes
 
 Adding indexes is an expensive process that blocks INSERT and UPDATE queries for