Merge branch '24059-round-robin-repository-storage' into 'master'

Resolve "Introduce round-robin project creation to spread load over multiple shards"

## What does this MR do?

Allow multiple shards to be enabled in the admin settings page, balancing project creation across all enabled shards.

## Are there points in the code the reviewer needs to double check?

* `f.select ..., multiple: true` isn't the most beautiful UI in the world, but switching to `collection_check_boxes` (or a facsimile thereof) isn't trivial
* Should `pick_repository_storage` be a method of `ApplicationSetting`, or `Project`? It's going to accrete logic over time so perhaps it should be its own class already?
* This is written to avoid the need for a database migration, so it is`serialize :repository_storage` without `, Array`. This is tested, but alternatives include:
  * Add a database migration
  * Write a custom Coder that will accept a String or Array in `load` and always `dump an Array.

## Why was this MR needed?

## Screenshots (if relevant)

![Screen_Shot_2016-11-03_at_14.42.41](/uploads/7de15d6c1b3fa60bb7a34d6a7d9f00ce/Screen_Shot_2016-11-03_at_14.42.41.png)

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [X] Added for this feature/bug
  - [ ] 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?

Closes #24059

See merge request !7273

3 files changed, 6 insertions, 1 deletions

diff --git a/doc/administration/img/repository_storages_admin_ui.png b/doc/administration/img/repository_storages_admin_ui.png
index 599350bc098..6481baca1ad 100644
--- a/doc/administration/img/repository_storages_admin_ui.png
+++ b/doc/administration/img/repository_storages_admin_ui.png
diff --git a/doc/administration/repository_storages.md b/doc/administration/repository_storages.md
index 55b054fc1a4..ab70557b69a 100644
--- a/doc/administration/repository_storages.md
+++ b/doc/administration/repository_storages.md
@@ -91,6 +91,9 @@ be stored via the **Application Settings** in the Admin area.
 
 ![Choose repository storage path in Admin area](img/repository_storages_admin_ui.png)
 
+Beginning with GitLab 8.13.4, multiple paths can be chosen. New projects will be
+randomly placed on one of the selected paths.
+
 [ce-4578]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4578
 [restart gitlab]: restart_gitlab.md#installations-from-source
 [reconfigure gitlab]: restart_gitlab.md#omnibus-gitlab-reconfigure
diff --git a/doc/api/settings.md b/doc/api/settings.md
index f7ad3b4cc8e..218546aafea 100644
--- a/doc/api/settings.md
+++ b/doc/api/settings.md
@@ -42,6 +42,7 @@ Example response:
    "sign_in_text" : null,
    "container_registry_token_expire_delay": 5,
    "repository_storage": "default",
+   "repository_storages": ["default"],
    "koding_enabled": false,
    "koding_url": null
 }
@@ -73,7 +74,8 @@ PUT /application/settings
 | `user_oauth_applications` | boolean | no | Allow users to register any application to use GitLab as an OAuth provider |
 | `after_sign_out_path` | string | no | Where to redirect users after logout |
 | `container_registry_token_expire_delay` | integer | no | Container Registry token duration in minutes |
-| `repository_storage` | string | no | Storage path for new projects. The value should be the name of one of the repository storage paths defined in your gitlab.yml |
+| `repository_storages` | array of strings | no | A list of names of enabled storage paths, taken from `gitlab.yml`. New projects will be created in one of these stores, chosen at random. |
+| `repository_storage` | string | no | The first entry in `repository_storages`. Deprecated, but retained for compatibility reasons |
 | `enabled_git_access_protocol` | string | no | Enabled protocols for Git access. Allowed values are: `ssh`, `http`, and `nil` to allow both protocols. |
 | `koding_enabled` | boolean | no | Enable Koding integration. Default is `false`. |
 | `koding_url` | string | yes (if `koding_enabled` is `true`) |  The Koding instance URL for integration. |