Improve Features API and its docs and add a Changelog item

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/33929-allow-to-enable-perf-bar-for-a-group.yml

+---
+title: Allow to enable the performance bar per user or Feature group
+merge_request: 12362
+author:

doc/administration/monitoring/performance/performance_bar.md

 # Performance Bar
 >**Note:**
-Available since GitLab 9.4.
+Available since GitLab 9.4. For installations from source you'll have to
+configure it yourself.
 A Performance Bar can be displayed, to dig into the performance of a page. When
 activated, it looks as follows:
@@ -22,8 +23,9 @@ It allows you to:
 ## Enable the Performance Bar
-By default, the Performance Bar is disabled. You can enable it either for a
-given feature group or user.
+By default, the Performance Bar is disabled. You can enable it for a group
+and/or users. Note that it's possible to enable it for a group and for
+individual users at the same time.
 1. Edit `/etc/gitlab/gitlab.rb`
 1. Find the following line, and set it to the group's **full path** that should
@@ -47,7 +49,10 @@ The `performance_team` feature group maps to the group specified by the
 curl --data "feature_group=performance_team" --data "value=true" --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/features/performance_bar
 ```
-### Enable for a specific user
+### Enable for specific users
+
+It's possible to enable the Performance Bar for specific users in addition to a
+group, or even instead of a group:
 ```
 curl --data "user=my_username" --data "value=true" --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/features/performance_bar

doc/api/features.md

@@ -61,8 +61,8 @@ POST /features/:name
 | `feature_group` | string | no | A Feature group name |
 | `user` | string | no | A GitLab username |
-Note that `feature_group` and `user` are mutually exclusive, with
-`feature_group` taking priority.
+Note that you can enable or disable a feature for both a `feature_group` and a
+`user` with a single API call.
 ```bash
 curl --data "value=30" --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/features/new_library

doc/development/feature_flags.md

@@ -13,7 +13,8 @@ During runtime you can set the values for the gates via the
 Starting from GitLab 9.4 we support feature groups via
 [Flipper groups](https://github.com/jnunemaker/flipper/blob/v0.10.2/docs/Gates.md#2-group).
-Feature groups must be defined statically in `lib/feature.rb` (in the `.register_feature_groups` method), but their implementation can obviously be
+Feature groups must be defined statically in `lib/feature.rb` (in the
+`.register_feature_groups` method), but their implementation can obviously be
 dynamic (querying the DB etc.). You can see how the `performance_team` feature
 group for a concrete example.

lib/api/features.rb

@@ -14,14 +14,12 @@ module API
         end
       end
-      def gate_target(params)
-        if params[:feature_group]
-          Feature.group(params[:feature_group])
-        elsif params[:user]
-          User.find_by_username(params[:user])
-        else
-          gate_value(params)
-        end
+      def gate_targets(params)
+        targets = []
+        targets << Feature.group(params[:feature_group]) if params[:feature_group]
+        targets << User.find_by_username(params[:user]) if params[:user]
+
+        targets
       end
     end
@@ -42,18 +40,25 @@ module API
         requires :value, type: String, desc: '`true` or `false` to enable/disable, an integer for percentage of time'
         optional :feature_group, type: String, desc: 'A Feature group name'
         optional :user, type: String, desc: 'A GitLab username'
-        mutually_exclusive :feature_group, :user
       end
       post ':name' do
         feature = Feature.get(params[:name])
-        target = gate_target(params)
+        targets = gate_targets(params)
         value = gate_value(params)
         case value
         when true
-          feature.enable(target)
+          if targets.present?
+            targets.each { |target| feature.enable(target) }
+          else
+            feature.enable
+          end
         when false
-          feature.disable(target)
+          if targets.present?
+            targets.each { |target| feature.disable(target) }
+          else
+            feature.disable
+          end
         else
           feature.enable_percentage_of_time(value)
         end

spec/requests/api/features_spec.rb

@@ -113,6 +113,20 @@ describe API::Features do
               { 'key' => 'actors', 'value' => ["User:#{user.id}"] }
             ])
         end
+
+        it 'creates an enabled feature for the given user and feature group when passed user=username and feature_group=perf_team' do
+          post api("/features/#{feature_name}", admin), value: 'true', user: user.username, feature_group: 'perf_team'
+
+          expect(response).to have_http_status(201)
+          expect(json_response).to eq(
+            'name' => 'my_feature',
+            'state' => 'conditional',
+            'gates' => [
+              { 'key' => 'boolean', 'value' => false },
+              { 'key' => 'groups', 'value' => ['perf_team'] },
+              { 'key' => 'actors', 'value' => ["User:#{user.id}"] }
+            ])
+        end
       end
       it 'creates a feature with the given percentage if passed an integer' do