Add a unique and not null constraint on the project_features.project_id column

This commit has two migrations:

1. The first prunes duplicate rows in the project_features table and leaves
   the row with the highest ID.  Since the behavior was indeterministic before
   and depended on which row the database decided to use, this change at least
   makes the permissions consistent. For example, in some cases, the Wiki may
   have been disabled but enabled in another entry.

2. The second adds a non-null constraint on the project_features.project_id
   column.

Closes #37882

Fixes a significant part of gitlab-com/migration#408.

We found that we were overcounting Wikis because of these duplicates.
On GitLab.com, there are 56 rows with duplicate entries by project_id, and 16,661 rows with NULL project_id values.

3 files changed, 67 insertions, 3 deletions

diff --git a/db/post_migrate/20180511174224_add_unique_constraint_to_project_features_project_id.rb b/db/post_migrate/20180511174224_add_unique_constraint_to_project_features_project_id.rb
new file mode 100644
index 00000000000..88a9f5f8256
--- /dev/null
+++ b/db/post_migrate/20180511174224_add_unique_constraint_to_project_features_project_id.rb
@@ -0,0 +1,43 @@
+class AddUniqueConstraintToProjectFeaturesProjectId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class ProjectFeature < ActiveRecord::Base
+    self.table_name = 'project_features'
+
+    include EachBatch
+  end
+
+  def up
+    remove_duplicates
+
+    add_concurrent_index :project_features, :project_id, unique: true, name: 'index_project_features_on_project_id_unique'
+    remove_concurrent_index_by_name :project_features, 'index_project_features_on_project_id'
+    rename_index :project_features, 'index_project_features_on_project_id_unique', 'index_project_features_on_project_id'
+  end
+
+  def down
+    rename_index :project_features, 'index_project_features_on_project_id', 'index_project_features_on_project_id_old'
+    add_concurrent_index :project_features, :project_id
+    remove_concurrent_index_by_name :project_features, 'index_project_features_on_project_id_old'
+  end
+
+  private
+
+  def remove_duplicates
+    features = ProjectFeature
+               .select('MAX(id) AS max, COUNT(id), project_id')
+               .group(:project_id)
+               .having('COUNT(id) > 1')
+
+    features.each do |feature|
+      ProjectFeature
+        .where(project_id: feature['project_id'])
+        .where('id <> ?', feature['max'])
+        .each_batch { |batch| batch.delete_all }
+    end
+  end
+end
diff --git a/db/post_migrate/20180512061621_add_not_null_constraint_to_project_features_project_id.rb b/db/post_migrate/20180512061621_add_not_null_constraint_to_project_features_project_id.rb
new file mode 100644
index 00000000000..5a6d6ff4a10
--- /dev/null
+++ b/db/post_migrate/20180512061621_add_not_null_constraint_to_project_features_project_id.rb
@@ -0,0 +1,21 @@
+class AddNotNullConstraintToProjectFeaturesProjectId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  class ProjectFeature < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'project_features'
+  end
+
+  def up
+    ProjectFeature.where(project_id: nil).delete_all
+
+    change_column_null :project_features, :project_id, false
+  end
+
+  def down
+    change_column_null :project_features, :project_id, true
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 4abca1789a0..ed29d202f91 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180511090724) do
+ActiveRecord::Schema.define(version: 20180512061621) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1494,7 +1494,7 @@ ActiveRecord::Schema.define(version: 20180511090724) do
   add_index "project_deploy_tokens", ["project_id", "deploy_token_id"], name: "index_project_deploy_tokens_on_project_id_and_deploy_token_id", unique: true, using: :btree
 
   create_table "project_features", force: :cascade do |t|
-    t.integer "project_id"
+    t.integer "project_id", null: false
     t.integer "merge_requests_access_level"
     t.integer "issues_access_level"
     t.integer "wiki_access_level"
@@ -1505,7 +1505,7 @@ ActiveRecord::Schema.define(version: 20180511090724) do
     t.integer "repository_access_level", default: 20, null: false
   end
 
-  add_index "project_features", ["project_id"], name: "index_project_features_on_project_id", using: :btree
+  add_index "project_features", ["project_id"], name: "index_project_features_on_project_id", unique: true, using: :btree
 
   create_table "project_group_links", force: :cascade do |t|
     t.integer "project_id", null: false