Ensure HasWiki constructs wikis with users as owners

This adds a concept of `default_actor` that `HasWiki` instances must
implement. This is implemented differently for projects and groups.
The contract is that `default_actor` must return a User with ownership
privileges over the object.

Wiki Owner differs from Owner of these models as wiki_owner must have
the type `User`, but `Project#owner` can be a `Group` for instance.
Groups can have many owners, so we select the first one, before
ascending the chain of parent groups.

This is vital since otherwise `HasWiki#wiki` will return wikis that
cannot save their own pages.

app/models/concerns/has_wiki.rb

@@ -17,7 +17,7 @@ module HasWiki
 
   def wiki
     strong_memoize(:wiki) do
-      Wiki.for_container(self, self.owner)
+      Wiki.for_container(self, self.default_owner)
     end
   end
 

app/models/group.rb

@@ -563,6 +563,10 @@ class Group < Namespace
     all_projects.update_all(shared_runners_enabled: false)
   end
 
+  def default_owner
+    owners.first || parent&.default_owner || owner
+  end
+
   private
 
   def update_two_factor_requirement

app/models/project.rb

@@ -1395,6 +1395,16 @@ class Project < ApplicationRecord
     group || namespace.try(:owner)
   end
 
+  def default_owner
+    obj = owner
+
+    if obj.respond_to?(:default_owner)
+      obj.default_owner
+    else
+      obj
+    end
+  end
+
   def to_ability_name
     model_name.singular
   end

app/models/wiki.rb

@@ -35,6 +35,7 @@ class Wiki
   def initialize(container, user = nil)
     @container = container
     @user = user
+    raise ArgumentError, "user must be a User, got #{user.class}" if user && !user.is_a?(User)
   end
 
   def path

spec/models/group_spec.rb

@@ -1541,4 +1541,48 @@ RSpec.describe Group do
       end
     end
   end
+
+  describe '#default_owner' do
+    let(:group) { build(:group) }
+
+    context 'the group has owners' do
+      before do
+        group.add_owner(create(:user))
+        group.add_owner(create(:user))
+      end
+
+      it 'is the first owner' do
+        expect(group.default_owner)
+          .to eq(group.owners.first)
+          .and be_a(User)
+      end
+    end
+
+    context 'the group has a parent' do
+      let(:parent) { build(:group) }
+
+      before do
+        group.parent = parent
+        parent.add_owner(create(:user))
+      end
+
+      it 'is the first owner of the parent' do
+        expect(group.default_owner)
+          .to eq(parent.default_owner)
+          .and be_a(User)
+      end
+    end
+
+    context 'we fallback to group.owner' do
+      before do
+        group.owner = build(:user)
+      end
+
+      it 'is the group.owner' do
+        expect(group.default_owner)
+          .to eq(group.owner)
+          .and be_a(User)
+      end
+    end
+  end
 end

spec/models/project_spec.rb

@@ -1084,6 +1084,30 @@ RSpec.describe Project do
     end
   end
 
+  describe '#default_owner' do
+    let_it_be(:owner)     { create(:user) }
+    let_it_be(:namespace) { create(:namespace, owner: owner) }
+
+    context 'the project does not have a group' do
+      let(:project) { build(:project, namespace: namespace) }
+
+      it 'is the namespace owner' do
+        expect(project.default_owner).to eq(owner)
+      end
+    end
+
+    context 'the project is in a group' do
+      let(:group)   { build(:group) }
+      let(:project) { build(:project, group: group, namespace: namespace) }
+
+      it 'is the group owner' do
+        allow(group).to receive(:default_owner).and_return(Object.new)
+
+        expect(project.default_owner).to eq(group.default_owner)
+      end
+    end
+  end
+
   describe '#external_wiki' do
     let(:project) { create(:project) }