Show disabled fork button for user without enough permissions

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/259693

**Problem**

We currently hide both the fork button and the fork count on
project's page when the user cannot fork the project. It is not
aligned with other pages where the user can see the fork count number.

**Solution**

* Mark fork button as disabled
* Show tooltip with an explanation

Changelog: changed

app/helpers/projects_helper.rb

@@ -405,6 +405,16 @@ module ProjectsHelper
     project.path_with_namespace
   end
 
+  def fork_button_disabled_tooltip(project)
+    return unless current_user
+
+    if !current_user.can?(:fork_project, project)
+      s_("ProjectOverview|You don't have permission to fork this project")
+    elsif !current_user.can?(:create_fork)
+      s_('ProjectOverview|You have reached your project limit')
+    end
+  end
+
   private
 
   def tab_ability_map

app/views/projects/buttons/_fork.html.haml

 - unless @project.empty_repo?
-  - if current_user && can?(current_user, :fork_project, @project)
+  - if current_user
     .count-badge.btn-group
       - if current_user.already_forked?(@project) && current_user.manageable_namespaces.size < 2
         = link_to namespace_project_path(current_user, current_user.fork_of(@project)), title: s_('ProjectOverview|Go to your fork'), class: 'gl-button btn btn-default btn-sm has-tooltip fork-btn' do
           = sprite_icon('fork', css_class: 'icon')
           %span= s_('ProjectOverview|Fork')
       - else
-        - can_create_fork = current_user.can?(:create_fork)
-        - disabled_fork_tooltip = s_('ProjectOverview|You have reached your project limit')
-        %span.btn-group.has-tooltip{ title: (disabled_fork_tooltip unless can_create_fork) }
-          = link_to new_project_fork_path(@project), class: "gl-button btn btn-default btn-sm fork-btn #{' disabled' unless can_create_fork }", 'aria-label' => (disabled_fork_tooltip unless can_create_fork) do
+        - disabled_tooltip = fork_button_disabled_tooltip(@project)
+        - count_class = 'disabled' unless can?(current_user, :download_code, @project)
+        - button_class = 'disabled' if disabled_tooltip
+
+        %span.btn-group{ class: ('has-tooltip' if disabled_tooltip), title: disabled_tooltip }
+          = link_to new_project_fork_path(@project), class: "gl-button btn btn-default btn-sm fork-btn #{button_class}" do
             = sprite_icon('fork', css_class: 'icon')
             %span= s_('ProjectOverview|Fork')
-      = link_to project_forks_path(@project), title: n_(s_('ProjectOverview|Forks'), s_('ProjectOverview|Forks'), @project.forks_count), class: 'gl-button btn btn-default btn-sm count has-tooltip' do
+      = link_to project_forks_path(@project), title: n_(s_('ProjectOverview|Forks'), s_('ProjectOverview|Forks'), @project.forks_count), class: "gl-button btn btn-default btn-sm count has-tooltip #{count_class}" do
         = @project.forks_count

locale/gitlab.pot

@@ -27018,6 +27018,9 @@ msgstr ""
 msgid "ProjectOverview|Unstar"
 msgstr ""
 
+msgid "ProjectOverview|You don't have permission to fork this project"
+msgstr ""
+
 msgid "ProjectOverview|You have reached your project limit"
 msgstr ""
 

spec/features/projects/fork_spec.rb

@@ -59,10 +59,11 @@ RSpec.describe 'Project fork' do
     context 'forking is disabled' do
       let(:forking_access_level) { ProjectFeature::DISABLED }
 
-      it 'does not render fork button' do
+      it 'render a disabled fork button' do
         visit project_path(project)
 
-        expect(page).not_to have_css('a', text: 'Fork')
+        expect(page).to have_css('a.disabled', text: 'Fork')
+        expect(page).to have_css('a.count', text: '0')
       end
 
       it 'does not render new project fork page' do
@@ -80,10 +81,11 @@ RSpec.describe 'Project fork' do
       end
 
       context 'user is not a team member' do
-        it 'does not render fork button' do
+        it 'render a disabled fork button' do
           visit project_path(project)
 
-          expect(page).not_to have_css('a', text: 'Fork')
+          expect(page).to have_css('a.disabled', text: 'Fork')
+          expect(page).to have_css('a.count', text: '0')
         end
 
         it 'does not render new project fork page' do
@@ -102,6 +104,7 @@ RSpec.describe 'Project fork' do
           visit project_path(project)
 
           expect(page).to have_css('a', text: 'Fork')
+          expect(page).to have_css('a.count', text: '0')
           expect(page).not_to have_css('a.disabled', text: 'Fork')
         end
 

spec/helpers/projects_helper_spec.rb

@@ -991,4 +991,31 @@ RSpec.describe ProjectsHelper do
       expect(subject).to eq(project.path_with_namespace)
     end
   end
+
+  describe '#fork_button_disabled_tooltip' do
+    using RSpec::Parameterized::TableSyntax
+
+    subject { helper.fork_button_disabled_tooltip(project) }
+
+    where(:has_user, :can_fork_project, :can_create_fork, :expected) do
+      false | false | false | nil
+      true | true | true | nil
+      true | false | true | 'You don\'t have permission to fork this project'
+      true | true | false | 'You have reached your project limit'
+    end
+
+    with_them do
+      before do
+        current_user = user if has_user
+
+        allow(helper).to receive(:current_user).and_return(current_user)
+        allow(user).to receive(:can?).with(:fork_project, project).and_return(can_fork_project)
+        allow(user).to receive(:can?).with(:create_fork).and_return(can_create_fork)
+      end
+
+      it 'returns tooltip text when user lacks privilege' do
+        expect(subject).to eq(expected)
+      end
+    end
+  end
 end