Fix GitLab plugins not working without hooks configured

https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/31741, shipped
in GitLab 12.2, reduced Gitaly RPCs by generating commit data only if
hooks or services were actually active. However, GitLab plugins
(https://docs.gitlab.com/ee/administration/plugins.html), are executable
programs that get launched inside `SystemHooksService#execute_hooks`
regardless of whether any hooks are actually active. As a result of this
change, users have to configure a Webhook for plugins to fire.

We restore the previous behavior by checking whether plugins are
available and executing `SystemHooksService#execute_hooks` if any
plugins are available.

Closes https://gitlab.com/gitlab-org/gitlab/issues/194131

app/models/project.rb

@@ -1318,7 +1318,7 @@ class Project < ApplicationRecord
   end
 
   def has_active_hooks?(hooks_scope = :push_hooks)
-    hooks.hooks_for(hooks_scope).any? || SystemHook.hooks_for(hooks_scope).any?
+    hooks.hooks_for(hooks_scope).any? || SystemHook.hooks_for(hooks_scope).any? || Gitlab::Plugin.any?
   end
 
   def has_active_services?(hooks_scope = :push_hooks)

changelogs/unreleased/sh-fix-plugins-not-executing.yml

+---
+title: Fix GitLab plugins not working without hooks configured
+merge_request: 22409
+author:
+type: fixed

lib/gitlab/plugin.rb

@@ -2,10 +2,16 @@
 
 module Gitlab
   module Plugin
+    def self.any?
+      plugin_glob.any? { |entry| File.file?(entry) }
+    end
+
     def self.files
-      Dir.glob(Rails.root.join('plugins/*')).select do |entry|
-        File.file?(entry)
-      end
+      plugin_glob.select { |entry| File.file?(entry) }
+    end
+
+    def self.plugin_glob
+      Dir.glob(Rails.root.join('plugins/*'))
     end
 
     def self.execute_all_async(data)

spec/lib/gitlab/plugin_spec.rb

@@ -3,22 +3,59 @@
 require 'spec_helper'
 
 describe Gitlab::Plugin do
+  let(:plugin) { Rails.root.join('plugins', 'test.rb') }
+  let(:tmp_file) { Tempfile.new('plugin-dump') }
+
+  let(:plugin_source) do
+    <<~EOS
+      #!/usr/bin/env ruby
+      x = STDIN.read
+      File.write('#{tmp_file.path}', x)
+    EOS
+  end
+
+  context 'with plugins present' do
+    before do
+      File.write(plugin, plugin_source)
+    end
+
+    after do
+      FileUtils.rm(plugin)
+    end
+
+    describe '.any?' do
+      it 'returns true' do
+        expect(described_class.any?).to be true
+      end
+    end
+
+    describe '.files?' do
+      it 'returns a list of plugins' do
+        expect(described_class.files).to match_array([plugin.to_s])
+      end
+    end
+  end
+
+  context 'without any plugins' do
+    describe '.any?' do
+      it 'returns false' do
+        expect(described_class.any?).to be false
+      end
+    end
+
+    describe '.files' do
+      it 'returns an empty list' do
+        expect(described_class.files).to be_empty
+      end
+    end
+  end
+
   describe '.execute' do
     let(:data) { Gitlab::DataBuilder::Push::SAMPLE_DATA }
-    let(:plugin) { Rails.root.join('plugins', 'test.rb') }
-    let(:tmp_file) { Tempfile.new('plugin-dump') }
     let(:result) { described_class.execute(plugin.to_s, data) }
     let(:success) { result.first }
     let(:message) { result.last }
 
-    let(:plugin_source) do
-      <<~EOS
-        #!/usr/bin/env ruby
-        x = STDIN.read
-        File.write('#{tmp_file.path}', x)
-      EOS
-    end
-
     before do
       File.write(plugin, plugin_source)
     end

spec/models/project_spec.rb

@@ -4721,6 +4721,13 @@ describe Project do
       expect(project.has_active_hooks?(:merge_request_events)).to be_falsey
       expect(project.has_active_hooks?).to be_truthy
     end
+
+    it 'returns true when a plugin exists' do
+      expect(Gitlab::Plugin).to receive(:any?).twice.and_return(true)
+
+      expect(project.has_active_hooks?(:merge_request_events)).to be_truthy
+      expect(project.has_active_hooks?).to be_truthy
+    end
   end
 
   describe '#has_active_services?' do