use safer .hooks_for instead of .public_send

with .public_send we can't make sure that the scope on the model
actually exists.

app/models/concerns/triggerable_hooks.rb

@@ -17,6 +17,12 @@ module TriggerableHooks
   class_methods do
     attr_reader :triggerable_hooks
 
+    def hooks_for(trigger)
+      return none unless self::TRIGGERS.keys.include?(trigger)
+
+      public_send(trigger) # rubocop:disable GitlabSecurity/PublicSend
+    end
+
     private
 
     def triggerable_hooks(hooks)

app/models/project.rb

@@ -970,7 +970,7 @@ class Project < ActiveRecord::Base
 
   def execute_hooks(data, hooks_scope = :push_hooks)
     run_after_commit_or_now do
-      hooks.public_send(hooks_scope).each do |hook| # rubocop:disable GitlabSecurity/PublicSend
+      hooks.hooks_for(hooks_scope).each do |hook|
         hook.async_execute(data, hooks_scope.to_s)
       end
     end

app/services/system_hooks_service.rb

@@ -8,7 +8,7 @@ class SystemHooksService
   end
 
   def execute_hooks(data, hooks_scope = :all)
-    SystemHook.public_send(hooks_scope).find_each do |hook| # rubocop:disable GitlabSecurity/PublicSend
+    SystemHook.hooks_for(hooks_scope).find_each do |hook|
       hook.async_execute(data, 'system_hooks')
     end
   end

spec/models/concerns/triggerable_hooks_spec.rb

@@ -14,4 +14,22 @@ RSpec.describe TriggerableHooks do
       expect(TestableHook).not_to respond_to :tag_push_hooks
     end
   end
+
+  describe '.hooks_for' do
+    context 'the model has the required trigger scope' do
+      it 'returns the record' do
+        hook = TestableHook.create!(url: 'http://example.com', push_events: true)
+
+        expect(TestableHook.hooks_for(:push_hooks)).to eq [hook]
+      end
+    end
+
+    context 'the model does not have the required trigger scope' do
+      it 'returns an empty relation' do
+        TestableHook.create!(url: 'http://example.com', push_events: true)
+
+        expect(TestableHook.hooks_for(:tag_push_hooks)).to eq []
+      end
+    end
+  end
 end