Add pending builds model and implement transitions

This commit adds a pending build model that will create a resource in
the database when a build transitions to the pending state. The resource
will be removed when an associated build gets picked by a runner.

app/models/ci/build.rb

@@ -295,11 +295,18 @@ module Ci
       end
 
       after_transition any => [:pending] do |build|
+        Ci::UpdateBuildQueueService.queue_push!(build)
+
         build.run_after_commit do
           BuildQueueWorker.perform_async(id)
         end
       end
 
+      after transition pending: any do |build|
+        # TODO ensure that there is no race condition here, add test for this
+        Ci::UpdateBuildQueueService.queue_pop!(build)
+      end
+
       after_transition pending: :running do |build|
         build.deployment&.run
 

app/models/ci/pending_build.rb

+# frozen_string_literal: true
+
+module Ci
+  class PendingBuild < ApplicationRecord
+    extend Gitlab::Ci::Model
+
+    belongs_to :project
+    belongs_to :build, class_name: 'Ci::Build'
+  end
+end

app/services/ci/update_build_queue_service.rb

@@ -2,12 +2,44 @@
 
 module Ci
   class UpdateBuildQueueService
+    ##
+    # Add a build to the pending builds queue
+    #
+    def queue_push!(build, metrics = ::Gitlab::Ci::Queue::Metrics)
+      in_transaction do
+        ::Ci::PendingBuild.create!(build: build, project: project)
+
+        # TODO increment pending builds counter
+      end
+    end
+
+    ##
+    # Remove a build from the pending builds queue
+    #
+    def queue_pop!(build, metrics = ::Gitlab::Ci::Queue::Metrics)
+      in_transaction do
+        ::Ci::PendingBuild.find(build.id).destroy!
+
+        # TODO decrement pending builds counter
+      end
+    end
+
+    ##
+    # Unblock runner associated with given project / build
+    #
     def execute(build, metrics = ::Gitlab::Ci::Queue::Metrics)
       tick_for(build, build.project.all_runners, metrics)
     end
 
     private
 
+    def in_transaction
+      # TODO ensure that state machine transition transaction is open
+      #
+      yield
+    end
+
+
     def tick_for(build, runners, metrics)
       runners = runners.with_recent_runner_queue
       runners = runners.with_tags if Feature.enabled?(:ci_preload_runner_tags, default_enabled: :yaml)