Merge branch 'refactor-builds-creation-service' into 'master'

Refactor pipeline creation service

## What does this MR do?
This refactors GitLab CI build processing: all builds for pipeline are pre-created when a pipeline object is created. 
The builds are created with a new introduced status `created`.
The builds are then automatically promoted to `pending` when a previous stage do succeed.
This significantly simplifies pipeline processing code solving a lot of problems of lazily initialisation of previous approach (builds were created on-demand).

## Why was this MR needed?
The previous mechanism had a lot of flows (shown in related issues) in how it work, but also in code design. Removing cross model-service-library dependencies.

The current approach moves a build creation to single place `CreatePipelineService` and removes a dynamic dependency on `config_processor` significantly simplifying a build creation and pipeline processing. Pipeline processing is implemented in `ProcessPipelineService`.

This also allows to easily extend GitLab with Manual Actions which is part of 8.10 direction issue.

## Migration problem
~~This MR removes the a on-demand creation of builds in pipelines.
 Pipelines that are running and are in mid-stage (some stages started, but not all) will not be fully evaluated after application restart. 
This happens, because the code responsible for on-demand creation is removed. 
There's no easy way to migrate existing pipelines, other than doing offline migration and putting pipeline processing in migration code (which seems to be a really bad idea).~~

To support old pipelines I added a lazy initialization of builds if none is found.

## What are the relevant issue numbers?
Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/12839
Solves: https://gitlab.com/gitlab-org/gitlab-ce/issues/18644 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/289
Allows to easily implement: https://gitlab.com/gitlab-org/gitlab-ce/issues/17010

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)


See merge request !5295

2 files changed, 10 insertions, 0 deletions

diff --git a/db/migrate/20160716115711_add_queued_at_to_ci_builds.rb b/db/migrate/20160716115711_add_queued_at_to_ci_builds.rb
new file mode 100644
index 00000000000..756910a1fa0
--- /dev/null
+++ b/db/migrate/20160716115711_add_queued_at_to_ci_builds.rb
@@ -0,0 +1,9 @@
+class AddQueuedAtToCiBuilds < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    add_column :ci_builds, :queued_at, :timestamp
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 059c923bb93..6c85e1e9dba 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -170,6 +170,7 @@ ActiveRecord::Schema.define(version: 20160810142633) do
     t.integer  "artifacts_size"
     t.string   "when"
     t.text     "yaml_variables"
+    t.datetime "queued_at"
   end
 
   add_index "ci_builds", ["commit_id", "stage_idx", "created_at"], name: "index_ci_builds_on_commit_id_and_stage_idx_and_created_at", using: :btree