Add documentation for the GitLab pipelines configuration

Signed-off-by: NRémy Coutable <remy@rymai.me>

doc/development/README.md

@@ -18,6 +18,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Generate a changelog entry with `bin/changelog`](changelog.md)
 - [Code review guidelines](code_review.md) for reviewing code and having code reviewed
 - [Database review guidelines](database_review.md) for reviewing database-related changes and complex SQL queries
+- [Pipelines for the GitLab project](pipelines.md)
 - [Automatic CE->EE merge](automatic_ce_ee_merge.md)
 - [Guidelines for implementing Enterprise Edition features](ee_features.md)
 - [Security process for developers](https://gitlab.com/gitlab-org/release/docs/blob/master/general/security/developer.md#security-releases-critical-non-critical-as-a-developer)

doc/development/pipelines.md

+# Pipelines for the GitLab project
+
+Pipelines for `gitlab-org/gitlab-ce` and `gitlab-org/gitlab-ce` (as well as the
+`dev` instance's mirrors) are configured in the usual
+[`.gitlab-ci.yml`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab-ci.yml)
+which itself includes files under
+[`.gitlab/ci/`](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/.gitlab/ci)
+for easier maintenance.
+
+We're striving to [dogfood](https://about.gitlab.com/handbook/engineering/#dogfooding)
+GitLab [CI/CD features and best-practices](../ci/yaml/README.md)
+as much as possible.
+
+## Stages
+
+The current stages are:
+
+- `prepare`: This stage includes jobs that prepare artifacts that are needed by
+  jobs in subsequent stages.
+- `quick-test`: This stage includes test jobs that should run first and fail the
+  pipeline early (currently used to run Geo tests when the branch name starts
+  with `geo-`, `geo/`, or ends with `-geo`).
+- `test`: This stage includes most of the tests, DB/migration jobs, and static analysis jobs.
+- `review-prepare`: This stage includes a job that build the CNG images that are
+  later used by the (Helm) Review App deployment (see
+  [Review Apps](testing_guide/review_apps.md) for details).
+- `review`: This stage includes jobs that deploy the GitLab and Docs Review Apps.
+- `qa`: This stage includes jobs that perform QA tasks against the Review App
+  that is deployed in the previous stage.
+- `post-test`: This stage includes jobs that build reports or gather data from
+  the previous stages' jobs (e.g. coverage, Knapsack metadata etc.).
+- `pages`: This stage includes a job that deploy the various reports as
+  GitLab pages (e.g. <https://gitlab-org.gitlab.io/gitlab-ce/coverage-ruby/>,
+  <https://gitlab-org.gitlab.io/gitlab-ce/coverage-javascript/>,
+  <https://gitlab-org.gitlab.io/gitlab-ce/webpack-report/>)
+
+## Default image
+
+The default image is currently
+`dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.6.3-golang-1.11-git-2.22-chrome-73.0-node-12.x-yarn-1.16-postgresql-9.6-graphicsmagick-1.3.33`.
+It includes Ruby 2.6.3, Go 1.11, Git 2.22, Chrome 73, Node 12, Yarn 1.16,
+PostgreSQL 9.6, and Graphics Magick 1.3.33.
+
+The images used in our pipelines are configured in the
+[`gitlab-org/gitlab-build-images`](https://gitlab.com/gitlab-org/gitlab-build-images)
+project, which is push-mirrored to <https://dev.gitlab.org/gitlab/gitlab-build-images>
+for redundancy.
+
+The current version of the build images can be found in the
+["Used by GitLab CE/EE section"](https://gitlab.com/gitlab-org/gitlab-build-images/blob/master/.gitlab-ci.yml).
+
+## Default variables
+
+In addition to the [predefined variables](../ci/variables/predefined_variables.md),
+each pipeline includes the following [variables](../ci/variables/README.md):
+
+- `RAILS_ENV: "test"`
+- `NODE_ENV: "test"`
+- `SIMPLECOV: "true"`
+- `GIT_DEPTH: "20"`
+- `GIT_SUBMODULE_STRATEGY: "none"`
+- `GET_SOURCES_ATTEMPTS: "3"`
+- `KNAPSACK_RSPEC_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/rspec_report-master.json`
+- `EE_KNAPSACK_RSPEC_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/rspec_report-master-ee.json`
+- `FLAKY_RSPEC_SUITE_REPORT_PATH: rspec_flaky/report-suite.json`
+- `BUILD_ASSETS_IMAGE: "false"`
+- `ES_JAVA_OPTS: "-Xms256m -Xmx256m"`
+- `ELASTIC_URL: "http://elastic:changeme@docker.elastic.co-elasticsearch-elasticsearch:9200"`
+
+## Common job definitions
+
+Most of the jobs [extend from a few CI definitions](../ci/yaml/README.md#extends)
+that are scoped to a single
+[configuration parameter](../ci/yaml/README.md#configuration-parameters).
+
+These common definitions are:
+
+- `.default-tags`: Ensures a job has the `gitlab-org` tag to ensure it's using
+  our dedicated runners.
+- `.default-retry`: Allows a job to retry upon `unknown_failure`, `api_failure`,
+  `runner_system_failure`.
+- `.default-before_script`: Allows a job to use a default `before_script` definition
+  suitable for Ruby/Rails tasks that may need a database running (e.g. tests).
+- `.default-cache`: Allows a job to use a default `cache` definition suitable for
+  Ruby/Rails and frontend tasks.
+- `.default-only`: Restricts the cases where a job is created. This currently
+  includes `master`, `/^[\d-]+-stable(-ee)?$/` (stable branches),
+  `/^\d+-\d+-auto-deploy-\d+$/` (security branches), `merge_requests`, `tags`.
+  Note that jobs won't be created for branches with this default configuration.
+- `.only-review`: Only creates a job for the `gitlab-org` namespace and if
+  Kubernetes integration is available. Also, prevents a job from being created
+  for `master` and auto-deploy branches.
+- `.only-review-schedules`: Same as `.only-review` but also restrict a job to
+  only run for [schedules](../user/project/pipelines/schedules.md).
+- `.use-pg`: Allows a job to use the `postgres:9.6.14` and `redis:alpine` services.
+- `.use-pg-10`: Allows a job to use the `postgres:10.9` and `redis:alpine` services.
+- `.only-ee`: Only creates a job for the `gitlab-ee` project.
+
+## Changes detection
+
+If a job extends from `.default-only` (and most of the jobs should), it can restrict
+the cases where it should be created
+[based on the changes](../ci/yaml/README.md#onlychangesexceptchanges)
+from a commit or MR by extending from the following CI definitions:
+
+- `.only-code-changes`: Allows a job to only be created upon code-related changes
+- `.only-qa-changes`: Allows a job to only be created upon QA-related changes
+- `.only-docs-changes`: Allows a job to only be created upon docs-related changes
+- `.only-code-qa-changes`: Allows a job to only be created upon code-related or QA-related changes
+- `.only-code-docs-changes`: Allows a job to only be created upon code-related or docs-related changes
+
+**See <https://gitlab.com/gitlab-org/gitlab-ee/blob/master/.gitlab/ci/global.gitlab-ci.yml>
+for the list of exact patterns.**
+
+## Directed acyclic graph
+
+We're using the [`needs:`](../ci/yaml/README.md#needs) keyword to
+execute jobs out-of-order for the following jobs:
+
+```mermaid
+graph RL;
+  A[setup-test-env];
+  B[gitlab:assets:compile];
+  C[gitlab:assets:compile pull-cache];
+  D[cache gems];
+  E[review-build-cng];
+  F[build-qa-image];
+
+subgraph "`prepare` stage"
+    A
+    end
+
+subgraph "`test` stage"
+    B
+    C
+    D
+    F
+    downtime_check -- needs --> A;
+    db:* -- needs --> A;
+    gitlab:setup -- needs --> A;
+    migration:upgrade-pg-ce-to-ee -- needs --> A;
+    end
+
+subgraph "`review-prepare` stage"
+    E
+    schedule:review-build-cng -- needs --> B;
+    end
+
+  B -- needs --> A;
+  C -- needs --> A;
+
+  D -- needs --> A;
+  E -- needs --> C;
+
+subgraph "`review` stage"
+    review-deploy -- needs --> E;
+    end
+
+subgraph "`qa` stage"
+    package-and-qa-manual -- needs --> C;
+    package-and-qa-manual -- needs --> F;
+    package-and-qa-manual:master -- needs --> B;
+    package-and-qa-manual:master -- needs --> F;
+    package-and-qa -- needs --> C;
+    package-and-qa -- needs --> F;
+    end
+```
+
+## Test jobs
+
+Consult [GitLab tests in the Continuous Integration (CI) context](testing_guide/ci.md)
+for more information.
+
+## Review app jobs
+
+Consult the [Review Apps](testing_guide/review_apps.md) dedicated page for more information.
+
+---
+
+[Return to Development documentation](README.md)