Expose CI_PIPELINE_SOURCE on CI jobs

It was missing and expected it wouldn't hurt anyone

app/models/ci/pipeline.rb

@@ -393,7 +393,8 @@ module Ci
     def predefined_variables
       [
         { key: 'CI_PIPELINE_ID', value: id.to_s, public: true },
-        { key: 'CI_CONFIG_PATH', value: ci_yaml_file_path, public: true }
+        { key: 'CI_CONFIG_PATH', value: ci_yaml_file_path, public: true },
+        { key: 'CI_PIPELINE_SOURCE', value: source.to_s, public: true }
       ]
     end

changelogs/unreleased/zj-add-pipeline-source-variable.yml

+---
+title: Add CI_PIPELINE_SOURCE variable on CI Jobs
+merge_request:
+author:
+type: added

doc/ci/variables/README.md

@@ -57,6 +57,7 @@ future GitLab releases.**
 | **CI_RUNNER_TAGS**              | 8.10   | 0.5    | The defined runner tags |
 | **CI_PIPELINE_ID**              | 8.10   | 0.5    | The unique id of the current pipeline that GitLab CI uses internally |
 | **CI_PIPELINE_TRIGGERED**       | all    | all    | The flag to indicate that job was [triggered] |
+| **CI_PIPELINE_SOURCE**          | 10.0   | all    | The source for this pipeline, one of: push, web, trigger, schedule, api, external. Pipelines created before 9.5 will have unknown as source |
 | **CI_PROJECT_DIR**              | all    | all    | The full path where the repository is cloned and where the job is run |
 | **CI_PROJECT_ID**               | all    | all    | The unique id of the current project that GitLab CI uses internally |
 | **CI_PROJECT_NAME**             | 8.10   | 0.5    | The project name that is currently being built (actually it is project folder name) |

spec/models/ci/pipeline_spec.rb

@@ -2,7 +2,7 @@ require 'spec_helper'
 describe Ci::Pipeline, :mailer do
   let(:user) { create(:user) }
-  let(:project) { create(:project) }
+  set(:project) { create(:project) }
   let(:pipeline) do
     create(:ci_empty_pipeline, status: :created, project: project)
@@ -159,6 +159,18 @@ describe Ci::Pipeline, :mailer do
     end
   end
+  describe '#predefined_variables' do
+    subject { pipeline.predefined_variables }
+
+    it { is_expected.to be_an(Array) }
+
+    it 'includes the defined keys' do
+      keys = subject.map { |v| v[:key] }
+
+      expect(keys).to include('CI_PIPELINE_ID', 'CI_CONFIG_PATH', 'CI_PIPELINE_SOURCE')
+    end
+  end
+
   describe '#auto_canceled?' do
     subject { pipeline.auto_canceled? }