Add `CI_COMMIT_BRANCH` variable

This significantly simplifies
handling between Tags/Branches/Merge Requests
and makes `workflow:rules` easy to use.

app/models/ci/pipeline.rb

@@ -638,6 +638,7 @@ module Ci
         variables.append(key: 'CI_COMMIT_BEFORE_SHA', value: before_sha)
         variables.append(key: 'CI_COMMIT_REF_NAME', value: source_ref)
         variables.append(key: 'CI_COMMIT_REF_SLUG', value: source_ref_slug)
+        variables.append(key: 'CI_COMMIT_BRANCH', value: ref) if branch?
         variables.append(key: 'CI_COMMIT_TAG', value: ref) if tag?
         variables.append(key: 'CI_COMMIT_MESSAGE', value: git_commit_message.to_s)
         variables.append(key: 'CI_COMMIT_TITLE', value: git_commit_full_title.to_s)

changelogs/unreleased/make-workflow-rules-to-work.yml

 ---
 title: Make `workflow:rules` to work well with Merge Requests
-merge_request:
+merge_request: 21742
 author:
 type: changed

doc/ci/variables/predefined_variables.md

@@ -37,6 +37,7 @@ future GitLab releases.**
 | `CI_COMMIT_REF_SLUG`                          | 9.0    | all    | `$CI_COMMIT_REF_NAME` lowercased, shortened to 63 bytes, and with everything except `0-9` and `a-z` replaced with `-`. No leading / trailing `-`. Use in URLs, host names and domain names.                                                                                                                                                                |
 | `CI_COMMIT_SHA`                               | 9.0    | all    | The commit revision for which project is built                                                                                                                                                                                                                                                                                                             |
 | `CI_COMMIT_SHORT_SHA`                         | 11.7   | all    | The first eight characters of `CI_COMMIT_SHA`                                                                                                                                                                                                                                                                                                              |
+| `CI_COMMIT_BRANCH`                            | 12.6   | 0.5    | The commit branch name. Present only when building branches.                                                                                                                                                                                                                                                                                                      |
 | `CI_COMMIT_TAG`                               | 9.0    | 0.5    | The commit tag name. Present only when building tags.                                                                                                                                                                                                                                                                                                      |
 | `CI_COMMIT_TITLE`                             | 10.8   | all    | The title of the commit - the full first line of the message                                                                                                                                                                                                                                                                                               |
 | `CI_CONCURRENT_ID`                            | all    | 11.10  | Unique ID of build execution within a single executor.                                                                                                                                                                                                                                                                                                     |

spec/models/ci/build_spec.rb

@@ -2589,6 +2589,19 @@ describe Ci::Build do
       it { is_expected.to include(job_variable) }
     end
 
+    context 'when build is for branch' do
+      let(:branch_variable) do
+        { key: 'CI_COMMIT_BRANCH', value: 'master', public: true, masked: false }
+      end
+
+      before do
+        build.update(tag: false)
+        pipeline.update(tag: false)
+      end
+
+      it { is_expected.to include(branch_variable) }
+    end
+
     context 'when build is for tag' do
       let(:tag_variable) do
         { key: 'CI_COMMIT_TAG', value: 'master', public: true, masked: false }