feat(CI checks): Add support for GitLab CI checks

Check `GITLAB_CI` environment variable and then verify
`CI_COMMIT_REF_NAME` matches the given branch.

Includes tests

Closes #88  re #32

docs/automatic-releases/index.rst

@@ -43,6 +43,14 @@ Checks for circle-ci to ensure that the build is not a pull-request and on the c
 The branch check, checks against the branch that circle-ci said it checked out, not the current
 branch.
 
+GitLab CI
+^^^^^^^^
+*Condition:* Environment variable ``GITLAB_CI`` is ``'true'``
+
+Checks for gitlab-ci to ensure that the build is on the correct branch.
+The branch check, checks against the branch that gitlab-ci said it checked out, not the current
+branch.
+
 Publish with CI
 ~~~~~~~~~~~~~~~
 Add ``python setup.py publish`` or ``semantic-release publish`` as an after success task on your

semantic_release/ci_checks.py

@@ -72,6 +72,18 @@ def circle(branch):
     assert not os.environ.get('CI_PULL_REQUEST')
 
 
+@checker
+def gitlab(branch):
+    """
+    Performs necessary checks to ensure that the gitlab build is one
+    that should create releases.
+
+    :param branch: The branch the environment should be running against.
+    """
+    assert os.environ.get('CI_COMMIT_REF_NAME') == branch
+    # TODO - don't think there's a merge request indicator variable
+
+
 def check(branch='master'):
     """
     Detects the current CI environment, if any, and performs necessary
@@ -88,3 +100,5 @@ def check(branch='master'):
         return frigg(branch)
     elif os.environ.get('CIRCLECI') == 'true':
         return circle(branch)
+    elif os.environ.get('GITLAB_CI') == 'true':
+        return gitlab(branch)

tests/ci_checks/test_gitlab.py

@@ -0,0 +1,23 @@
+import pytest
+
+from semantic_release import ci_checks
+from semantic_release.errors import CiVerificationError
+
+
+def test_gitlab_should_pass_if_branch_is_master(monkeypatch):
+    monkeypatch.setenv('CI_COMMIT_REF_NAME', 'master')
+
+    assert ci_checks.gitlab('master')
+
+
+def test_gitlab_should_pass_if_branch_is_correct(monkeypatch):
+    monkeypatch.setenv('CI_COMMIT_REF_NAME', 'other-branch')
+
+    assert ci_checks.gitlab('other-branch')
+
+
+def test_gitlab_should_raise_ci_verification_error_for_wrong_branch(monkeypatch):
+    monkeypatch.setenv('CI_COMMIT_REF_NAME', 'other-branch')
+
+    with pytest.raises(CiVerificationError):
+        ci_checks.gitlab('master')