Detect circular dependenies in CI/CD `extends:` entry

lib/gitlab/ci/config/extendable/collection.rb

@@ -6,6 +6,7 @@ module Gitlab
           include Enumerable
 
           ExtensionError = Class.new(StandardError)
+          CircularDependencyError = Class.new(ExtensionError)
 
           def initialize(hash)
             @hash = hash

lib/gitlab/ci/config/extendable/entry.rb

@@ -5,9 +5,9 @@ module Gitlab
         class Entry
           attr_reader :key
 
-          def initialize(key, hash, parent = nil)
+          def initialize(key, context, parent = nil)
             @key = key
-            @hash = hash
+            @context = context
             @parent = parent
           end
 
@@ -16,12 +16,12 @@ module Gitlab
           end
 
           def value
-            @value ||= @hash.fetch(@key)
+            @value ||= @context.fetch(@key)
           end
 
           def base
             Extendable::Entry
-              .new(extends, @hash, self)
+              .new(extends, @context, self)
               .extend!
           end
 
@@ -33,9 +33,17 @@ module Gitlab
             value.fetch(:extends).to_sym
           end
 
+          def path
+            Array(@parent&.path).compact.push(key)
+          end
+
           def extend!
+            if path.count(key) > 1
+              raise Extendable::Collection::CircularDependencyError
+            end
+
             if extensible?
-              @hash[key] = base.deep_merge(value)
+              @context[key] = base.deep_merge(value)
             else
               value
             end

spec/lib/gitlab/ci/config/extendable/collection_spec.rb

@@ -117,6 +117,33 @@ describe Gitlab::Ci::Config::Extendable::Collection do
     end
 
     pending 'when invalid `extends` is specified'
-    pending 'when circular dependecy has been detected'
+    context 'when circular dependecy has been detected' do
+      let(:hash) do
+        {
+          test: {
+            extends: 'something',
+            script: 'ls',
+            only: { refs: %w[master] }
+          },
+
+          something: {
+            extends: '.first',
+            script: 'deploy',
+            only: { variables: %w[$SOMETHING] }
+          },
+
+          '.first': {
+            extends: 'something',
+            script: 'run',
+            only: { kubernetes: 'active' }
+          }
+        }
+      end
+
+      it 'raises an error' do
+        expect { subject.extend! }
+          .to raise_error(described_class::CircularDependencyError)
+      end
+    end
   end
 end