Assume that unspecified CI config is undefined

We assume that when someone adds a key for the configuration entry, but
does not provide a valid value, which causes entry to be `nil`, then
entry should be considered as the undefined one. We also assume this is
semantically correct, this is also backwards compatible with legacy CI
config processor.

See issue #18775 for more details.

3 files changed, 12 insertions, 13 deletions

diff --git a/lib/gitlab/ci/config/node/configurable.rb b/lib/gitlab/ci/config/node/configurable.rb
index 25b5c2c9e21..0fb9092dafa 100644
--- a/lib/gitlab/ci/config/node/configurable.rb
+++ b/lib/gitlab/ci/config/node/configurable.rb
@@ -27,7 +27,6 @@ module Gitlab
 
           def create_node(key, factory)
             factory.with(value: @config[key], key: key)
-            factory.undefine! unless @config.has_key?(key)
             factory.create!
           end
 
diff --git a/lib/gitlab/ci/config/node/factory.rb b/lib/gitlab/ci/config/node/factory.rb
index 2271c386df6..647b0c82a79 100644
--- a/lib/gitlab/ci/config/node/factory.rb
+++ b/lib/gitlab/ci/config/node/factory.rb
@@ -18,16 +18,20 @@ module Gitlab
             self
           end
 
-          def undefine!
-            @attributes[:value] = @node.dup
-            @node = Node::Undefined
-            self
-          end
-
           def create!
             raise InvalidFactory unless @attributes.has_key?(:value)
 
-            @node.new(@attributes[:value]).tap do |entry|
+            ##
+            # We assume unspecified entry is undefined.
+            # See issue #18775.
+            #
+            if @attributes[:value].nil?
+              node, value = Node::Undefined, @node
+            else
+              node, value = @node, @attributes[:value]
+            end
+
+            node.new(value).tap do |entry|
               entry.description = @attributes[:description]
               entry.key = @attributes[:key]
             end
diff --git a/lib/gitlab/ci/config/node/variables.rb b/lib/gitlab/ci/config/node/variables.rb
index fd3ce8715a9..5f813f81f55 100644
--- a/lib/gitlab/ci/config/node/variables.rb
+++ b/lib/gitlab/ci/config/node/variables.rb
@@ -9,11 +9,7 @@ module Gitlab
           include Validatable
 
           validations do
-            validates :value, variables: true
-          end
-
-          def value
-            @config || self.class.default
+            validates :config, variables: true
           end
 
           def self.default