1 files changed, 96 insertions, 0 deletions
diff --git a/lib/gitlab/ci/config/interpolation/inputs/base_input.rb b/lib/gitlab/ci/config/interpolation/inputs/base_input.rb
new file mode 100644
index 00000000000..5648c4d31ea
--- /dev/null
+++ b/lib/gitlab/ci/config/interpolation/inputs/base_input.rb
@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    class Config
+      module Interpolation
+        class Inputs
+          ##
+          # This is a common abstraction for all input types
+          class BaseInput
+            ArgumentNotValidError = Class.new(StandardError)
+
+            # Checks whether the class matches the type in the specification
+            def self.matches?(spec)
+              raise NotImplementedError
+            end
+
+            # Human readable type used in error messages
+            def self.type_name
+              raise NotImplementedError
+            end
+
+            # Checks whether the provided value is of the given type
+            def valid_value?(value)
+              raise NotImplementedError
+            end
+
+            attr_reader :errors, :name, :spec, :value
+
+            def initialize(name:, spec:, value:)
+              @name = name
+              @errors = []
+
+              # Treat minimal spec definition (nil) as a valid hash:
+              #   spec:
+              #     inputs:
+              #       website:
+              @spec = spec || {} # specification from input definition
+              @value = value     # actual value provided by the user
+
+              validate!
+            end
+
+            def to_hash
+              raise ArgumentNotValidError unless valid?
+
+              { name => actual_value }
+            end
+
+            def valid?
+              @errors.none?
+            end
+
+            private
+
+            def validate!
+              return error('required value has not been provided') if required_input? && value.nil?
+
+              # validate default value
+              if !required_input? && !valid_value?(default)
+                return error("default value is not a #{self.class.type_name}")
+              end
+
+              # validate provided value
+              error("provided value is not a #{self.class.type_name}") unless valid_value?(actual_value)
+            end
+
+            def error(message)
+              @errors.push("`#{name}` input: #{message}")
+            end
+
+            def actual_value
+              # nil check is to support boolean values.
+              value.nil? ? default : value
+            end
+
+            # An input specification without a default value is required.
+            # For example:
+            # ```yaml
+            # spec:
+            #   inputs:
+            #     website:
+            # ```
+            def required_input?
+              !spec.key?(:default)
+            end
+
+            def default
+              spec[:default]
+            end
+          end
+        end
+      end
+    end
+  end
+end