Don't pass undefined context-values to Labkit

Don't pass values on to labkit that weren't explicityly
defined. Otherwise, labkit will unintentionally overwrite any existing
values with nil.

Consider the following example:

  ApplicationContext.with_context(user: <User "jane-doe">) do
    ApplicationContext.with_context(
      project: <Project "gitlab-org/gitlab">
    ) do
      # The inner context lives here
    end
  end

We want the inner context for Labkit to contain this hash:

  {
    user: "jane-doe",
    project: "gitlab-org/gitlab",
    root_namespace: "gitlab-org"
  }

But before this change, we'd overwrite the `user` value when creating
the inner context, even though the inner context did not contain any
information that should override the user info.

lib/gitlab/application_context.rb

@@ -5,6 +5,14 @@ module Gitlab
   class ApplicationContext
     include Gitlab::Utils::LazyAttributes
 
+    Attribute = Struct.new(:name, :type)
+
+    APPLICATION_ATTRIBUTES = [
+      Attribute.new(:project, Project),
+      Attribute.new(:namespace, Namespace),
+      Attribute.new(:user, User)
+    ].freeze
+
     def self.with_context(args, &block)
       application_context = new(**args)
       Labkit::Context.with_context(application_context.to_lazy_hash, &block)
@@ -15,21 +23,36 @@ module Gitlab
       Labkit::Context.push(application_context.to_lazy_hash)
     end
 
-    def initialize(user: nil, project: nil, namespace: nil)
-      @user, @project, @namespace = user, project, namespace
+    def initialize(**args)
+      unknown_attributes = args.keys - APPLICATION_ATTRIBUTES.map(&:name)
+      raise ArgumentError, "#{unknown_attributes} are not known keys" if unknown_attributes.any?
+
+      @set_values = args.keys
+
+      assign_attributes(args)
     end
 
     def to_lazy_hash
-      { user: -> { username },
-        project: -> { project_path },
-        root_namespace: -> { root_namespace_path } }
+      {}.tap do |hash|
+        hash[:user] = -> { username } if set_values.include?(:user)
+        hash[:project] = -> { project_path } if set_values.include?(:project)
+        hash[:root_namespace] = -> { root_namespace_path } if include_namespace?
+      end
     end
 
     private
 
-    lazy_attr_reader :user, type: User
-    lazy_attr_reader :project, type: Project
-    lazy_attr_reader :namespace, type: Namespace
+    attr_reader :set_values
+
+    APPLICATION_ATTRIBUTES.each do |attr|
+      lazy_attr_reader attr.name, type: attr.type
+    end
+
+    def assign_attributes(values)
+      values.slice(*APPLICATION_ATTRIBUTES.map(&:name)).each do |name, value|
+        instance_variable_set("@#{name}", value)
+      end
+    end
 
     def project_path
       project&.full_path
@@ -46,5 +69,9 @@ module Gitlab
         project&.full_path_components&.first
       end
     end
+
+    def include_namespace?
+      set_values.include?(:namespace) || set_values.include?(:project)
+    end
   end
 end

spec/lib/gitlab/application_context_spec.rb

@@ -28,7 +28,7 @@ describe Gitlab::ApplicationContext do
   describe '.push' do
     it 'passes the expected context on to labkit' do
       fake_proc = duck_type(:call)
-      expected_context = hash_including(user: fake_proc, project: fake_proc, root_namespace: fake_proc)
+      expected_context = { user: fake_proc }
 
       expect(Labkit::Context).to receive(:push).with(expected_context)
 
@@ -78,5 +78,27 @@ describe Gitlab::ApplicationContext do
       expect(result(context))
         .to include(project: project.full_path, root_namespace: project.full_path_components.first)
     end
+
+    context 'only include values for which an option was specified' do
+      using RSpec::Parameterized::TableSyntax
+
+      where(:provided_options, :expected_context_keys) do
+        [:user, :namespace, :project] | [:user, :project, :root_namespace]
+        [:user, :project]             | [:user, :project, :root_namespace]
+        [:user, :namespace]           | [:user, :root_namespace]
+        [:user]                       | [:user]
+        []                            | []
+      end
+
+      with_them do
+        it do
+          # Build a hash that has all `provided_options` as keys, and `nil` as value
+          provided_values = provided_options.map { |key| [key, nil] }.to_h
+          context = described_class.new(provided_values)
+
+          expect(context.to_lazy_hash.keys).to contain_exactly(*expected_context_keys)
+        end
+      end
+    end
   end
 end