feat: Provide an access object for environment variables (#49)

lib/google/cloud/env/variables.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+module Google
+  module Cloud
+    class Env
+      ##
+      # Access to system environment variables.
+      #
+      # This is a hashlike object that controls access to environment variable
+      # data. It supports temporarily changing the data source (i.e. swapping
+      # ::ENV out for a different set of data) for mocking.
+      #
+      class Variables
+        ##
+        # Create an enviroment variables access object. This is initially
+        # backed by the actual environment variables (i.e. ENV).
+        #
+        def initialize
+          @backing_data = ::ENV
+        end
+
+        ##
+        # Fetch the given environment variable from the backing data.
+        #
+        # @param key [String]
+        # @return [String,nil]
+        #
+        def [] key
+          @backing_data[key.to_s]
+        end
+        alias get []
+
+        ##
+        # The backing data is a hash or hash-like object that represents the
+        # environment variable data. This can either be the actual environment
+        # variables object (i.e. ENV) or a substitute hash used for mocking.
+        #
+        # @return [Hash{String=>String}]
+        #
+        attr_accessor :backing_data
+
+        ##
+        # Run the given block with the backing data replaced with the given
+        # hash. The original backing data is restored afterward. This is used
+        # for debugging/testing/mocking.
+        #
+        # @param temp_backing_data [Hash{String=>String}]
+        #
+        def with_backing_data temp_backing_data
+          old_backing_data = @backing_data
+          begin
+            @backing_data = temp_backing_data
+            yield
+          ensure
+            @backing_data = old_backing_data
+          end
+        end
+      end
+    end
+  end
+end

test/google/cloud/env/variables_test.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+require "helper"
+require "google/cloud/env/variables"
+
+describe Google::Cloud::Env::Variables do
+  let(:variables) { Google::Cloud::Env::Variables.new }
+  let(:fake_home) { "fake-home" }
+  let(:fake_data) { { "HOME" => fake_home } }
+  let(:empty_backing_data) { {} }
+
+  it "returns the value from the environment" do
+    assert_equal ENV["HOME"], variables["HOME"]
+  end
+
+  it "supports modifying the backing data" do
+    variables.backing_data = fake_data
+    assert_equal fake_home, variables["HOME"]
+    variables.backing_data = ENV
+    assert_equal ENV["HOME"], variables["HOME"]
+  end
+
+  it "supports modifying the backing data in a block" do
+    variables.with_backing_data fake_data do
+      assert_equal fake_home, variables["HOME"]
+    end
+    assert_equal ENV["HOME"], variables["HOME"]
+  end
+
+  it "completely replaces backing data" do
+    refute_nil variables["HOME"]
+    variables.backing_data = empty_backing_data
+    assert_nil variables["HOME"]
+    variables.backing_data = ENV
+    refute_nil variables["HOME"]
+  end
+end