test: os.Unsetenv affects global state (#5)

* test: os.Unsetenv affects global state

This change adds a helper function that guarantees the cleanup of
environment variables.

For each applicable test function, we run this at the start, and at the
end through a `defer` keyword. Previously, we were unsetting the
environment variable within the test, though there is a risk that since
global state is being manipulated, we may run into a flakey test where
an environment variable is inadvertently lingering.

* test: use unsetEnv helper to manage global state

---------

Co-authored-by: James Telfer <[email protected]>

src/plugin/config_test.go

@@ -13,8 +13,7 @@ func TestFailOnMissingEnvironment(t *testing.T) {
 	var config plugin.Config
 	fetcher := plugin.EnvironmentConfigFetcher{}
 
-	t.Setenv("BUILDKITE_PLUGIN_EXAMPLE_GO_MESSAGE", "")
-	os.Unsetenv("BUILDKITE_PLUGIN_EXAMPLE_GO_MESSAGE")
+	unsetEnv(t, "BUILDKITE_PLUGIN_EXAMPLE_GO_MESSAGE")
 
 	err := fetcher.Fetch(&config)
 
@@ -33,3 +32,20 @@ func TestFetchConfigFromEnvironment(t *testing.T) {
 	require.NoError(t, err, "fetch should not error")
 	assert.Equal(t, "test-message", config.Message, "fetched message should match environment")
 }
+
+func unsetEnv(t *testing.T, key string) {
+	t.Helper()
+
+	// ensure state is restored correctly
+	currValue, exists := os.LookupEnv(key)
+	t.Cleanup(func() {
+		if exists {
+			os.Setenv(key, currValue)
+		} else {
+			os.Unsetenv(key)
+		}
+	})
+
+	// clear the value
+	os.Unsetenv(key)
+}