Tests: Add unit coverage for rest_output_link_header()

tests/phpunit/tests/rest-api/restOutputLinkHeader.php

@@ -0,0 +1,56 @@
+<?php
+
+/**
+ * Tests for rest_output_link_header().
+ *
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ * @group restapi
+ * @group xdebug
+ *
+ * @covers ::rest_output_link_header
+ */
+class Tests_REST_API_RestOutputLinkHeader extends WP_UnitTestCase {
+
+	/**
+	 * @ticket 65819
+	 */
+	public function test_outputs_nothing_to_body() {
+		ob_start();
+		rest_output_link_header();
+		$output = ob_get_clean();
+
+		$this->assertSame( '', $output );
+	}
+
+	/**
+	 * @ticket 65819
+	 *
+	 * @requires function xdebug_get_headers
+	 */
+	public function test_returns_early_when_rest_url_is_empty() {
+		add_filter( 'rest_url', '__return_empty_string' );
+
+		$headers_before = xdebug_get_headers();
+
+		rest_output_link_header();
+
+		$this->assertSame( $headers_before, xdebug_get_headers() );
+	}
+
+	/**
+	 * @ticket 65819
+	 *
+	 * @requires function xdebug_get_headers
+	 */
+	public function test_sends_rest_api_link_header() {
+		$api_root = sanitize_url( get_rest_url() );
+
+		rest_output_link_header();
+
+		$headers  = xdebug_get_headers();
+		$expected = sprintf( 'Link: <%s>; rel="https://api.w.org/"', $api_root );
+
+		$this->assertContains( $expected, $headers );
+	}
+}