jetpack: Use locking in CI coverage test (#45851)

Jetpack_Sync_Themes_Test creates some files on class setup and deletes
them on teardown. When the CI coverage test tries to run multiple copies
in parallen (for normal and multisite), these can wind up interfering
with each other.

Avoid this by locking a file when running the CI coverage test.

Author: anomiex

projects/plugins/jetpack/changelog/fix-jetpack-coverage-theme-test-collision

@@ -0,0 +1,5 @@
+Significance: patch
+Type: other
+Comment: CI: Use locking to avoid multiple instances of Jetpack_Sync_Themes_Test from interfering with each other.
+
+

projects/plugins/jetpack/tests/action-test-coverage.sh

@@ -2,6 +2,8 @@
 
 set -eo pipefail
 
+export PHPUNIT_JETPACK_TESTSUITE_IS_PARALLEL=true
+
 # To run the PHP tests in parallel, we need to create custom configs for each.
 for test in lfs multi; do
 	P="${WP_TESTS_CONFIG_FILE_PATH%.php}.$test.php"

projects/plugins/jetpack/tests/php/sync/Jetpack_Sync_Themes_Test.php

@@ -51,12 +51,32 @@ class Jetpack_Sync_Themes_Test extends Jetpack_Sync_TestBase {
 		'theme-file-sync-child',
 	);
 
+	/**
+	 * Lock file.
+	 *
+	 * @var resource|null
+	 */
+	protected static $lockfile = null;
+
 	/**
 	 * Move Dummy Themes to proper location for testing.
+	 *
+	 * @throws RuntimeException If locking is needed and fails.
 	 */
 	public static function set_up_before_class() {
 		parent::set_up_before_class();
 
+		// For CI coverage tests, use a lock file to avoid multiple copies of this test from interfering with each other.
+		if ( getenv( 'PHPUNIT_JETPACK_TESTSUITE_IS_PARALLEL' ) === 'true' ) {
+			static::$lockfile = fopen( WP_CONTENT_DIR . '/themes/.jplock', 'c+' );
+			if ( ! static::$lockfile ) {
+				throw new RuntimeException( 'Failed to open lockfile ' . WP_CONTENT_DIR . '/themes/.jplock' );
+			}
+			if ( ! flock( static::$lockfile, LOCK_EX ) ) {
+				throw new RuntimeException( 'Failed to lock lockfile ' . WP_CONTENT_DIR . '/themes/.jplock' );
+			}
+		}
+
 		// Copy themes from tests/php/files/ to wp-content/themes.
 		foreach ( static::$themes as $theme ) {
 			$source_dir = __DIR__ . '/../files/' . $theme;
@@ -86,6 +106,10 @@ public static function tear_down_after_class() {
 
 			rmdir( $dest_dir );
 		}
+
+		if ( static::$lockfile ) {
+			fclose( static::$lockfile );
+		}
 	}
 
 	/**