Persist URL fragment during client-side mobile redirection and guard …

…against infinite redirect (#5775)

assets/src/mobile-redirection.js

@@ -53,19 +53,27 @@
 		return;
 	}
 
-	const url = new URL( location.href );
+	const locationUrlObject = new URL( location.href );
+	const amphtmlUrlObject = new URL( ampUrl );
 
-	if ( url.searchParams.has( noampQueryVarName ) && noampQueryVarValue === url.searchParams.get( noampQueryVarName ) ) {
+	// Persist the URL fragment when redirecting to the AMP version. This is needed because the server-generated amphtml
+	// link has no awareness of the client-side URL target.
+	amphtmlUrlObject.hash = locationUrlObject.hash;
+
+	if ( locationUrlObject.searchParams.has( noampQueryVarName ) && noampQueryVarValue === locationUrlObject.searchParams.get( noampQueryVarName ) ) {
 		// If the noamp query param is present, remember that redirection should be disabled.
 		sessionStorage.setItem( disabledStorageKey, '1' );
-	} else {
+	} else if ( amphtmlUrlObject.href !== locationUrlObject.href ) {
 		// Otherwise, since JS is running then we know it's not an AMP page and we need to redirect to the AMP version.
+		// Nevertheless, the `url.href !== location.href` condition was added for the edge case where a caching plugin
+		// is erroneously serving a cached non-AMP page at the AMP URL, so the condition prevents an infinite redirect
+		// from ensuing. See <https://github.com/ampproject/amp-wp/issues/5767>.
 		window.stop(); // Stop loading the page! This should cancel all loading resources.
 
 		// Replace the current page with the AMP version.
-		location.replace( ampUrl );
+		location.replace( amphtmlUrlObject.href );
 	}
 }(
-	// Note: The argument here is replaced with JSON in PHP by \AmpProject\AmpWP\MobileRedirection::add_mobile_redirect_script().
+	// Note: The argument here is replaced with a JSON object literal in PHP by \AmpProject\AmpWP\MobileRedirection::add_mobile_redirect_script().
 	AMP_MOBILE_REDIRECTION,
 ) );