Domains: allow different variations of root domain for supported records

Before, we only support an empty name (subdomain) as means of inputting the root
domain. Since some users might by mistake or out of habit input other
variations, this change will treat the following the same as empty
subdomain and add a root domain (for records that it's allowed):
@
@.example.com
@.example.com.
example.com
example.com.

Fixes #3318

client/lib/domains/dns/index.js

@@ -5,22 +5,23 @@ const includes = require( 'lodash/collection/includes' ),
 	mapValues = require( 'lodash/object/mapValues' ),
 	endsWith = require( 'lodash/string/endsWith' );
 
-function validateAllFields( fieldValues ) {
+function validateAllFields( fieldValues, domainName ) {
 	return mapValues( fieldValues, ( value, fieldName ) => {
 		const isValid = validateField( {
 			name: fieldName,
 			value: value,
 			type: fieldValues.type,
+			domainName: domainName
 		} );
 
 		return isValid ? [] : [ 'Invalid' ];
 	} );
 }
 
-function validateField( { name, value, type } ) {
+function validateField( { name, value, type, domainName } ) {
 	switch ( name ) {
 		case 'name':
-			return isValidName( value, type );
+			return isValidName( value, type, domainName );
 		case 'target':
 			return isValidDomain( value, type );
 		case 'data':
@@ -45,8 +46,8 @@ function isValidDomain( name ) {
 	return /^([a-z0-9-_]{1,63}\.)*[a-z0-9-]{1,63}\.[a-z]{2,63}$/i.test( name );
 }
 
-function isValidName( name, type ) {
-	if ( isNameEmptyAndItsAllowed( name, type ) ) {
+function isValidName( name, type, domainName ) {
+	if ( isRootDomainAndItsAllowed( name, type, domainName ) ) {
 		return true;
 	}
 
@@ -87,8 +88,8 @@ function getNormalizedData( record, selectedDomainName ) {
 function getNormalizedName( name, type, selectedDomainName ) {
 	const endsWithDomain = endsWith( name, '.' + selectedDomainName );
 
-	if ( isNameEmptyAndItsAllowed( name, type ) ) {
-		return name;
+	if ( isRootDomainAndItsAllowed( name, type, selectedDomainName ) ) {
+		return '';
 	}
 
 	if ( endsWithDomain ) {
@@ -106,8 +107,15 @@ function isIpRecord( type ) {
 	return includes( [ 'A', 'AAAA' ], type );
 }
 
-function isNameEmptyAndItsAllowed( name, type ) {
-	return ! name && includes( [ 'MX', 'SRV', 'TXT' ], type );
+function isRootDomainAndItsAllowed( name, type, domainName ) {
+	const rootDomainVariations = [
+		'@',
+		domainName,
+		domainName + '.',
+		'@.' + domainName,
+		'@.' + domainName + '.' ];
+	return ( ! name || includes( rootDomainVariations, name ) ) &&
+			includes( [ 'MX', 'SRV', 'TXT' ], type );
 }
 
 function getFieldWithDot( field ) {

client/my-sites/upgrades/domain-management/dns/dns-add-new.jsx

@@ -66,7 +66,7 @@ const DnsAddNew = React.createClass( {
 			initialFields: this.getInitialFields(),
 			onNewState: this.setFormState,
 			validatorFunction: ( fieldValues, onComplete ) => {
-				onComplete( null, validateAllFields( fieldValues ) );
+				onComplete( null, validateAllFields( fieldValues, this.props.selectedDomainName ) );
 			}
 		}	);