When I set $wgReadOnly='reaonly reason', I recieve a error when logging in.

Error stack from log:

[exception] [928a82ffc7fbeb742806306f] /index.php?title=--- Wikimedia\Rdbms\DBReadOnlyError from line 1121 of /etc/mediawiki/includes/libs/rdbms/database/Database.php: Database is read-only: This wiki is in readonly mode.
#0 /etc/mediawiki/includes/libs/rdbms/database/Database.php(2010): Wikimedia\Rdbms\Database->query(string, string)
#1 /etc/mediawiki/extensions/LdapAuthentication/LdapAuthenticationPlugin.php(2170): Wikimedia\Rdbms\Database->update(string, array, array, string)
#2 /etc/mediawiki/extensions/LdapAuthentication/LdapAuthenticationPlugin.php(1260): LdapAuthenticationPlugin::saveDomain(User, string)
#3 /etc/mediawiki/includes/auth/AuthPluginPrimaryAuthenticationProvider.php(145): LdapAuthenticationPlugin->updateUser(User)
#4 /etc/mediawiki/includes/Hooks.php(177): MediaWiki\Auth\AuthPluginPrimaryAuthenticationProvider->onUserLoggedIn(User)
#5 /etc/mediawiki/includes/Hooks.php(205): Hooks::callHook(string, array, array, NULL)
#6 /etc/mediawiki/includes/auth/AuthManager.php(2392): Hooks::run(string, array)
#7 /etc/mediawiki/includes/auth/AuthManager.php(691): MediaWiki\Auth\AuthManager->setSessionDataForUser(User, boolean)
#8 /etc/mediawiki/includes/auth/AuthManager.php(383): MediaWiki\Auth\AuthManager->continueAuthentication(array)
#9 /etc/mediawiki/includes/specialpage/AuthManagerSpecialPage.php(353): MediaWiki\Auth\AuthManager->beginAuthentication(array, string)
#10 /etc/mediawiki/includes/specialpage/AuthManagerSpecialPage.php(482): AuthManagerSpecialPage->performAuthenticationStep(string, array)
#11 /etc/mediawiki/includes/htmlform/HTMLForm.php(660): AuthManagerSpecialPage->handleFormSubmit(array, VFormHTMLForm)
#12 /etc/mediawiki/includes/specialpage/AuthManagerSpecialPage.php(416): HTMLForm->trySubmit()
#13 /etc/mediawiki/includes/specialpage/LoginSignupSpecialPage.php(316): AuthManagerSpecialPage->trySubmit()
#14 /etc/mediawiki/includes/specialpage/SpecialPage.php(522): LoginSignupSpecialPage->execute(NULL)
#15 /etc/mediawiki/includes/specialpage/SpecialPageFactory.php(568): SpecialPage->run(NULL)
#16 /etc/mediawiki/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#17 /etc/mediawiki/includes/MediaWiki.php(861): MediaWiki->performRequest()
#18 /etc/mediawiki/includes/MediaWiki.php(524): MediaWiki->main()
#19 /etc/mediawiki/index.php(42): MediaWiki->run()
#20 {main}

Looks like the login itself works well for existing users but the problem is extension tries to update domain for user even when the domain is the same as it was before.

Probably we can just add another if to check whether we need to update $olddomain to $domain:

static function saveDomain( $user, $domain ) {
	$user_id = $user->getId();
	if ( $user_id != 0 ) {
		$dbw = wfGetDB( DB_MASTER );
		$olddomain = self::loadDomain( $user );
		if ( $olddomain ) {
			//Check whether we need to update domain in database or not
			if( $olddomain != $domain ){
				return $dbw->update(
					'ldap_domains',
					[ 'domain' => $domain ],
					[ 'user_id' => $user_id ],
					__METHOD__
				);
			}
		} else {
			return $dbw->insert(
				'ldap_domains',
				[
					'domain' => $domain,
					'user_id' => $user_id
				],
				__METHOD__
			);
		}
	}
	return false;
}

The change fixed problem on my lab.