Pull Request for Issue #18543 .

Summary of Changes

This implements be possible to define multiple tags for an element's in an update server definition.

Testing Instructions

Do NOT do this on a production site! This is for testing only!

Joomla Update

• install 3.8.2
• apply this patch
• set this update server: https://joomla.septdir.ru/updateserver/list.xml
• I propose a fake update to 3.8.3
• the update should fail as the primary download url is not available
• joomla will take turns trying to download the update files until it downloads the available
• joomla will report a successful update

Extensions Update

• install 3.8.2
• apply this patch
• Download test plugin
• I propose a fake update to 1.1.0
• joomla will take turns trying to download the update files until it downloads the available
• joomla will report a successful update

Expected result

Joomla can be updated from other download url

Technical Details

On the update server part we need to define the fallback like this:

Joomla Update

<?xml version="1.0" ?>
<updates>
	<update>
		<name>Joomla! 3.8</name>
		<description>Joomla! 3.8 CMS</description>
		<element>joomla</element>
		<type>file</type>
		<version>3.8.3</version>
		<infourl title="Joomla!">https://www.joomla.org/announcements/release-news/5716-joomla-3-8-2-release.html
		</infourl>
		<downloads>
			<downloadurl type="full" format="zip">https://downloads.joomla.org/fail.zip</downloadurl>
			<downloadurl type="full" format="zip">https://downloads.joomla.org/cms/joomla3/3-8-2/Joomla_3.8.2-Stable-Update_Package.zip</downloadurl>
			<downloadurl type="full" format="zip">https://github.com/joomla/joomla-cms/releases/download/3.8.2/Joomla_3.8.2-Stable-Update_Package.zip</downloadurl>
			<downloadurl type="full" format="zip">https://downloads.joomla.org/fail2.zip</downloadurl>
			<downloadurl type="full" format="zip">https://downloads.joomla.org/fail3.zip</downloadurl>
		</downloads>
		<tags>
			<tag>stable</tag>
		</tags>
		<maintainer>Joomla! PLT</maintainer>
		<maintainerurl>https://www.joomla.org</maintainerurl>
		<section>STS</section>
		<targetplatform name="joomla" version="3.[78]"/>
		<php_minimum>5.3.10</php_minimum>
	</update>
</updates>

Extensions Update

<updates>
	<update>
		<name>Test Plguin</name>
		<description>Плагин для тестов</description>
		<element>test</element>
		<type>plugin</type>
		<folder>system</folder>
		<client>0</client>
		<version>1.1.0</version>
		<infourl title="Test Plguin">
			https://joomla.septdir.ru/updateserver/plugin.xml
		</infourl>
		<downloads>
			<downloadurl type="full" format="zip">https://joomla.septdir.ru/updateserver/fail.zip</downloadurl>
			<downloadurl type="full" format="zip">https://joomla.septdir.ru/updateserver/plg_system_test_1.0.0.zip</downloadurl>
			<downloadurl type="full" format="zip">https://joomla.septdir.ru/updateserver/fail2.zip</downloadurl>
		</downloads>
		<tags>
			<tag>stable</tag>
		</tags>
		<maintainer></maintainer>
		<maintainerurl></maintainerurl>
		<targetplatform name="joomla" version="3.[0,1,2,3,4,5,6,7,8,9]"/>
	</update>
</updates>