display-plugin-updates: Inherited plugin version not detected

[kwin]

When cloning https://github.com/apache/sling-parent and executing mvn org.codehaus.mojo:versions-maven-plugin:2.9.0:display-plugin-updates it emits

[WARNING] The following plugins do not have their version specified:
[WARNING]   maven-jar-plugin ......................................... 3.2.2

for the module sling-bundle-parent, although the version of this plugin is managed in https://github.com/apache/sling-parent/blob/b7104da953b090821549d23f9bdf1ea1d002a1d8/sling-parent/pom.xml#L249 (the parent).

[stefanseifert]

it also happens with version 2.8.1 of the plugin.

strange issue. it works for other plugins which are declared in the same way.

from time to time i've observed this in very few other projects as well, e.g. https://github.com/wcm-io/io.wcm.maven.parent
in that context it seems to be related when the same plugin is also used in profiles without version number (which is correct but seems to be misinterpreted by the plugin). but it's not the same case here.

[kwin]

I have only seen this in projects where the pom.xml in the parent folder is not the parent pom.xml, i.e. separated reactor and parent, but in the example from above the reference to the parent is definitely correct (https://github.com/apache/sling-parent/blob/b7104da953b090821549d23f9bdf1ea1d002a1d8/sling-bundle-parent/pom.xml#L22-L25).

[stefanseifert]

#562 is my proposal to fix this issue.

while looking at the details i discovered that this issue it not an edge case, but version defined in parent POMs for plugins where never handled properly if a plugin is actually used in the local POM without a version.

#562 effectively skips the version update check for all plugins which have their version not defined in the current POM but in a parent POM. one could argue that it would be nice to also check the plugin versions defined in the parent POMS (similar to what display-dependency-updates is doing), but this would be a much bigger refactoring of this quite old and complex logic - this should be done in a separate enhancement issue if required (-> #533). so this fixes only the problem at hand to not report false positives as it does currently.