The 2.4 Release Breaks Previously-Working Builds

The plugin-plugin works in other cases so the problem is likely caused by something specific to your project/environment. Could you please attach a debug log of Maven and if possible some test project to reproduce this? Just from your description it's really hard to start the bug search.

As a side note: For best build reproducibility, consider to always specify fixed versions for the plugins you employ during the build. This adds a little more clutter to the POM but frees you from such surprises. If you have a parent POM, its <pluginManagement> section is a good place to do this centrally.

Here's an example project.

Yes, I'm currently in the process of locking down all versions of our project's plugins. This seems to be quite tedious. I was trying to use the enforcer plugin, but it has its own problems:

http://jira.codehaus.org/browse/MENFORCER-25

Is there an easy way to ask "what are all my plugins I'm using and what are the current versions"? Better yet, I'd love to be able to do:

mvn plugin:lockdown-current

By the way, the Tapestry folks also ran into this issue. Have a look:

https://issues.apache.org/jira/browse/TAPESTRY-1914

Caused by change for MPLUGIN-103 applied in r543359.

Other comments on your test case:

Adding the following

    <reporting>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-plugin-plugin</artifactId>
                <version>2.4</version>
            </plugin>
        </plugins>
    </reporting>

mvn site fails with a NPE on maven-plugin-plugin 2.3 or bellow but produces your error on 2.4

I have the same error running archetypes with 2.4

The log is this:

[INFO] Apache MyFaces Trinidad Archetype
[INFO] MyFaces Archetype
[INFO] MyFaces Archetype Facelets
[INFO] MyFaces Portlet Archetype
[INFO] ------------------------------------------------------------------------
[INFO] Building Apache MyFaces Buildtools Maven2 Archetype Parent
[INFO] task-segment: [clean, install]
[INFO] ------------------------------------------------------------------------
[INFO] [clean:clean]
[INFO] [site:attach-descriptor]
[INFO] [install:install]
[INFO] Installing C:\GSOC\workspace\maven2-archetypes\pom.xml to C:\Documents and Settings\Leonardo\.m2\repository\org\apache\myfaces\buildtools\myfaces-archetype-parent\1.0.1-SNAPSHOT\myfaces-archetype-parent-1.0.1-SNAPSHOT.pom
[INFO] ------------------------------------------------------------------------
[INFO] Building JSF Components Library Archetype
[INFO] task-segment: [clean, install]
[INFO] ------------------------------------------------------------------------
[INFO] [clean:clean]
[INFO] Deleting directory C:\GSOC\workspace\maven2-archetypes\myfaces-archetype-jsfcomponents\target
[INFO] [plugin:descriptor]
[INFO] Using 2 extractors.
[INFO] Applying extractor for language: java
[INFO] Extractor for language: java found 0 mojo descriptors.
[INFO] Applying extractor for language: bsh
[INFO] Extractor for language: bsh found 0 mojo descriptors.
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error extracting plugin descriptor: 'No mojo descriptors found in plugin.'

[INFO] ------------------------------------------------------------------------
[INFO] Trace
org.apache.maven.lifecycle.LifecycleExecutionException: Error extracting plugin descriptor: 'No mojo descriptors found in plugin.'
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:564)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:480)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:459)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:311)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:278)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:143)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:333)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:126)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:282)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
Caused by: org.apache.maven.plugin.MojoExecutionException: Error extracting plugin descriptor: 'No mojo descriptors found in plugin.'
at org.apache.maven.plugin.plugin.AbstractGeneratorMojo.execute(AbstractGeneratorMojo.java:149)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:447)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:539)
... 16 more
Caused by: org.apache.maven.plugin.descriptor.InvalidPluginDescriptorException: No mojo descriptors found in plugin.
at org.apache.maven.tools.plugin.scanner.DefaultMojoScanner.populatePluginDescriptor(DefaultMojoScanner.java:107)
at org.apache.maven.plugin.plugin.AbstractGeneratorMojo.execute(AbstractGeneratorMojo.java:137)
... 18 more
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 5 seconds
[INFO] Finished at: Wed Mar 19 20:32:46 COT 2008
[INFO] Final Memory: 11M/254M
[INFO] ------------------------------------------------------------------------

getting back to 2.3 make this work again

James, why do you have this in the pom of your archetype?

<packaging>maven-plugin</packaging>

That's what is pulling in the plugin-plugin. But your code doesn't have any plugin in it so plugin-plugin (correctly IMO) reports the error.

Because I followed the directions here when I created it:

http://maven.apache.org/guides/mini/guide-creating-archetypes.html

It turns out that that page is not correct. You should change the packaging to "jar" to solve your problem. I've committed a change to said document in svn.

But, it's not a jar module. It's an archetype. It seems like there might be support for a "maven-archetype" packaging:

http://jira.codehaus.org/browse/ARCHETYPE-88

However, this worked in the past just fine and apparently was the way we were supposed to do it. So, I guess this is a backward compatibility issue? If the packaging must change because of this code change, maybe the error message could give us a little hint?

Error extracting plugin descriptor: 'No mojo descriptors found in plugin (for archetypes, please use packaging jar/maven-archetype).'

I did go and ask the developers about this before I replied.

You can read the thread here:
http://www.nabble.com/Is-an-archetype-really-a-plugin--to16198757s177.html

Ok, so the old documentation was wrong? That means that there are probably lots (as many people who were willing to write their own archetypes for maven) of folks out there using maven-plugin as their packaging in their archetypes. The unfortunate thing is that this actually worked in the past and now it doesn't.

We're restoring the old functionality for one release with a huge warning:

[INFO] Extractor for language: bsh found 0 mojo descriptors.
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING] *******************************************************
[WARNING] Deprecation Alert:
[WARNING] No mojo descriptors were found in this project, which has a packaging type of maven-plugin.
[WARNING] In future versions of the plugin tools, this will fail the build.
[WARNING] If this project is an archetype, change the packaging type from maven-plugin to maven-archetype.
[WARNING] ********************************************************
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]
[WARNING]

I like it! Thanks.