[MNG-6983] Plugin key can get out of sync with artifactId and groupId

The plugin key is build as combination of artifactId and groupId but not
updated if either of these two ids change.
This can be a problem if artifactId or groupId is a variable. The calculated
key will likely contain the unresolved property and is not updated once the
variable is interpolated.

This closes #372

maven-core/src/test/java/org/apache/maven/project/ProjectBuilderTest.java

@@ -34,6 +34,7 @@ import java.util.Properties;
 import org.apache.maven.AbstractCoreMavenComponentTestCase;
 import org.apache.maven.artifact.InvalidArtifactRTException;
 import org.apache.maven.execution.MavenSession;
+import org.apache.maven.model.Plugin;
 import org.apache.maven.model.building.FileModelSource;
 import org.apache.maven.model.building.ModelBuildingRequest;
 import org.apache.maven.model.building.ModelSource;
@@ -328,4 +329,17 @@ public class ProjectBuilderTest
         assertEquals( 1, project.getMailingLists().size() );
         assertEquals( 1, project.getResources().size() );
     }
+
+    public void testPropertyInPluginManagementGroupId()
+            throws Exception
+    {
+        File pom = getProject( "MNG-6983" );
+
+        MavenSession session = createMavenSession( pom );
+        MavenProject project = session.getCurrentProject();
+
+        for (Plugin buildPlugin : project.getBuildPlugins()) {
+            assertNotNull( "Missing version for build plugin " + buildPlugin.getKey(), buildPlugin.getVersion() );
+        }
+    }
 }

maven-core/src/test/projects/project-builder/MNG-6983/parent-pom.xml

@@ -0,0 +1,38 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.example</groupId>
+    <artifactId>parent</artifactId>
+    <version>0.0.1-SNAPSHOT</version>
+    <packaging>pom</packaging>
+
+    <properties>
+        <codehaus.groupId>org.codehaus.mojo</codehaus.groupId>
+    </properties>
+
+    <build>
+        <pluginManagement>
+            <plugins>
+                <plugin>
+                    <groupId>${codehaus.groupId}</groupId>
+                    <artifactId>build-helper-maven-plugin</artifactId>
+                    <version>3.2.0</version>
+                    <executions>
+                        <execution>
+                            <id>add-source-config</id>
+                            <phase>generate-sources</phase>
+                            <goals>
+                                <goal>add-source</goal>
+                            </goals>
+                            <configuration>
+                                <sources>
+                                    <source>.</source>
+                                </sources>
+                            </configuration>
+                        </execution>
+                    </executions>
+                </plugin>
+            </plugins>
+        </pluginManagement>
+    </build>
+</project>

maven-core/src/test/projects/project-builder/MNG-6983/pom.xml

@@ -0,0 +1,22 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>org.example</groupId>
+        <artifactId>parent</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>./parent-pom.xml</relativePath>
+    </parent>
+
+    <artifactId>child</artifactId>
+    <packaging>jar</packaging>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>${codehaus.groupId}</groupId>
+                <artifactId>build-helper-maven-plugin</artifactId>
+            </plugin>
+        </plugins>
+    </build>
+</project>

maven-model/src/main/mdo/maven.mdo

@@ -2314,18 +2314,12 @@
         return id.toString();
     }
 
-    //TODO we shall reset key variable when groupId/artifactId change
-    private String key = null;
     /**
      * @return the key of the plugin, ie <code>groupId:artifactId</code>
      */
     public String getKey()
     {
-        if ( key == null )
+        return constructKey( groupId, artifactId );
-        {
-            key = constructKey( groupId, artifactId );
-        }
-        return key;
     }
 
     /**