Fix detection of plugin updates requiring newer Maven for building th…

…an for running (#527)

Separate minimal Maven version detection for projects from requirements of plugins
This closes #526

src/it-repo/dummy-maven-plugin-3.1.pom

@@ -119,6 +119,27 @@
         </plugin>
       </plugins>
     </pluginManagement>
+    <plugins>
+      <plugin>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <version>3.0.0</version>
+        <executions>
+          <execution>
+            <id>enforce-maven</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <requireMavenVersion>
+                  <version>3.8.4</version>
+                </requireMavenVersion>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
   </build>
 
   <repositories>

src/it/it-display-plugin-updates-010-issue-526/invoker.properties

@@ -0,0 +1 @@
+invoker.goals=${project.groupId}:${project.artifactId}:${project.version}:display-plugin-updates

src/it/it-display-plugin-updates-010-issue-526/pom.xml

@@ -0,0 +1,45 @@
+<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>localhost</groupId>
+  <artifactId>it-101</artifactId>
+  <version>1.0</version>
+  <packaging>pom</packaging>
+  <name>display-plugin-updates</name>
+  <description>IT for https://github.com/mojohaus/versions-maven-plugin/issues/526</description>
+
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <groupId>localhost</groupId>
+          <artifactId>dummy-maven-plugin</artifactId>
+          <version>1.0</version>
+        </plugin><!-- defines older compatible Maven runtime compatibility than build compatibility -->
+      </plugins>
+    </pluginManagement>
+    <plugins>
+      <plugin>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <version>3.0.0</version>
+        <executions>
+          <execution>
+            <id>enforce-maven</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <requireMavenVersion>
+                  <version>3.3.9</version>
+                </requireMavenVersion>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+
+</project>

src/it/it-display-plugin-updates-010-issue-526/verify.bsh

@@ -0,0 +1,33 @@
+import java.io.*;
+import org.codehaus.plexus.util.FileUtils;
+import java.util.regex.*;
+
+try
+{
+    File file = new File( basedir, "build.log" );
+    String buf = FileUtils.fileRead( file );
+
+    Pattern p1 = Pattern.compile( "\\QNo plugins require a newer version of Maven than specified by the pom.\\E" );
+    Matcher m1 = p1.matcher( buf.toString() );
+    Pattern p2 = Pattern.compile( "\\Qlocalhost:dummy-maven-plugin\\E\\s*\\.*\\s*\\Q1.0 -> 3.1\\E" );
+    Matcher m2 = p2.matcher( buf.toString() );
+    if ( !m1.find() )
+    {
+        System.out.println( "Did not correctly detect minimum Maven build version 3.0 of this project" );
+        return false;
+    }
+    if ( !m2.find() )
+    {
+        System.out.println( "Did not suggest updating dummy-maven-plugin to version 3.1" );
+        return false;
+    }
+    System.out.println( m1.group( 0 ) );
+    System.out.println( m2.group( 0 ) );
+}
+catch( Throwable t )
+{
+    t.printStackTrace();
+    return false;
+}
+
+return true;

src/main/java/org/codehaus/mojo/versions/DisplayPluginUpdatesMojo.java

@@ -38,6 +38,7 @@
 import org.apache.maven.lifecycle.mapping.LifecycleMapping;
 import org.apache.maven.model.Model;
 import org.apache.maven.model.Plugin;
+import org.apache.maven.model.Prerequisites;
 import org.apache.maven.model.Profile;
 import org.apache.maven.model.ReportPlugin;
 import org.apache.maven.model.io.xpp3.MavenXpp3Writer;
@@ -382,7 +383,7 @@ public void execute()
         List<String> pluginUpdates = new ArrayList<>();
         List<String> pluginLockdowns = new ArrayList<>();
         ArtifactVersion curMavenVersion = runtimeInformation.getApplicationVersion();
-        ArtifactVersion specMavenVersion = new DefaultArtifactVersion( getRequiredMavenVersion( getProject(), "2.0" ) );
+        ArtifactVersion specMavenVersion = MinimalMavenBuildVersionFinder.find( getProject(), "2.0", getLog() );
         ArtifactVersion minMavenVersion = null;
         boolean superPomDrivingMinVersion = false;
         // if Maven prerequisite upgraded to a version, Map<plugin compact key, latest compatible plugin vesion>
@@ -433,8 +434,7 @@ public void execute()
                         getHelper().resolveArtifact( probe, true );
                         MavenProject pluginMavenProject =
                             projectBuilder.buildFromRepository( probe, remotePluginRepositories, localRepository );
-                        ArtifactVersion pluginRequires =
-                            new DefaultArtifactVersion( getRequiredMavenVersion( pluginMavenProject, "2.0" ) );
+                        ArtifactVersion pluginRequires = getPrerequisitesMavenVersion( pluginMavenProject );
                         if ( artifactVersion == null && compare( specMavenVersion, pluginRequires ) >= 0 )
                         {
                             // ok, newer version compatible with current specMavenVersion
@@ -492,8 +492,7 @@ public void execute()
                         getHelper().resolveArtifact( probe, true );
                         MavenProject mavenProject =
                             projectBuilder.buildFromRepository( probe, remotePluginRepositories, localRepository );
-                        ArtifactVersion requires =
-                            new DefaultArtifactVersion( getRequiredMavenVersion( mavenProject, "2.0" ) );
+                        ArtifactVersion requires = getPrerequisitesMavenVersion( mavenProject );
                         if ( minMavenVersion == null || compare( minMavenVersion, requires ) < 0 )
                         {
                             minMavenVersion = requires;
@@ -595,30 +594,14 @@ && new DefaultArtifactVersion( effectiveVersion ).compareTo( new DefaultArtifact
         logLine( false, "" );
 
         // information on minimum Maven version
-        boolean noMavenMinVersion = getRequiredMavenVersion( getProject(), null ) == null;
-        boolean noExplicitMavenMinVersion =
-            getProject().getPrerequisites() == null || getProject().getPrerequisites().getMaven() == null;
+        boolean noMavenMinVersion = MinimalMavenBuildVersionFinder.find( getProject(), null, getLog() ) == null;
         if ( noMavenMinVersion )
         {
-            getLog().warn( "Project does not define minimum Maven version, default is: 2.0" );
-        }
-        else if ( noExplicitMavenMinVersion )
-        {
-            logLine( false, "Project inherits minimum Maven version as: " + specMavenVersion );
+            getLog().warn( "Project does not define minimum Maven version required for build, default is: 2.0" );
         }
         else
         {
-            ArtifactVersion explicitMavenVersion =
-                new DefaultArtifactVersion( getProject().getPrerequisites().getMaven() );
-            if ( compare( explicitMavenVersion, specMavenVersion ) < 0 )
-            {
-                logLine( true, "Project's effective minimum Maven (from parent) is: " + specMavenVersion );
-                logLine( true, "Project defines minimum Maven version as: " + explicitMavenVersion );
-            }
-            else
-            {
-                logLine( false, "Project defines minimum Maven version as: " + specMavenVersion );
-            }
+            logLine( false, "Project requires minimum Maven version for build of: " + specMavenVersion );
         }
         logLine( false, "Plugins require minimum Maven version of: " + minMavenVersion );
         if ( superPomDrivingMinVersion )
@@ -822,13 +805,6 @@ private String compactKey( String groupId, String artifactId )
         return groupId + ":" + artifactId;
     }
 
-    private String getRequiredMavenVersion( MavenProject mavenProject, String defaultValue )
-    {
-        ArtifactVersion requiredMavenVersion = new RequiredMavenVersionFinder( mavenProject ).find();
-
-        return requiredMavenVersion == null ? defaultValue : requiredMavenVersion.toString();
-    }
-
     private static final class StackState
     {
         private final String path;
@@ -936,6 +912,26 @@ else if ( event.isEndElement() )
 
     // -------------------------- OTHER METHODS --------------------------
 
+    /**
+     * Get the minimum required Maven version of the given plugin
+     * Same logic as in https://github.com/apache/maven-plugin-tools/blob/c8ddcdcb10d342a5a5e2f38245bb569af5730c7c/maven-plugin-plugin/src/main/java/org/apache/maven/plugin/plugin/PluginReport.java#L711
+     * @param pluginProject the plugin for which to retrieve the minimum Maven version which is required
+     * @return The minimally required Maven version (never {@code null})
+     */
+    private ArtifactVersion getPrerequisitesMavenVersion( MavenProject pluginProject ) {
+        Prerequisites prerequisites = pluginProject.getPrerequisites();
+        if (null == prerequisites) {
+            return new DefaultArtifactVersion("2.0");
+        }
+
+        String prerequisitesMavenValue = prerequisites.getMaven();
+        if (null == prerequisitesMavenValue) {
+            return new DefaultArtifactVersion("2.0");
+        }
+
+        return new DefaultArtifactVersion(prerequisitesMavenValue);
+    }
+
     /**
      * Gets the build plugins of a specific project.
      *