Do not use a "persistent" cache which is the root cause MNG-6530

The root of the problem is MNG-5312 which is a performance issue because a ModelCache was not always involved during a model build.  This lead to MNG-6030 (memory consumption issue because of the usage of multiple caches). The fix for MNG-6030 lead to MNG-6311 (problem with changes not being picked up in M2E).
The root problem is that the fix for MNG-5312 was wrong.  The caches and its data should be managed at the RepositorySystemSession level, especially as the data is related to a given session parameters (maven global settings).
This commit brings back the cache at the session level, and a default cache is now set on the session by the MavenCli.  The cache is used later by DefaultArtifactDescriptorReader and DefaultProjectBuilder to build a DefaultModelCache.

* use the cache on the MavenExecutionRequest
* get rid of the hack for  MNG-6530.
* MavenCli configures a cache for the whole execution request
* merge ReactorModelCache into DefaultModelCache

maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java

@@ -63,13 +63,15 @@
 import org.apache.maven.model.building.ModelBuildingException;
 import org.apache.maven.model.building.ModelBuildingRequest;
 import org.apache.maven.model.building.ModelBuildingResult;
+import org.apache.maven.model.building.ModelCache;
 import org.apache.maven.model.building.ModelProblem;
 import org.apache.maven.model.building.ModelProcessor;
 import org.apache.maven.model.building.ModelSource;
 import org.apache.maven.model.building.StringModelSource;
 import org.apache.maven.model.building.TransformerContext;
 import org.apache.maven.model.resolution.ModelResolver;
 import org.apache.maven.repository.internal.ArtifactDescriptorUtils;
+import org.apache.maven.repository.internal.DefaultModelCache;
 import org.codehaus.plexus.logging.Logger;
 import org.codehaus.plexus.util.Os;
 import org.codehaus.plexus.util.StringUtils;
@@ -91,9 +93,6 @@ public class DefaultProjectBuilder
     implements ProjectBuilder
 {
 
-    public static final String DISABLE_GLOBAL_MODEL_CACHE_SYSTEM_PROPERTY =
-            "maven.defaultProjectBuilder.disableGlobalModelCache";
-
     private final Logger logger;
 
     private final ModelBuilder modelBuilder;
@@ -110,8 +109,6 @@ public class DefaultProjectBuilder
 
     private final ProjectDependenciesResolver dependencyResolver;
 
-    private final ReactorModelCache modelCache;
-
     @Inject
     @SuppressWarnings( "checkstyle:parameternumber" )
     public DefaultProjectBuilder(
@@ -133,7 +130,6 @@ public DefaultProjectBuilder(
         this.repoSystem = repoSystem;
         this.repositoryManager = repositoryManager;
         this.dependencyResolver = dependencyResolver;
-        this.modelCache = new ReactorModelCache();
     }
 
     // ----------------------------------------------------------------------
@@ -145,20 +141,15 @@ public ProjectBuildingResult build( File pomFile, ProjectBuildingRequest request
         throws ProjectBuildingException
     {
         return build( pomFile, new FileModelSource( pomFile ),
-                new InternalConfig( request, null, useGlobalModelCache() ? getModelCache() : null ) );
-    }
-
-    private boolean useGlobalModelCache()
-    {
-        return !Boolean.getBoolean( DISABLE_GLOBAL_MODEL_CACHE_SYSTEM_PROPERTY );
+                new InternalConfig( request, null ) );
     }
 
     @Override
     public ProjectBuildingResult build( ModelSource modelSource, ProjectBuildingRequest request )
         throws ProjectBuildingException
     {
         return build( null, modelSource,
-                 new InternalConfig( request, null, useGlobalModelCache() ? getModelCache() : null ) );
+                 new InternalConfig( request, null ) );
     }
 
     private ProjectBuildingResult build( File pomFile, ModelSource modelSource, InternalConfig config )
@@ -301,6 +292,8 @@ private ModelBuildingRequest getModelBuildingRequest( InternalConfig config )
             new ProjectModelResolver( config.session, trace, repoSystem, repositoryManager, config.repositories,
                                       configuration.getRepositoryMerging(), config.modelPool );
 
+        ModelCache modelCache = DefaultModelCache.newInstance( config.session );
+
         request.setValidationLevel( configuration.getValidationLevel() );
         request.setProcessPlugins( configuration.isProcessPlugins() );
         request.setProfiles( configuration.getProfiles() );
@@ -310,7 +303,7 @@ private ModelBuildingRequest getModelBuildingRequest( InternalConfig config )
         request.setUserProperties( configuration.getUserProperties() );
         request.setBuildStartTime( configuration.getBuildStartTime() );
         request.setModelResolver( resolver );
-        request.setModelCache( config.modelCache );
+        request.setModelCache( modelCache );
         request.setTransformerContext( (TransformerContext) config.session.getData().get( TransformerContext.KEY ) );
 
         return request;
@@ -330,7 +323,7 @@ public ProjectBuildingResult build( Artifact artifact, boolean allowStubModel, P
         org.eclipse.aether.artifact.Artifact pomArtifact = RepositoryUtils.toArtifact( artifact );
         pomArtifact = ArtifactDescriptorUtils.toPomArtifact( pomArtifact );
 
-        InternalConfig config = new InternalConfig( request, null, useGlobalModelCache() ? getModelCache() : null );
+        InternalConfig config = new InternalConfig( request, null );
 
         boolean localProject;
 
@@ -427,8 +420,7 @@ public Model getRawModel( String groupId, String artifactId )
             request.getRepositorySession().getData().set( TransformerContext.KEY, context );
         }
 
-        InternalConfig config = new InternalConfig( request, modelPool,
-                useGlobalModelCache() ? getModelCache() : new ReactorModelCache() );
+        InternalConfig config = new InternalConfig( request, modelPool );
 
         Map<String, MavenProject> projectIndex = new HashMap<>( 256 );
 
@@ -1110,13 +1102,10 @@ class InternalConfig
 
         private final ReactorModelPool modelPool;
 
-        private final ReactorModelCache modelCache;
-
-        InternalConfig( ProjectBuildingRequest request, ReactorModelPool modelPool, ReactorModelCache modelCache )
+        InternalConfig( ProjectBuildingRequest request, ReactorModelPool modelPool )
         {
             this.request = request;
             this.modelPool = modelPool;
-            this.modelCache = modelCache;
             session =
                 LegacyLocalRepositoryManager.overlay( request.getLocalRepository(), request.getRepositorySession(),
                                                       repoSystem );
@@ -1125,9 +1114,4 @@ class InternalConfig
 
     }
 
-    private ReactorModelCache getModelCache()
-    {
-        return this.modelCache;
-    }
-
 }

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

@@ -140,7 +140,6 @@ public void testDontResolveDependencies()
     }
 
     public void testReadModifiedPoms() throws Exception {
-        String initialValue = System.setProperty( DefaultProjectBuilder.DISABLE_GLOBAL_MODEL_CACHE_SYSTEM_PROPERTY, Boolean.toString( true ) );
         // TODO a similar test should be created to test the dependency management (basically all usages
         // of DefaultModelBuilder.getCache() are affected by MNG-6530
 
@@ -167,14 +166,6 @@ public void testReadModifiedPoms() throws Exception {
         }
         finally
         {
-            if ( initialValue == null )
-            {
-                System.clearProperty( DefaultProjectBuilder.DISABLE_GLOBAL_MODEL_CACHE_SYSTEM_PROPERTY );
-            }
-            else
-            {
-                System.setProperty( DefaultProjectBuilder.DISABLE_GLOBAL_MODEL_CACHE_SYSTEM_PROPERTY, initialValue );
-            }
             FileUtils.deleteDirectory( tempDir.toFile() );
         }
     }

maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java

@@ -82,6 +82,7 @@
 import org.codehaus.plexus.logging.LoggerManager;
 import org.codehaus.plexus.util.StringUtils;
 import org.codehaus.plexus.util.xml.pull.XmlPullParserException;
+import org.eclipse.aether.DefaultRepositoryCache;
 import org.eclipse.aether.transfer.TransferListener;
 import org.slf4j.ILoggerFactory;
 import org.slf4j.Logger;
@@ -977,6 +978,11 @@ private int execute( CliRequest cliRequest )
     {
         MavenExecutionRequest request = executionRequestPopulator.populateDefaults( cliRequest.request );
 
+        if ( cliRequest.request.getRepositoryCache() == null )
+        {
+            cliRequest.request.setRepositoryCache( new DefaultRepositoryCache() );
+        }
+
         eventSpyDispatcher.onEvent( request );
 
         MavenExecutionResult result = maven.execute( request );