[MNG-5102] Add support for POM mixins

Author: gnodet

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

@@ -112,6 +112,20 @@
           </association>
         </field>
 
+        <!-- ====================================================================== -->
+        <!-- Mixins                                                                 -->
+        <!-- ====================================================================== -->
+
+        <field xdoc.separator="blank">
+          <name>mixins</name>
+          <version>4.1.0+</version>
+          <description>Mixins...</description>
+          <association>
+            <type>Parent</type>
+            <multiplicity>*</multiplicity>
+          </association>
+        </field>
+
         <!-- ====================================================================== -->
         <!-- groupId/artifactId/Version/Packaging                                   -->
         <!-- ====================================================================== -->

compat/maven-model-builder/src/main/java/org/apache/maven/model/building/FileToRawModelMerger.java

@@ -138,6 +138,12 @@ protected void mergeModel_Profiles(
                 .collect(Collectors.toList()));
     }
 
+    @Override
+    protected void mergeModel_Mixins(
+            Model.Builder builder, Model target, Model source, boolean sourceDominant, Map<Object, Object> context) {
+        // don't merge
+    }
+
     @Override
     protected void mergeModelBase_Dependencies(
             ModelBase.Builder builder,

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java

@@ -210,6 +210,7 @@ static Model transformNonPom(Model model, MavenProject project) {
                                 .preserveModelVersion(false)
                                 .root(false)
                                 .parent(null)
+                                .mixins(null)
                                 .build(null),
                         model)
                 .mailingLists(null)

impl/maven-impl/src/main/java/org/apache/maven/api/services/model/ModelResolver.java

@@ -87,6 +87,13 @@ record ModelResolverRequest(
             @Nonnull String version,
             @Nullable String classifier)
             implements Request<Session> {
+        public ModelResolverRequest {
+            Objects.requireNonNull(session, "session cannot be null");
+            Objects.requireNonNull(groupId, "groupId cannot be null");
+            Objects.requireNonNull(artifactId, "artifactId cannot be null");
+            Objects.requireNonNull(version, "version cannot be null");
+        }
+
         @Nonnull
         @Override
         public Session getSession() {

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultInheritanceAssembler.java

@@ -194,6 +194,16 @@ private void concatPath(StringBuilder url, String path) {
             }
         }
 
+        @Override
+        protected void mergeModel_Mixins(
+                Model.Builder builder,
+                Model target,
+                Model source,
+                boolean sourceDominant,
+                Map<Object, Object> context) {
+            // do not merge
+        }
+
         @Override
         protected void mergeModelBase_Properties(
                 ModelBase.Builder builder,

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelBuilder.java

@@ -840,12 +840,11 @@ void buildEffectiveModel(Collection<String> importIds) throws ModelBuilderExcept
             }
         }
 
-        Model readParent(Model childModel, DefaultProfileActivationContext profileActivationContext) {
+        Model readParent(Model childModel, Parent parent, DefaultProfileActivationContext profileActivationContext) {
             Model parentModel;
 
-            Parent parent = childModel.getParent();
             if (parent != null) {
-                parentModel = resolveParent(childModel, profileActivationContext);
+                parentModel = resolveParent(childModel, parent, profileActivationContext);
 
                 if (!"pom".equals(parentModel.getPackaging())) {
                     add(
@@ -870,23 +869,24 @@ Model readParent(Model childModel, DefaultProfileActivationContext profileActiva
             return parentModel;
         }
 
-        private Model resolveParent(Model childModel, DefaultProfileActivationContext profileActivationContext)
+        private Model resolveParent(
+                Model childModel, Parent parent, DefaultProfileActivationContext profileActivationContext)
                 throws ModelBuilderException {
             Model parentModel = null;
             if (isBuildRequest()) {
-                parentModel = readParentLocally(childModel, profileActivationContext);
+                parentModel = readParentLocally(childModel, parent, profileActivationContext);
             }
             if (parentModel == null) {
-                parentModel = resolveAndReadParentExternally(childModel, profileActivationContext);
+                parentModel = resolveAndReadParentExternally(childModel, parent, profileActivationContext);
             }
             return parentModel;
         }
 
-        private Model readParentLocally(Model childModel, DefaultProfileActivationContext profileActivationContext)
+        private Model readParentLocally(
+                Model childModel, Parent parent, DefaultProfileActivationContext profileActivationContext)
                 throws ModelBuilderException {
             ModelSource candidateSource;
 
-            Parent parent = childModel.getParent();
             String parentPath = parent.getRelativePath();
             if (request.getRequestType() == ModelBuilderRequest.RequestType.BUILD_PROJECT) {
                 if (parentPath != null && !parentPath.isEmpty()) {
@@ -927,10 +927,9 @@ private Model readParentLocally(Model childModel, DefaultProfileActivationContex
             String version = getVersion(candidateModel);
 
             // Ensure that relative path and GA match, if both are provided
-            if (groupId == null
-                    || !groupId.equals(parent.getGroupId())
-                    || artifactId == null
-                    || !artifactId.equals(parent.getArtifactId())) {
+            if (parent.getGroupId() != null && (groupId == null || !groupId.equals(parent.getGroupId()))
+                    || parent.getArtifactId() != null
+                            && (artifactId == null || !artifactId.equals(parent.getArtifactId()))) {
                 mismatchRelativePathAndGA(childModel, groupId, artifactId);
                 return null;
             }
@@ -1002,13 +1001,12 @@ private void wrongParentRelativePath(Model childModel) {
             add(Severity.FATAL, Version.BASE, buffer.toString(), parent.getLocation(""));
         }
 
-        Model resolveAndReadParentExternally(Model childModel, DefaultProfileActivationContext profileActivationContext)
+        Model resolveAndReadParentExternally(
+                Model childModel, Parent parent, DefaultProfileActivationContext profileActivationContext)
                 throws ModelBuilderException {
             ModelBuilderRequest request = this.request;
             setSource(childModel);
 
-            Parent parent = childModel.getParent();
-
             String groupId = parent.getGroupId();
             String artifactId = parent.getArtifactId();
             String version = parent.getVersion();
@@ -1150,7 +1148,8 @@ private Model readEffectiveModel() throws ModelBuilderException {
                 profileActivationContext.setUserProperties(profileProps);
             }
 
-            Model parentModel = readParent(activatedFileModel, profileActivationContext);
+            Model parentModel =
+                    readParent(activatedFileModel, activatedFileModel.getParent(), profileActivationContext);
 
             // Now that we have read the parent, we can set the relative
             // path correctly if it was not set in the input model
@@ -1172,6 +1171,12 @@ private Model readEffectiveModel() throws ModelBuilderException {
 
             Model model = inheritanceAssembler.assembleModelInheritance(inputModel, parentModel, request, this);
 
+            // Mixins
+            for (Parent mixin : model.getMixins()) {
+                Model parent = resolveParent(model, mixin, profileActivationContext);
+                model = inheritanceAssembler.assembleModelInheritance(model, parent, request, this);
+            }
+
             // model normalization
             model = modelNormalizer.mergeDuplicates(model, request, this);
 
@@ -1544,7 +1549,7 @@ Model readAsParentModel(DefaultProfileActivationContext profileActivationContext
         private Model doReadAsParentModel(DefaultProfileActivationContext profileActivationContext)
                 throws ModelBuilderException {
             Model raw = readRawModel();
-            Model parentData = readParent(raw, profileActivationContext);
+            Model parentData = readParent(raw, raw.getParent(), profileActivationContext);
             Model parent = new DefaultInheritanceAssembler(new DefaultInheritanceAssembler.InheritanceModelMerger() {
                         @Override
                         protected void mergeModel_Modules(

impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelValidator.java

@@ -356,6 +356,11 @@ public void validateFileModel(Model m, int validationLevel, ModelProblemCollecto
             }
         }
 
+        for (Parent mixin : m.getMixins()) {
+            // TODO: validate mixin
+            mixin.getId();
+        }
+
         if (validationLevel == ModelValidator.VALIDATION_LEVEL_MINIMAL) {
             // profiles: they are essential for proper model building (may contribute profiles, dependencies...)
             HashSet<String> minProfileIds = new HashSet<>();