Plugins - use a source level annotation to provide compiler hint. Fixes

#9910

Currently an interface is used to provide a compiler hint that the application belongs to a plugin. This creates problems because the plugin then cannot be used in an earlier version of Grails. This fixes that by using a source level annotation instead.

grails-bootstrap/src/main/groovy/grails/plugins/metadata/PluginSource.java

@@ -0,0 +1,32 @@
+/*
+* Copyright 2016 the original author or authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*      http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+package grails.plugins.metadata;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Source level annotation that indicates that a given source file is part of a plugin
+ *
+ * @author Graeme Rocher
+ * @since 3.1.7
+ */
+@Retention(RetentionPolicy.SOURCE)
+@Target({ElementType.TYPE})
+public @interface PluginSource {
+}

grails-web-boot/src/main/groovy/org/grails/compiler/boot/BootInitializerClassInjector.groovy

@@ -4,6 +4,7 @@ import grails.boot.GrailsPluginApplication
 import grails.boot.config.GrailsAutoConfiguration
 import grails.compiler.ast.AstTransformer
 import grails.compiler.ast.GlobalClassInjectorAdapter
+import grails.plugins.metadata.PluginSource
 import grails.util.BuildSettings
 import groovy.transform.CompileStatic
 import org.codehaus.groovy.ast.ClassHelper
@@ -51,12 +52,18 @@ import java.lang.reflect.Modifier
 class BootInitializerClassInjector extends GlobalClassInjectorAdapter {
 
     public static final ClassNode GRAILS_CONFIGURATION_CLASS_NODE = ClassHelper.make(GrailsAutoConfiguration)
+    public static final ClassNode PLUGIN_SOURCE_ANNOTATION = ClassHelper.make(PluginSource)
 
     @Override
     void performInjectionInternal(SourceUnit source, ClassNode classNode) {
+        // if this is a plugin source, then exit
+        if( classNode.getAnnotations(PLUGIN_SOURCE_ANNOTATION) ) {
+            return
+        }
         // don't generate for plugins
         if( classNode.getNodeMetaData('isPlugin') ) return
 
+
         if(GrailsASTUtils.isAssignableFrom(GRAILS_CONFIGURATION_CLASS_NODE, classNode) && !GrailsASTUtils.isSubclassOfOrImplementsInterface(classNode, GrailsPluginApplication.name)) {
             def methods = classNode.getMethods("main")
             for(MethodNode mn in methods) {