Build: Fix Java9 MR build (#29312)

Correctly setup classpath/dependencies and fix checkstyle task that was partly broken because delayed setup of Java9 sourcesets. This also cleans packaging of META-INF. It also prepares forbiddenapis 2.6 upgrade

relates #29292

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -551,7 +551,7 @@ class BuildPlugin implements Plugin<Project> {
                 if (project.licenseFile == null || project.noticeFile == null) {
                     throw new GradleException("Must specify license and notice file for project ${project.path}")
                 }
-                jarTask.into('META-INF') {
+                jarTask.metaInf {
                     from(project.licenseFile.parent) {
                         include project.licenseFile.name
                     }

buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/PrecommitTasks.groovy

@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.gradle.precommit
 
+import de.thetaphi.forbiddenapis.gradle.CheckForbiddenApis
 import de.thetaphi.forbiddenapis.gradle.ForbiddenApisPlugin
 import org.gradle.api.Project
 import org.gradle.api.Task
@@ -83,17 +84,14 @@ class PrecommitTasks {
                               getClass().getResource('/forbidden/es-all-signatures.txt')]
             suppressAnnotations = ['**.SuppressForbidden']
         }
-        Task mainForbidden = project.tasks.findByName('forbiddenApisMain')
-        if (mainForbidden != null) {
-            mainForbidden.configure {
-                signaturesURLs += getClass().getResource('/forbidden/es-server-signatures.txt')
-            }
-        }
-        Task testForbidden = project.tasks.findByName('forbiddenApisTest')
-        if (testForbidden != null) {
-            testForbidden.configure {
-                signaturesURLs += getClass().getResource('/forbidden/es-test-signatures.txt')
-                signaturesURLs += getClass().getResource('/forbidden/http-signatures.txt')
+        project.tasks.withType(CheckForbiddenApis) {
+            // we do not use the += operator to add signatures, as conventionMappings of Gradle do not work when it's configured using withType:
+            if (name.endsWith('Test')) {
+                signaturesURLs = project.forbiddenApis.signaturesURLs +
+                    [ getClass().getResource('/forbidden/es-test-signatures.txt'), getClass().getResource('/forbidden/http-signatures.txt') ]
+            } else {
+                signaturesURLs = project.forbiddenApis.signaturesURLs +
+                    [ getClass().getResource('/forbidden/es-server-signatures.txt') ]
             }
         }
         Task forbiddenApis = project.tasks.findByName('forbiddenApis')
@@ -144,21 +142,15 @@ class PrecommitTasks {
             ]
             toolVersion = 7.5
         }
-        for (String taskName : ['checkstyleMain', 'checkstyleJava9', 'checkstyleTest']) {
-            Task task = project.tasks.findByName(taskName)
-            if (task != null) {
-                project.tasks['check'].dependsOn.remove(task)
-                checkstyleTask.dependsOn(task)
-                task.dependsOn(copyCheckstyleConf)
-                task.inputs.file(checkstyleSuppressions)
-                task.reports {
-                    html.enabled false
-                }
-            }
-        }
 
-        project.tasks.withType(Checkstyle) {
-            dependsOn(copyCheckstyleConf)
+        project.tasks.withType(Checkstyle) { task ->
+            project.tasks[JavaBasePlugin.CHECK_TASK_NAME].dependsOn.remove(task)
+            checkstyleTask.dependsOn(task)
+            task.dependsOn(copyCheckstyleConf)
+            task.inputs.file(checkstyleSuppressions)
+            task.reports {
+                html.enabled false
+            }
         }
 
         return checkstyleTask

server/build.gradle

@@ -45,14 +45,30 @@ if (!isEclipse && !isIdea) {
       }
     }
   }
+  
+  configurations {
+    java9Compile.extendsFrom(compile)
+  }
+  
+  dependencies {
+    java9Compile sourceSets.main.output
+  }
 
   compileJava9Java {
     sourceCompatibility = 9
     targetCompatibility = 9
   }
+  
+  /* Enable this when forbiddenapis was updated to 2.6.
+   * See: https://github.com/elastic/elasticsearch/issues/29292
+  forbiddenApisJava9 {
+    targetCompatibility = 9
+  }
+  */
 
   jar {
-    into('META-INF/versions/9') {
+    metaInf {
+      into 'versions/9'
       from sourceSets.java9.output
     }
     manifest.attributes('Multi-Release': 'true')