Fix loggerUsageCheck after clean

The `loggerUsageCheck` can only run on directories that exist. It was
checking whether or not the directories exists before they were built
built and then deciding to do no work. But only if you are building in
a cleaned environment which CI does, but people rarely do locally.

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

@@ -38,7 +38,7 @@ public class LoggerUsageTask extends LoggedExec {
 
     private FileCollection classpath;
 
-    private List<File> classDirectories;
+    private FileCollection classDirectories;
 
     public LoggerUsageTask() {
         project.afterEvaluate {
@@ -46,15 +46,25 @@ public class LoggerUsageTask extends LoggedExec {
             description = "Runs LoggerUsageCheck on ${classDirectories}"
             executable = new File(project.javaHome, 'bin/java')
             if (classDirectories == null) {
-                classDirectories = []
-                if (project.sourceSets.findByName("main") && project.sourceSets.main.output.classesDir.exists()) {
-                    classDirectories += [project.sourceSets.main.output.classesDir]
+                // Default to main and test class files
+                List files = []
+                // But only if the source sets that will make them exist
+                if (project.sourceSets.findByName("main")) {
+                    files.add(project.sourceSets.main.output.classesDir)
                     dependsOn project.tasks.classes
                 }
-                if (project.sourceSets.findByName("test") && project.sourceSets.test.output.classesDir.exists()) {
-                    classDirectories += [project.sourceSets.test.output.classesDir]
+                if (project.sourceSets.findByName("test")) {
+                    files.add(project.sourceSets.test.output.classesDir)
                     dependsOn project.tasks.testClasses
                 }
+                /* In an extra twist, it isn't good enough that the source set
+                 * exists. Empty source sets won't make a classes directory
+                 * which will cause the check to fail. We have to filter the
+                 * empty directories out manually. This filter is done right
+                 * before the actual logger usage check giving the rest of the
+                 * build the opportunity to actually build the directory.
+                 */
+                classDirectories = project.files(files).filter { it.exists() }
             }
             doFirst({
                 args('-cp', getClasspath().asPath, 'org.elasticsearch.test.loggerusage.ESLoggerUsageChecker')
@@ -79,11 +89,11 @@ public class LoggerUsageTask extends LoggedExec {
     }
 
     @InputFiles
-    List<File> getClassDirectories() {
+    FileCollection getClassDirectories() {
         return classDirectories
     }
 
-    void setClassDirectories(List<File> classDirectories) {
+    void setClassDirectories(FileCollection classDirectories) {
         this.classDirectories = classDirectories
     }