Forbid empty testing tasks (#36259)

Closes #34820

With this change we allow for no tests being ran in randomized testing
task, and forbid empty testing tasks from the testing conventions task.
We will no longer have to disable the task if all tests are muted.

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

@@ -863,7 +863,6 @@ class BuildPlugin implements Plugin<Project> {
         project.tasks.withType(RandomizedTestingTask) {task ->
             jvm "${project.runtimeJavaHome}/bin/java"
             parallelism System.getProperty('tests.jvms', project.rootProject.ext.defaultParallel)
-            ifNoTests System.getProperty('tests.ifNoTests', 'fail')
             onNonEmptyWorkDirectory 'wipe'
             leaveTemporary true
 

buildSrc/src/main/java/org/elasticsearch/gradle/precommit/TestingConventionsTasks.java

@@ -25,7 +25,6 @@ import org.gradle.api.file.FileCollection;
 import org.gradle.api.file.FileTree;
 import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.OutputFile;
-import org.gradle.api.tasks.SkipWhenEmpty;
 import org.gradle.api.tasks.TaskAction;
 import org.gradle.api.tasks.testing.Test;
 import org.gradle.api.tasks.util.PatternFilterable;
@@ -103,6 +102,22 @@ public class TestingConventionsTasks extends DefaultTask {
             final Map<String, Set<File>> classFilesPerRandomizedTestingTask = classFilesPerRandomizedTestingTask(allTestClassFiles);
             final Map<String, Set<File>> classFilesPerGradleTestTask = classFilesPerGradleTestTask();
 
+            Map<String, Set<Class<?>>> testClassesPerTask =
+                Stream.concat(
+                    classFilesPerGradleTestTask.entrySet().stream(),
+                    classFilesPerRandomizedTestingTask.entrySet().stream()
+                )
+                    .collect(
+                        Collectors.toMap(
+                            Map.Entry::getKey,
+                            entry -> entry.getValue().stream()
+                                .map(classes::get)
+                                .filter(implementsNamingConvention)
+                                .collect(Collectors.toSet())
+                        )
+                    );
+
+
             problems = collectProblems(
                 checkNoneExists(
                     "Test classes implemented by inner classes will not run",
@@ -118,6 +133,16 @@ public class TestingConventionsTasks extends DefaultTask {
                         .filter(this::seemsLikeATest)
                         .filter(implementsNamingConvention.negate())
                 ),
+                collectProblems(
+                    testClassesPerTask.entrySet().stream()
+                    .map( entry ->
+                        checkAtLeastOneExists(
+                            "test class in " + entry.getKey(),
+                            entry.getValue().stream()
+                        )
+                    )
+                    .collect(Collectors.joining())
+                ),
                 checkNoneExists(
                     "Test classes are not included in any enabled task (" +
                         Stream.concat(
@@ -215,7 +240,6 @@ public class TestingConventionsTasks extends DefaultTask {
     }
 
     @Input
-    @SkipWhenEmpty
     public Map<String, File> getTestClassNames() {
         if (testClassNames == null) {
             testClassNames = Boilerplate.getJavaSourceSets(getProject()).getByName("test").getOutput().getClassesDirs()
@@ -243,6 +267,14 @@ public class TestingConventionsTasks extends DefaultTask {
         }
     }
 
+    private String checkAtLeastOneExists(String message, Stream<? extends Class<?>> stream) {
+        if (stream.findAny().isPresent()) {
+            return "";
+        } else {
+            return "Expected at least one " + message + ", but found none.\n";
+        }
+    }
+
     private boolean seemsLikeATest(Class<?> clazz) {
         try {
             ClassLoader classLoader = clazz.getClassLoader();