Allow TestEngines to be completely dynamic

Prior to this commit, it was impossible to create a custom TestEngine
that was completely dynamic in nature (i.e., that did not discover any
tests during the discovery phase) since such an engine would get
pruned from the test plan.

This commit addresses this issue by ensuring that engines without
statically registered children are not pruned by the Launcher.

Closes: #367

Author: sbrannen

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/Root.java

@@ -71,11 +71,10 @@ void applyPostDiscoveryFilters(LauncherDiscoveryRequest discoveryRequest) {
 	 * that do not have executable tests.
 	 *
 	 * <p>If a {@link TestEngine} ends up with no {@code TestDescriptors} after
-	 * pruning, it will be removed.
+	 * pruning, it will <strong>not</strong> be removed.
 	 */
 	void prune() {
 		acceptInAllTestEngines(REMOVE_DESCRIPTORS_WITHOUT_TESTS);
-		pruneEmptyTestEngines();
 	}
 
 	private boolean isExcluded(TestDescriptor descriptor, Filter<TestDescriptor> postDiscoveryFilter) {
@@ -86,8 +85,4 @@ private void acceptInAllTestEngines(TestDescriptor.Visitor visitor) {
 		this.testEngineDescriptors.values().forEach(descriptor -> descriptor.accept(visitor));
 	}
 
-	private void pruneEmptyTestEngines() {
-		this.testEngineDescriptors.values().removeIf(descriptor -> descriptor.getChildren().isEmpty());
-	}
-
 }

platform-tests/src/test/java/org/junit/platform/launcher/core/DefaultLauncherTests.java

@@ -35,7 +35,6 @@
 import org.junit.platform.engine.test.TestEngineSpy;
 import org.junit.platform.launcher.PostDiscoveryFilter;
 import org.junit.platform.launcher.PostDiscoveryFilterStub;
-import org.junit.platform.launcher.TagFilter;
 import org.junit.platform.launcher.TestIdentifier;
 import org.junit.platform.launcher.TestPlan;
 
@@ -71,7 +70,7 @@ void discoverEmptyTestPlanWithEngineWithoutAnyTests() {
 
 		TestPlan testPlan = launcher.discover(request().build());
 
-		assertThat(testPlan.getRoots()).isEmpty();
+		assertThat(testPlan.getRoots()).hasSize(1);
 	}
 
 	@Test
@@ -288,19 +287,4 @@ void withoutConfigurationParameters_LookupFallsBackToSystemProperty() {
 		}
 	}
 
-	@Test
-	void engineDescriptorsWithoutAnyChildrenAreNotFilteredButPruned() {
-		DefaultLauncher launcher = createLauncher(new DummyTestEngine("emptyEngine"));
-
-		// @formatter:off
-		TestPlan testPlan = launcher.discover(
-				request()
-						.selectors(selectPackage("any"))
-						.filters(TagFilter.includeTags("foo"))
-						.build());
-		// @formatter:on
-
-		assertThat(testPlan.getRoots()).isEmpty();
-	}
-
 }

platform-tests/src/test/java/org/junit/platform/runner/JUnitPlatformRunnerTests.java

@@ -418,15 +418,7 @@ public String getId() {
 
 		@Override
 		public TestDescriptor discover(EngineDiscoveryRequest discoveryRequest, UniqueId uniqueId) {
-			EngineDescriptor engineDescriptor = new EngineDescriptor(uniqueId, "Dynamic Engine");
-
-			// If we don't add this pseudo-container, the whole engine will get pruned because
-			// it otherwise won't have any (static) children.
-			TestDescriptor dynamicTestContainer = new DynamicTestContainerTestDescriptor(
-				uniqueId.append("pseudo-container", "0"), "dynamic test pseudo-container");
-			engineDescriptor.addChild(dynamicTestContainer);
-
-			return engineDescriptor;
+			return new EngineDescriptor(uniqueId, "Dynamic Engine");
 		}
 
 		@Override
@@ -462,28 +454,6 @@ public void execute(ExecutionRequest request) {
 
 	}
 
-	private static class DynamicTestContainerTestDescriptor extends AbstractTestDescriptor {
-
-		DynamicTestContainerTestDescriptor(UniqueId uniqueId, String displayName) {
-			super(uniqueId, displayName);
-		}
-
-		@Override
-		public boolean isContainer() {
-			return true;
-		}
-
-		@Override
-		public boolean isTest() {
-			return false;
-		}
-
-		@Override
-		public boolean hasTests() {
-			return true;
-		}
-	}
-
 	private static class DemoContainerTestDescriptor extends AbstractTestDescriptor {
 
 		DemoContainerTestDescriptor(UniqueId uniqueId, String displayName) {