Adjust tests to be able to build / test when using IBM J9 / OpenJ9 (#8900)

Motivation:

We should run a CI job using J9 to ensure netty also works when using different JVMs.

Modifications:

- Adjust PooledByteBufAllocatorTest to be able to complete faster when using a JVM which takes longer when joining Threads (this seems to be the case with J9).
- Skip UDT tests on J9 as UDT is not supported there.

Result:

Be able to run CI against J9.

buffer/src/test/java/io/netty/buffer/PooledByteBufAllocatorTest.java

@@ -430,8 +430,14 @@ public class PooledByteBufAllocatorTest extends AbstractByteBufAllocatorTest<Poo
                 Thread.sleep(100);
             }
         } finally {
+            // First mark all AllocationThreads to complete their work and then wait until these are complete
+            // and rethrow if there was any error.
             for (AllocationThread t : threads) {
-                t.finish();
+                t.markAsFinished();
+            }
+
+            for (AllocationThread t: threads) {
+                t.joinAndCheckForError();
             }
         }
     }
@@ -494,14 +500,17 @@ public class PooledByteBufAllocatorTest extends AbstractByteBufAllocatorTest<Poo
             }
         }
 
-        public boolean isFinished() {
+        boolean isFinished() {
             return finish.get() != null;
         }
 
-        public void finish() throws Throwable {
+        void markAsFinished() {
+            finish.compareAndSet(null, Boolean.TRUE);
+        }
+
+        void joinAndCheckForError() throws Throwable {
             try {
                 // Mark as finish if not already done but ensure we not override the previous set error.
-                finish.compareAndSet(null, Boolean.TRUE);
                 join();
             } finally {
                 releaseBuffers();
@@ -509,7 +518,7 @@ public class PooledByteBufAllocatorTest extends AbstractByteBufAllocatorTest<Poo
             checkForError();
         }
 
-        public void checkForError() throws Throwable {
+        void checkForError() throws Throwable {
             Object obj = finish.get();
             if (obj instanceof Throwable) {
                 throw (Throwable) obj;

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -74,6 +74,7 @@ public final class PlatformDependent {
 
     private static final boolean IS_WINDOWS = isWindows0();
     private static final boolean IS_OSX = isOsx0();
+    private static final boolean IS_J9_JVM = isJ9Jvm0();
 
     private static final boolean MAYBE_SUPER_USER;
 
@@ -998,6 +999,19 @@ public final class PlatformDependent {
         }
     }
 
+    /**
+     * Returns {@code true} if the running JVM is either <a href="https://developer.ibm.com/javasdk/">IBM J9</a> or
+     * <a href="https://www.eclipse.org/openj9/">Eclipse OpenJ9</a>, {@code false} otherwise.
+     */
+    public static boolean isJ9Jvm() {
+        return IS_J9_JVM;
+    }
+
+    private static boolean isJ9Jvm0() {
+        String vmName = SystemPropertyUtil.get("java.vm.name", "").toLowerCase();
+        return vmName.startsWith("ibm j9") || vmName.startsWith("eclipse openj9");
+    }
+
     private static long maxDirectMemory0() {
         long maxDirectMemory = 0;
 

testsuite/src/main/java/io/netty/testsuite/transport/udt/UDTClientServerConnectionTest.java

@@ -35,6 +35,8 @@ import io.netty.util.CharsetUtil;
 import io.netty.util.NetUtil;
 import io.netty.util.concurrent.DefaultThreadFactory;
 import io.netty.util.concurrent.GlobalEventExecutor;
+import io.netty.util.internal.PlatformDependent;
+import org.junit.Assume;
 import org.junit.Test;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -338,6 +340,8 @@ public class UDTClientServerConnectionTest {
      */
     @Test
     public void connection() throws Exception {
+        Assume.assumeFalse("Not supported on J9 JVM", PlatformDependent.isJ9Jvm());
+
         log.info("Starting server.");
         // Using LOCALHOST4 as UDT transport does not support IPV6 :(
         final Server server = new Server(new InetSocketAddress(NetUtil.LOCALHOST4, 0));