Improve documentation and add test for virtual thread progress in closed event loop

Author: franz1981

core/src/main/java/io/netty/loom/EventLoopScheduler.java

@@ -26,10 +26,9 @@ public EventLoopScheduler get() {
     }
 
     /**
-     * PerCarrier Read-Sub-Pollers are not created through the vThreadFactory and although the NettyScheduler
-     * can dynamically assign them to this scheduler, while running, won't see this scheduler as the current one.
-     * This means that if they try to unpark some virtual thread which belong to this scheduler,
-     * they can still wakeup the carrier thread. It's more a performance defect, but won't affect correctness.
+     * We currently lack a way to query the current thread's assigned scheduler: this is a workaround using ScopedValues.<br>
+     * The same functionality could have been achieved (in a less hacky way) using ThreadLocals, which are way more expensive, for VirtualThreads.<br>
+     * Read sub-poller(s) sadly won't work as expected with this, because are not created using the event loop scheduler factory.
      */
     public record SchedulingContext(long vThreadId, SharedRef scheduler) {
 
@@ -163,8 +162,8 @@ private void virtualThreadSchedulerLoop() {
             parkedCarrierThread = null;
          }
       }
-      // make sure the event loop thread is fully terminated
-      while (eventLoopThread.isAlive()) {
+      // make sure the event loop thread is fully terminated and all tasks are run
+      while (eventLoopThread.isAlive() || !canBlock()) {
          runExternalContinuations(RUNNING_YIELD_US);
          runEventLoopContinuation();
       }

core/src/test/java/io/netty/loom/VirtualMultithreadIoEventLoopGroupTest.java

@@ -2,7 +2,6 @@
 
 import static java.util.concurrent.StructuredTaskScope.Joiner.allSuccessfulOrThrow;
 import static org.junit.jupiter.api.Assertions.*;
-import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 import java.net.InetSocketAddress;
 import java.net.URI;
@@ -367,6 +366,22 @@ void schedulerIsNotInheritedByForkedVT() throws InterruptedException, ExecutionE
         }
     }
 
+    @Test
+    void virtualThreadCanMakeProgressEvenIfEventLoopIsClosed() throws InterruptedException, ExecutionException, BrokenBarrierException, TimeoutException {
+        var group = new VirtualMultithreadIoEventLoopGroup(1, LocalIoHandler.newFactory());
+        final var barrier = new CyclicBarrier(2);
+        final var vThreadFactory = group.submit(group::vThreadFactory).get();
+        vThreadFactory.newThread(() -> {
+            try {
+                group.shutdownGracefully().get();
+                barrier.await();
+            } catch (Throwable e) {
+                // ignore
+            }
+        }).start();
+        barrier.await(5, TimeUnit.SECONDS);
+    }
+
    @Test
    void eventLoopSchedulerCanMakeProgressIfTheEventLoopIsBlocked() throws BrokenBarrierException, InterruptedException, TimeoutException {
       var group = new VirtualMultithreadIoEventLoopGroup(1, NioIoHandler.newFactory());