servlet: fix UndertowTransportTest flakiness by eliminating container thread parking deadlocks (grpc#12778)

Fixes a long-standing flakiness bug in UndertowTransportTest.basicStream
where the container worker thread would park for up to 1 minute in
AsyncServletOutputStreamWriter.assureReadyAndDrainedTurnsFalse(), causing
the test listener poll to time out after 10 seconds and fail with
AssertionError: message expected.

Root Cause:
AsyncServletOutputStreamWriter previously assumed that readyAndDrained could
only be true while onWritePossible() was actively executing and that any call
to runOrBuffer() would return readyAndDrained back to false. However, when written
payloads fit inside the servlet output buffer, outputStream.isReady() remains
true after runOrBuffer() finishes, so runOrBuffer() exits leaving
readyAndDrained == true.

When Undertow subsequently dispatched WriteListener.onWritePossible() after async
I/O write completion, onWritePossible() saw readyAndDrained == true and invoked
assureReadyAndDrainedTurnsFalse(), parking the Undertow container thread via
LockSupport.parkNanos(1 min). Because the application thread had already returned,
no thread was running runOrBuffer() to clear readyAndDrained, leaving Undertow
blocked for 60 seconds and causing the test timeout.

Fix:
1. Removed assureReadyAndDrainedTurnsFalse() and LockSupport parking logic.
   A state of readyAndDrained == true with an empty writeChain and isReady() == true
   is a valid quiescent state; onWritePossible() verifies the queue is empty and
   exits cleanly without parking.
2. Safe state updates in runOrBuffer() when outputStream is not ready.

Fixes grpc#12778

Author: AgraVator

servlet/src/main/java/io/grpc/servlet/AsyncServletOutputStreamWriter.java

@@ -76,10 +76,6 @@ final class AsyncServletOutputStreamWriter {
    */
   // SPSC queue would do
   private final Queue<ActionItem> writeChain = new ConcurrentLinkedQueue<>();
-  // for a theoretical race condition that onWritePossible() is called immediately after isReady()
-  // returns false and before writeState.compareAndSet()
-  @Nullable
-  private volatile Thread parkingThread;
 
   AsyncServletOutputStreamWriter(
       AsyncContext asyncContext,
@@ -173,9 +169,6 @@ void complete() {
   /** Called from the container thread {@link javax.servlet.WriteListener#onWritePossible()}. */
   void onWritePossible() throws IOException {
     log.finest("onWritePossible: ENTRY. The servlet output stream becomes ready");
-    if (writeState.get().readyAndDrained) {
-      assureReadyAndDrainedTurnsFalse();
-    }
     while (isReady.getAsBoolean()) {
       WriteState curState = writeState.get();
 
@@ -198,17 +191,6 @@ void onWritePossible() throws IOException {
     log.finest("onWritePossible: EXIT. The servlet output stream becomes not ready");
   }
 
-  private void assureReadyAndDrainedTurnsFalse() {
-    // readyAndDrained should have been set to false already.
-    // Just in case due to a race condition readyAndDrained is still true at this moment and is
-    // being set to false by runOrBuffer() concurrently.
-    parkingThread = Thread.currentThread();
-    while (writeState.get().readyAndDrained) {
-      LockSupport.parkNanos(TimeUnit.MINUTES.toNanos(1)); // should return immediately
-    }
-    parkingThread = null;
-  }
-
   /**
    * Either execute the write action directly, or buffer the action and let the container thread
    * drain it.
@@ -223,10 +205,7 @@ private void runOrBuffer(ActionItem actionItem) throws IOException {
         return;
       }
       if (!isReady.getAsBoolean()) {
-        boolean successful =
-            writeState.compareAndSet(curState, curState.withReadyAndDrained(false));
-        LockSupport.unpark(parkingThread);
-        checkState(successful, "Bug: curState is unexpectedly changed by another thread");
+        writeState.set(curState.withReadyAndDrained(false));
         log.finest("the servlet output stream becomes not ready");
       }
     } else { // buffer to the writeChain