servlet: fix write when not ready in AsyncServletOutputStreamWriter

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

@@ -222,12 +222,27 @@ private void runOrBuffer(ActionItem actionItem) throws IOException {
       if (actionItem == completeAction) {
         return;
       }
-      if (!isReady.getAsBoolean()) {
+      if (actionItem == writeAction) {
+        // For writeBytes, always set readyAndDrained to false even when isReady()
+        // returns true. Tomcat requires onWritePossible() to fire between writes,
+        // even if isReady() is still true. For flush, keep the original behavior
+        // since flush is less latency-sensitive and can safely wait for
+        // onWritePossible.
         boolean successful =
             writeState.compareAndSet(curState, curState.withReadyAndDrained(false));
         LockSupport.unpark(parkingThread);
         checkState(successful, "Bug: curState is unexpectedly changed by another thread");
         log.finest("the servlet output stream becomes not ready");
+      } else {
+        // For flush, only set to false if isReady() returns false.
+        // If isReady() is still true, keep readyAndDrained true so flush goes direct.
+        if (!isReady.getAsBoolean()) {
+          boolean successful =
+              writeState.compareAndSet(curState, curState.withReadyAndDrained(false));
+          LockSupport.unpark(parkingThread);
+          checkState(successful, "Bug: curState is unexpectedly changed by another thread");
+          log.finest("the servlet output stream becomes not ready");
+        }
       }
     } else { // buffer to the writeChain
       writeChain.offer(actionItem);
@@ -274,9 +289,10 @@ private static final class WriteState {
      * check of {@link javax.servlet.ServletOutputStream#isReady()} is true.
      *
      * <p>readyAndDrained turns from true to false when:
-     * {@code runOrBuffer()} exits while either the action item is written directly to the
-     * servlet output stream and the check of {@link javax.servlet.ServletOutputStream#isReady()}
-     * right after that returns false, or the action item is buffered into the writeChain.
+     * {@code runOrBuffer()} exits after writing directly to the servlet output stream.
+     * For writeBytes actions, it always transitions to false. For flush actions,
+     * it transitions to false only when {@link javax.servlet.ServletOutputStream#isReady()}
+     * returns false, or when the action is buffered into the writeChain.
      */
     final boolean readyAndDrained;