servlet: fix write when not ready in AsyncServletOutputStreamWriter

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

@@ -222,12 +222,24 @@ private void runOrBuffer(ActionItem actionItem) throws IOException {
       if (actionItem == completeAction) {
         return;
       }
-      if (!isReady.getAsBoolean()) {
+      if (actionItem == flushAction) {
+        // flush path: only set readyAndDrained=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 {
+        // writeBytes path: always set readyAndDrained=false even when isReady()
+        // returns true. Tomcat requires onWritePossible() to fire between writes.
         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");
+        log.finest("direct write: cleared readyAndDrained, next writes buffered");
       }
     } else { // buffer to the writeChain
       writeChain.offer(actionItem);
@@ -274,9 +286,10 @@ 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;
 

servlet/src/test/java/io/grpc/servlet/AsyncServletOutputStreamWriterTest.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2026 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc.servlet;
+
+import static org.junit.Assert.assertEquals;
+
+import io.grpc.servlet.AsyncServletOutputStreamWriter.ActionItem;
+import io.grpc.servlet.AsyncServletOutputStreamWriter.Log;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.BiFunction;
+import java.util.function.BooleanSupplier;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * Unit test for {@link AsyncServletOutputStreamWriter} with a mock isReady supplier.
+ * Tests two scenarios: (1) writes with isReady always true, where onWritePossible()
+ * is called between writes; (2) writes with isReady becoming false, where buffered
+ * writes are only drained when onWritePossible() is called.
+ */
+@RunWith(JUnit4.class)
+public class AsyncServletOutputStreamWriterTest {
+
+  /**
+   * Test that multiple consecutive writes succeed when isReady() always returns true
+   * and the container calls onWritePossible() between writes.
+   *
+   * <p>Each writeBytes() call is followed by onWritePossible() to drain any buffered
+   * writes. This validates that the writer can handle the Tomcat-style callback pattern
+   * where isReady() stays true but onWritePossible() fires between writes.
+   */
+  @Test
+  public void writeBytes_onWritePossibleBetweenWrites_succeeds() throws IOException {
+    AtomicBoolean isReady = new AtomicBoolean(true);
+    List<byte[]> writtenData = new ArrayList<>();
+
+    BiFunction<byte[], Integer, ActionItem> writeAction =
+        (bytes, numBytes) -> () -> {
+          writtenData.add(bytes);
+        };
+
+    ActionItem flushAction = () -> {};
+
+    ActionItem completeAction = () -> {};
+
+    BooleanSupplier isReadySupplier = () -> isReady.get();
+
+    AsyncServletOutputStreamWriter writer =
+        new AsyncServletOutputStreamWriter(writeAction, flushAction, completeAction, isReadySupplier, new Log() {});
+
+    // Initial onWritePossible to set readyAndDrained=true
+    writer.onWritePossible();
+
+    // Write 5 times, calling onWritePossible after each write.
+    // With isReady staying true, each onWritePossible should set readyAndDrained back to true,
+    // allowing the next write to go direct again.
+    for (int i = 0; i < 5; i++) {
+      byte[] data = new byte[]{(byte) i};
+      writer.writeBytes(data, 1);
+      writer.onWritePossible();
+    }
+
+    // Verify all 5 writes completed
+    assertEquals("All writes should complete", 5, writtenData.size());
+  }
+
+  /**
+   * Test that when isReady() returns false, writes are buffered and only drain
+   * when onWritePossible() is called.
+   */
+  @Test
+  public void writeBytes_isReadyFalse_buffersUntilOnWritePossible() throws IOException {
+    AtomicBoolean isReady = new AtomicBoolean(true);
+    List<String> actions = new ArrayList<>();
+
+    BiFunction<byte[], Integer, ActionItem> writeAction =
+        (bytes, numBytes) -> () -> {
+          actions.add("write");
+        };
+
+    ActionItem flushAction = () -> {
+      actions.add("flush");
+    };
+
+    ActionItem completeAction = () -> {};
+
+    BooleanSupplier isReadySupplier = () -> isReady.get();
+
+    AsyncServletOutputStreamWriter writer =
+        new AsyncServletOutputStreamWriter(writeAction, flushAction, completeAction, isReadySupplier, new Log() {});
+
+    // Initial onWritePossible to set readyAndDrained=true
+    writer.onWritePossible();
+
+    // First write - isReady is true, goes direct, readyAndDrained=false
+    byte[] data1 = new byte[]{1};
+    writer.writeBytes(data1, 1);
+
+    // Simulate isReady becoming false (container buffer full)
+    isReady.set(false);
+
+    // Second write - should be buffered since isReady=false
+    byte[] data2 = new byte[]{2};
+    writer.writeBytes(data2, 1);
+
+    // Only the first write should have executed
+    assertEquals("First write should complete, second buffered", 1, actions.size());
+
+    // Container calls onWritePossible to signal readiness
+    isReady.set(true);
+    writer.onWritePossible();
+
+    // Now both writes should have completed
+    assertEquals("Both writes should complete after onWritePossible", 2, actions.size());
+  }
+}