servlet: add unit test verifying AsyncServletOutputStreamWriter.onWritePossible non-blocking behavior (grpc#12778)

Author: AgraVator

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

@@ -0,0 +1,56 @@
+/*
+ * 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.assertTrue;
+
+import io.grpc.servlet.AsyncServletOutputStreamWriter.ActionItem;
+import io.grpc.servlet.AsyncServletOutputStreamWriter.Log;
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicBoolean;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link AsyncServletOutputStreamWriter}. */
+@RunWith(JUnit4.class)
+public class AsyncServletOutputStreamWriterTest {
+
+  @Test(timeout = 5000)
+  public void onWritePossibleWhenAlreadyReadyAndDrainedReturnsImmediately() throws IOException {
+    AtomicBoolean written = new AtomicBoolean(false);
+    ActionItem writeAction = () -> written.set(true);
+
+    AsyncServletOutputStreamWriter writer =
+        new AsyncServletOutputStreamWriter(
+            (bytes, len) -> writeAction,
+            () -> {},
+            () -> {},
+            () -> true,
+            new Log() {});
+
+    // Initial onWritePossible call turns readyAndDrained to true
+    writer.onWritePossible();
+
+    long startTime = System.nanoTime();
+    // Subsequent onWritePossible call when readyAndDrained is already true must return non-blockingly
+    writer.onWritePossible();
+    long durationMs = (System.nanoTime() - startTime) / 1_000_000;
+
+    assertTrue("onWritePossible must not park or block thread", durationMs < 1000);
+  }
+}