fix: stop append when writer is in shutdown state.

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1beta2/StreamWriter.java

@@ -251,6 +251,9 @@ private void setException(Throwable t) {
    * @return the message ID wrapped in a future.
    */
   public ApiFuture<AppendRowsResponse> append(AppendRowsRequest message) {
+    if (shutdown.get()) {
+      throw new IllegalStateException("Cannot append to a shutdown writer");
+    }
     appendAndRefreshAppendLock.lock();
     Preconditions.checkState(!shutdown.get(), "Cannot append on a shut-down writer.");
     Preconditions.checkNotNull(message, "Message is null.");
@@ -284,14 +287,9 @@ public ApiFuture<AppendRowsResponse> append(AppendRowsRequest message) {
    * @throws Exception
    */
   public void flushAll(long timeoutMillis) throws Exception {
-    appendAndRefreshAppendLock.lock();
-    try {
-      writeAllOutstanding();
-      synchronized (messagesWaiter) {
-        messagesWaiter.waitComplete(timeoutMillis);
-      }
-    } finally {
-      appendAndRefreshAppendLock.unlock();
+    writeAllOutstanding();
+    synchronized (messagesWaiter) {
+      messagesWaiter.waitComplete(timeoutMillis);
     }
     exceptionLock.lock();
     try {

google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1beta2/StreamWriterTest.java

@@ -1049,4 +1049,28 @@ public void testShutdownWithConnectionError() throws Exception {
       assertEquals("Request aborted due to previous failures", e.getCause().getMessage());
     }
   }
+
+  @Test
+  public void testAppendAfterShutdown() throws Exception {
+    StreamWriter writer =
+        getTestStreamWriterBuilder()
+            .setBatchingSettings(
+                StreamWriter.Builder.DEFAULT_BATCHING_SETTINGS
+                    .toBuilder()
+                    .setElementCountThreshold(1L)
+                    .build())
+            .build();
+    testBigQueryWrite.addResponse(
+        AppendRowsResponse.newBuilder()
+            .setAppendResult(
+                AppendRowsResponse.AppendResult.newBuilder().setOffset(Int64Value.of(1)).build())
+            .build());
+    writer.shutdown();
+    try {
+      ApiFuture<AppendRowsResponse> appendFuture1 = sendTestMessage(writer, new String[] {"A"});
+      fail("Should fail with exception");
+    } catch (IllegalStateException e) {
+      assertEquals("Cannot append to a shutdown writer", e.getMessage());
+    }
+  }
 }