fix(fcp): Avoid input-output lock-order deadlock

src/main/java/network/crypta/clients/fcp/FCPConnectionOutputHandler.java

@@ -160,9 +160,11 @@ private void flushOutput(OutputStream os) throws IOException {
   }
 
   private QueueAction nextQueueAction(boolean flushedSinceLastSend) throws InterruptedException {
-    synchronized (outQueue) {
-      while (true) {
-        boolean closed = handler.isClosed();
+    while (true) {
+      // Read the handler state before taking the queue monitor to keep lock ordering consistent
+      // with input-side code paths that already hold the handler lock and then inspect outQueue.
+      boolean closed = handler.isClosed();
+      synchronized (outQueue) {
         if (!outQueue.isEmpty()) {
           return QueueAction.message(outQueue.removeFirst());
         }

src/test/java/network/crypta/clients/fcp/FCPConnectionOutputHandlerTest.java

@@ -18,10 +18,12 @@
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.atLeastOnce;
+import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
@@ -91,6 +93,27 @@ void run_whenHandlerAlreadyClosed_flushesAndClosesStream() throws IOException {
     assertTrue(outputHandler.closedOutputQueue);
   }
 
+  @Test
+  void run_whenCheckingClosedState_doesNotHoldQueueMonitor() throws IOException {
+    when(handler.getSocket()).thenReturn(socket);
+    when(socket.getOutputStream()).thenReturn(outputStream);
+    doAnswer(
+            invocation -> {
+              assertFalse(Thread.holdsLock(outputHandler.outQueue));
+              return true;
+            })
+        .when(handler)
+        .isClosed();
+
+    outputHandler.run();
+
+    verify(outputStream, atLeastOnce()).flush();
+    verify(outputStream).close();
+    verify(handler).close();
+    verify(handler).closedOutput();
+    assertTrue(outputHandler.closedOutputQueue);
+  }
+
   @Test
   void run_whenMessageQueued_sendsMessageBeforeClosing() throws IOException {
     FCPMessage message = mock(FCPMessage.class);