feat(appserver): serialize websocket writes

Author: loong10k

src/main/java/io/github/easy4j/codex/appserver/CodexWebSocketSender.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2018-present, easy-4-java (https://github.com/easy-4-java).
+ *
+ * Licensed under the Apache License, Version 2.0.
+ */
+package io.github.easy4j.codex.appserver;
+
+import java.net.http.WebSocket;
+import java.util.Objects;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+
+/**
+ * Serializes text writes for one JDK {@link WebSocket}. The JDK WebSocket
+ * contract permits only one outstanding send at a time, so every write is
+ * chained after the previous write future.
+ */
+final class CodexWebSocketSender {
+
+    private final WebSocket socket;
+    private CompletionStage<WebSocket> tail;
+
+    CodexWebSocketSender(WebSocket socket) {
+        this.socket = Objects.requireNonNull(socket, "socket");
+        this.tail = CompletableFuture.completedFuture(socket);
+    }
+
+    synchronized CompletionStage<WebSocket> send(String text) {
+        Objects.requireNonNull(text, "text");
+        CompletionStage<WebSocket> next =
+                tail.thenCompose(ignored -> socket.sendText(text, true));
+        tail = next;
+        return next;
+    }
+}