Bound each WebSocket fragment by what is left of the limit

Every continuation frame could be as long as the whole message limit,
and the total was checked only after appending it, so a fragmented
message could take about twice the limit. Read each fragment against
what is left, with room for a control frame between them.

test/test.cc

@@ -26458,6 +26458,69 @@ TEST(WebSocketTest, CloseDoesNotStealBytesFromConcurrentRead) {
   }
 }
 
+TEST(WebSocketTest, AssemblesFragmentsAroundAPing) {
+  std::atomic<bool> done{false};
+  std::string received;
+
+  Server svr;
+  svr.set_websocket_ping_interval(0);
+  svr.WebSocket("/ws", [&](const Request &, ws::WebSocket &ws) {
+    std::string msg;
+    if (ws.read(msg) == ws::Binary) { received = msg; }
+    done = true;
+  });
+
+  auto port = svr.bind_to_any_port("127.0.0.1");
+  std::thread t([&]() { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+  });
+  svr.wait_until_ready();
+
+  auto sock = ::socket(AF_INET, SOCK_STREAM, 0);
+  ASSERT_NE(INVALID_SOCKET, sock);
+  auto se_sock = detail::scope_exit([&] { detail::close_socket(sock); });
+  detail::set_socket_opt_time(sock, SOL_SOCKET, SO_RCVTIMEO, 5, 0);
+
+  sockaddr_in addr{};
+  addr.sin_family = AF_INET;
+  addr.sin_port = htons(static_cast<uint16_t>(port));
+  ::inet_pton(AF_INET, "127.0.0.1", &addr.sin_addr);
+  ASSERT_EQ(0,
+            ::connect(sock, reinterpret_cast<sockaddr *>(&addr), sizeof(addr)));
+
+  // Frames with an all-zero mask key, so the payload goes out verbatim.
+  auto frame = [](uint8_t first_byte, const std::string &payload) {
+    std::string f;
+    f += static_cast<char>(first_byte);
+    f += static_cast<char>(0x80 | payload.size());
+    f.append(4, '\0');
+    return f + payload;
+  };
+  std::string data = "GET /ws HTTP/1.1\r\n"
+                     "Host: 127.0.0.1\r\n"
+                     "Upgrade: websocket\r\n"
+                     "Connection: Upgrade\r\n"
+                     "Sec-WebSocket-Key: AAAAAAAAAAAAAAAAAAAAAA==\r\n"
+                     "Sec-WebSocket-Version: 13\r\n"
+                     "\r\n";
+  data += frame(0x02, "Hel"); // Binary, more to come
+  data += frame(0x89, "p");   // FIN + Ping
+  data += frame(0x80, "lo");  // FIN + Continuation
+#ifdef _WIN32
+  ::send(sock, data.data(), static_cast<int>(data.size()), 0);
+#else
+  ::send(sock, data.data(), data.size(), 0);
+#endif
+
+  for (int i = 0; i < 500 && !done; i++) {
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  }
+  ASSERT_TRUE(done);
+  EXPECT_EQ("Hello", received);
+}
+
 #ifdef CPPHTTPLIB_OPENSSL_SUPPORT
 class WebSocketSSLIntegrationTest : public ::testing::Test {
 protected: