add 'network.find_free_port' function

src/network/Network.cpp

@@ -40,6 +40,8 @@ namespace network {
         int port,
         const ServerDatagramCallback& handler
     );
+
+    int find_free_port();
 }
 
 
@@ -90,6 +92,10 @@ Server* Network::getServer(u64id_t id, bool includePrivate) const {
     return found->second.get();
 }
 
+int Network::findFreePort() const {
+    return find_free_port();
+}
+
 u64id_t Network::connectTcp(
     const std::string& address,
     int port,

src/network/Network.hpp

@@ -88,6 +88,8 @@ namespace network {
         [[nodiscard]] Connection* getConnection(u64id_t id, bool includePrivate);
         [[nodiscard]] Server* getServer(u64id_t id, bool includePrivate) const;
 
+        int findFreePort() const;
+
         u64id_t connectTcp(
             const std::string& address,
             int port,

src/network/Sockets.cpp

@@ -443,6 +443,7 @@ public:
             closesocket(descriptor);
             throw std::runtime_error("could not bind port "+std::to_string(port));
         }
+        port = ntohs(address.sin_port);
         logger.info() << "opened server at port " << port;
         auto server =
             std::make_shared<SocketTcpServer>(id, network, descriptor, port);
@@ -721,4 +722,24 @@ namespace network {
     ) {
         return SocketUdpServer::openServer(id, network, port, handler);
     }
+
+    int find_free_port() {
+        SOCKET descriptor = socket(AF_INET, SOCK_STREAM, 0);
+        if (descriptor == -1) {
+            return -1;
+        }
+        sockaddr_in address;
+        address.sin_family = AF_INET;
+        address.sin_addr.s_addr = INADDR_ANY;
+        address.sin_port = 0;
+        if (bind(descriptor, (sockaddr*)&address, sizeof(address)) < 0) {
+            closesocket(descriptor);
+            return -1;
+        }
+        socklen_t len = sizeof(address);
+        getsockname(descriptor, (sockaddr*)&address, &len);
+        int port = ntohs(address.sin_port);
+        closesocket(descriptor);
+        return port;
+    }
 }