Manually configurable minimum protocol version

Partially address #13483.  Server operators can set a minimum
protocol version to match the game requirements (or any other
restriction they may want), and it's applied as an additional
constraint on top of the baseline compatiblity range, optional
strict_protocol_version_checking, and any kick-on-join used by
the game/mods.

builtin/settingtypes.txt

@@ -796,6 +796,14 @@ bind_address (Bind address) string
 #    to new servers, but they may not support all new features that you are expecting.
 strict_protocol_version_checking (Strict protocol checking) bool false
 
+#    Define the oldest clients allowed to connect.
+#    Older clients are compatible in the sense that they will not crash when connecting
+#    to new servers, but they may not support all new features that you are expecting.
+#    This allows more fine-grained control than strict_protocol_version_checking.
+#    Minetest may still enforce its own internal minimum, and enabling
+#    strict_protocol_version_checking will effectively override this.
+protocol_version_min (Protocol version minimum) int 1 1 65535
+
 #    Specifies URL from which client fetches media instead of using UDP.
 #    $filename should be accessible from $remote_media$filename via cURL
 #    (obviously, remote_media should end with a slash).

minetest.conf.example

@@ -760,6 +760,15 @@
 #    type: bool
 # strict_protocol_version_checking = false
 
+#    Define the oldest clients allowed to connect.
+#    Older clients are compatible in the sense that they will not crash when connecting
+#    to new servers, but they may not support all new features that you are expecting.
+#    This allows more fine-grained control than strict_protocol_version_checking.
+#    Minetest may still enforce its own internal minimum, and enabling
+#    strict_protocol_version_checking will effectively override this.
+#    type: int min: 0 max: 65535
+# protocol_version_min = 1
+
 #    Specifies URL from which client fetches media instead of using UDP.
 #    $filename should be accessible from $remote_media$filename via cURL
 #    (obviously, remote_media should end with a slash).

src/defaultsettings.cpp

@@ -363,6 +363,7 @@ void set_default_settings()
 	settings->setDefault("max_packets_per_iteration", "1024");
 	settings->setDefault("port", "30000");
 	settings->setDefault("strict_protocol_version_checking", "false");
+	settings->setDefault("protocol_version_min", "1");
 	settings->setDefault("player_transfer_distance", "0");
 	settings->setDefault("max_simultaneous_block_sends_per_client", "40");
 	settings->setDefault("time_send_interval", "5");

src/network/serverpackethandler.cpp

@@ -147,9 +147,11 @@ void Server::handleCommand_Init(NetworkPacket* pkt)
 
 	client->net_proto_version = net_proto_version;
 
-	if ((g_settings->getBool("strict_protocol_version_checking") &&
-			net_proto_version != LATEST_PROTOCOL_VERSION) ||
-			net_proto_version < SERVER_PROTOCOL_VERSION_MIN ||
+	u16 min_proto = g_settings->getU16("protocol_version_min");
+	if (g_settings->getBool("strict_protocol_version_checking"))
+			min_proto = LATEST_PROTOCOL_VERSION;
+	min_proto = rangelim(min_proto, SERVER_PROTOCOL_VERSION_MIN, SERVER_PROTOCOL_VERSION_MAX);
+	if (net_proto_version < min_proto ||
 			net_proto_version > SERVER_PROTOCOL_VERSION_MAX) {
 		actionstream << "Server: A mismatched client tried to connect from " <<
 			addr_s << " proto_max=" << (int)max_net_proto_version << std::endl;

src/serverlist.cpp

@@ -50,11 +50,14 @@ void sendAnnounce(AnnounceAction action,
 		server["address"] = g_settings->get("server_address");
 	}
 	if (action != AA_DELETE) {
+		u16 min_proto = g_settings->getU16("protocol_version_min");
+		if(min_proto < SERVER_PROTOCOL_VERSION_MIN)
+			min_proto = SERVER_PROTOCOL_VERSION_MIN;
 		bool strict_checking = g_settings->getBool("strict_protocol_version_checking");
 		server["name"]         = g_settings->get("server_name");
 		server["description"]  = g_settings->get("server_description");
 		server["version"]      = g_version_string;
-		server["proto_min"]    = strict_checking ? LATEST_PROTOCOL_VERSION : SERVER_PROTOCOL_VERSION_MIN;
+		server["proto_min"]    = strict_checking ? LATEST_PROTOCOL_VERSION : min_proto;
 		server["proto_max"]    = strict_checking ? LATEST_PROTOCOL_VERSION : SERVER_PROTOCOL_VERSION_MAX;
 		server["url"]          = g_settings->get("server_url");
 		server["creative"]     = g_settings->getBool("creative_mode");