Implement minetest.register_can_bypass_userlimit (#6369)

* Implement minetest.register_on_userlimit_check

This new callback permits to bypass the max_users parameter with new mods condition, based on player name or IP
Only one mod needs to permit it.

Move core part for builtin privileges checks to builtin

builtin/game/privileges.lua

@@ -86,3 +86,8 @@ core.register_privilege("debug", {
 	description = "Allows enabling various debug options that may affect gameplay",
 	give_to_singleplayer = false,
 })
+
+core.register_can_bypass_userlimit(function(name, ip)
+	local privs = core.get_player_privs(name)
+	return privs["server"] or privs["ban"] or privs["privs"] or privs["password"]
+end)

builtin/game/register.lua

@@ -581,6 +581,7 @@ core.registered_on_item_eats, core.register_on_item_eat = make_registration()
 core.registered_on_punchplayers, core.register_on_punchplayer = make_registration()
 core.registered_on_priv_grant, core.register_on_priv_grant = make_registration()
 core.registered_on_priv_revoke, core.register_on_priv_revoke = make_registration()
+core.registered_can_bypass_userlimit, core.register_can_bypass_userlimit = make_registration()
 
 --
 -- Compatibility for on_mapgen_init()

doc/lua_api.txt

@@ -2470,6 +2470,9 @@ Call these functions only at load time!
     * Called when `revoker` revokes the priv `priv` from `name`.
     * Note that the callback will be called twice if it's done by a player, once with revoker being the player name,
       and again with revoker being nil.
+* `minetest.register_can_bypass_userlimit(function(name, ip))`
+    * Called when `name` user connects with `ip`.
+    * Return `true` to by pass the player limit
 
 ### Other registration functions
 * `minetest.register_chatcommand(cmd, chatcommand definition)`

src/network/serverpackethandler.cpp

@@ -209,13 +209,10 @@ void Server::handleCommand_Init(NetworkPacket* pkt)
 			<< addr_s << " (peer_id=" << pkt->getPeerId() << ")" << std::endl;
 
 	// Enforce user limit.
-	// Don't enforce for users that have some admin right
+	// Don't enforce for users that have some admin right or mod permits it.
 	if (m_clients.isUserLimitReached() &&
-			!checkPriv(playername, "server") &&
+			playername != g_settings->get("name") &&
-			!checkPriv(playername, "ban") &&
+			!m_script->can_bypass_userlimit(playername, addr_s)) {
-			!checkPriv(playername, "privs") &&
-			!checkPriv(playername, "password") &&
-			playername != g_settings->get("name")) {
 		actionstream << "Server: " << playername << " tried to join from "
 				<< addr_s << ", but there" << " are already max_users="
 				<< g_settings->getU16("max_users") << " players." << std::endl;

src/script/cpp_api/s_player.cpp

@@ -21,6 +21,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "cpp_api/s_internal.h"
 #include "common/c_converter.h"
 #include "common/c_content.h"
+#include "debug.h"
 #include "util/string.h"
 
 void ScriptApiPlayer::on_newplayer(ServerActiveObject *player)
@@ -123,6 +124,20 @@ bool ScriptApiPlayer::on_prejoinplayer(
 	return false;
 }
 
+bool ScriptApiPlayer::can_bypass_userlimit(const std::string &name, const std::string &ip)
+{
+	SCRIPTAPI_PRECHECKHEADER
+
+	// Get core.registered_on_prejoinplayers
+	lua_getglobal(L, "core");
+	lua_getfield(L, -1, "registered_can_bypass_userlimit");
+	lua_pushstring(L, name.c_str());
+	lua_pushstring(L, ip.c_str());
+	runCallbacks(2, RUN_CALLBACKS_MODE_OR);
+	FATAL_ERROR_IF(!lua_isboolean(L, -1), "on_user_limitcheck must return a boolean");
+	return lua_toboolean(L, -1);
+}
+
 void ScriptApiPlayer::on_joinplayer(ServerActiveObject *player)
 {
 	SCRIPTAPI_PRECHECKHEADER

src/script/cpp_api/s_player.h

@@ -35,6 +35,7 @@ public:
 	bool on_respawnplayer(ServerActiveObject *player);
 	bool on_prejoinplayer(const std::string &name, const std::string &ip,
 			std::string *reason);
+	bool can_bypass_userlimit(const std::string &name, const std::string &ip);
 	void on_joinplayer(ServerActiveObject *player);
 	void on_leaveplayer(ServerActiveObject *player, bool timeout);
 	void on_cheat(ServerActiveObject *player, const std::string &cheat_type);