Add a trick to join servers with different version of the game

src/lang/english.txt

@@ -5324,3 +5324,6 @@ STR_STATION_RATING_TOOLTIP_WAITUNITS_5                          :{GREEN}{NUM}, +
 STR_STATION_RATING_TOOLTIP_STATUE                               :Statue in town (max 10%): {STRING}
 STR_STATION_RATING_TOOLTIP_STATUE_NO                            :{GOLD}no, 0%
 STR_STATION_RATING_TOOLTIP_STATUE_YES                           :{GREEN}yes, +10%
+
+STR_IGNORE_VERSION_CHECK_WARNING : WARNING! You're about to join server running a different version of the game!
+STR_IGNORE_VERSION_CHECK_WARNING_DETAILS : OpenTTD was never indended to work in such way. If you're lucky you may be able to play it but most likely something is going to break sooner or later. Desyncs or crashes are to be expected. So proceed at your own risk and don't report any errors you encounter there.

src/network/network_client.cpp

@@ -41,6 +41,12 @@
 /* This file handles all the client-commands */
 
 void SyncCMUser(const char *msg);
+static const std::map<std::string, uint32> OPENTTD_NEWGRF_VERSIONS = {
+	{"1.7.0", 386428096},
+	{"1.7.0-RC1", 385903751},
+	{"1.6.1", 370699225},
+	{"1.6.0", 369650572},
+};
 
 /** Read some packets, and when do use that data as initial load filter. */
 struct PacketReader : LoadFilter {
@@ -308,6 +314,8 @@ CompanyID _network_join_as;
 const char *_network_join_server_password = NULL;
 /** Company password from -P argument */
 const char *_network_join_company_password = NULL;
+/** Server revision to use when fooling revision check */
+const char *_network_join_server_revision = NULL;
 
 /** Make sure the server ID length is the same as a md5 hash. */
 assert_compile(NETWORK_SERVER_ID_LENGTH == 16 * 2 + 1);
@@ -337,8 +345,23 @@ NetworkRecvStatus ClientNetworkGameSocketHandler::SendJoin()
 	SetWindowDirty(WC_NETWORK_STATUS_WINDOW, WN_NETWORK_STATUS_WINDOW_JOIN);
 
 	Packet *p = new Packet(PACKET_CLIENT_JOIN);
-	p->Send_string(_openttd_revision);
-	p->Send_uint32(_openttd_newgrf_version);
+	if (_network_join_server_revision) {
+		auto r = _network_join_server_revision;
+		p->Send_string(r);
+		auto nr = OPENTTD_NEWGRF_VERSIONS.find(r);
+		int rev;
+		if (nr != OPENTTD_NEWGRF_VERSIONS.end()) {
+			p->Send_uint32(nr->second);
+		} else if (r[0] == 'r' && r[6] == 0 && (rev = atoi(r + 1)) > 0) {
+			// assume it's nightly
+			p->Send_uint32((rev >= 27840 ? 402653184u : 385875968u) | rev);
+		} else {
+			p->Send_uint32(_openttd_newgrf_version);
+		}
+	} else {
+		p->Send_string(_openttd_revision);
+		p->Send_uint32(_openttd_newgrf_version);
+	}
 	p->Send_string(_settings_client.network.client_name); // Client name
 	p->Send_uint8 (_network_join_as);     // PlayAs
 	p->Send_uint8 (NETLANG_ANY);          // Language

src/network/network_client.h

@@ -117,6 +117,7 @@ extern CompanyID _network_join_as;
 
 extern const char *_network_join_server_password;
 extern const char *_network_join_company_password;
+extern const char *_network_join_server_revision;
 
 #endif /* ENABLE_NETWORK */
 

src/network/network_gui.cpp

@@ -14,6 +14,7 @@
 #include "../strings_func.h"
 #include "../date_func.h"
 #include "../fios.h"
+#include "../error.h"
 #include "network_client.h"
 #include "network_gui.h"
 #include "network_gamelist.h"
@@ -613,7 +614,7 @@ public:
 		this->SetWidgetDisabledState(WID_NG_JOIN, sel == NULL || // no Selected Server
 				!sel->online || // Server offline
 				sel->info.clients_on >= sel->info.clients_max || // Server full
-				!sel->info.compatible); // Revision mismatch
+				(!sel->info.compatible && !_ctrl_pressed)); // Revision mismatch
 
 		/* 'NewGRF Settings' button invisible if no NewGRF is used */
 		this->GetWidget<NWidgetStacked>(WID_NG_NEWGRF_SEL)->SetDisplayedPlane(sel == NULL || !sel->online || sel->info.grfconfig == NULL);
@@ -772,6 +773,17 @@ public:
 				if (this->server != NULL) {
 					seprintf(_settings_client.network.last_host, lastof(_settings_client.network.last_host), "%s", this->server->address.GetHostname());
 					_settings_client.network.last_port = this->server->address.GetPort();
+					if (this->server->info.compatible) {
+						_network_join_server_revision = NULL;
+					} else {
+						_network_join_server_revision = this->server->info.server_revision;
+						ShowErrorMessage(STR_IGNORE_VERSION_CHECK_WARNING,
+						                 STR_IGNORE_VERSION_CHECK_WARNING_DETAILS,
+						                 WL_WARNING,
+						                 0, 0,
+						                 NULL, 0, NULL);
+						// ShowGoalQuestion(0, 2 /* WARNING */, 2 /* OK */, "Full-tile autoroad tool is deprecated and will be removed in next release.\n Use regular autoroad tool instead.");
+					}
 					ShowNetworkLobbyWindow(this->server);
 				}
 				break;