From e6a2c207de9ca2f2e21dbede197029c0b02d4896 Mon Sep 17 00:00:00 2001
From: Alibek Omarov <a1ba.omarov@gmail.com>
Date: Mon, 13 Jun 2022 03:42:20 +0300
Subject: [PATCH] engine: implement change game with execv in-engine. For now
 it enabled for all platforms, will probably disabled selectively

---
 engine/common/host.c   |  6 ++--
 engine/common/system.c | 69 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/engine/common/host.c b/engine/common/host.c
index d3ca35b8..fcac92dd 100644
--- a/engine/common/host.c
+++ b/engine/common/host.c
@@ -305,7 +305,9 @@ void Host_NewInstance( const char *name, const char *finalmsg )
 
 	host.change_game = true;
 	Q_strncpy( host.finalmsg, finalmsg, sizeof( host.finalmsg ));
-	pChangeGame( name ); // call from hl.exe
+
+	if( !Platform_ChangeGame( name ))
+		pChangeGame( name ); // call from hl.exe
 }
 
 /*
@@ -868,7 +870,7 @@ void Host_InitCommon( int argc, char **argv, const char *progname, qboolean bCha
 
 	host.enabledll = !Sys_CheckParm( "-nodll" );
 
-	host.change_game = bChangeGame;
+	host.change_game = bChangeGame || Sys_CheckParm( "-changegame" );
 	host.config_executed = false;
 	host.status = HOST_INIT; // initialzation started
 
diff --git a/engine/common/system.c b/engine/common/system.c
index c41854e8..f154f7ee 100644
--- a/engine/common/system.c
+++ b/engine/common/system.c
@@ -32,9 +32,14 @@ GNU General Public License for more details.
 #endif
 #endif
 
+#if XASH_WIN32
+#include <process.h>
+#endif
+
 #include "menu_int.h" // _UPDATE_PAGE macro
 
 #include "library.h"
+#include "whereami.h"
 
 qboolean	error_on_exit = false;	// arg for exit();
 #define DEBUG_BREAK
@@ -539,3 +544,67 @@ void Sys_Print( const char *pMsg )
 
 	Rcon_Print( pMsg );
 }
+
+/*
+==================
+Sys_ChangeGame
+
+This is a special function
+
+Here we restart engine with new -game parameter
+but since engine will be unloaded during this call
+it explicitly doesn't use internal allocation or string copy utils
+==================
+*/
+qboolean Sys_ChangeGame( const char *gamedir )
+{
+	int i = 0;
+	qboolean replacedArg = false;
+	size_t exelen;
+	char *exe, **newargs;
+
+	// don't use engine allocation utils here
+	// they will be freed after Host_Shutdown
+	newargs = calloc( host.argc + 4, sizeof( *newargs ));
+	while( i < host.argc )
+	{
+		newargs[i] = strdup( host.argv[i] );
+
+		// replace existing -game argument
+		if( !Q_stricmp( newargs[i], "-game" ))
+		{
+			newargs[i + 1] = strdup( gamedir );
+			replacedArg = true;
+			i += 2;
+		}
+		else i++;
+	}
+
+	if( !replacedArg )
+	{
+		newargs[i++] = strdup( "-game" );
+		newargs[i++] = strdup( gamedir );
+	}
+
+	newargs[i++] = strdup( "-changegame" );
+	newargs[i] = NULL;
+
+	exelen = wai_getExecutablePath( NULL, 0, NULL );
+	exe = malloc( exelen + 1 );
+	wai_getExecutablePath( exe, exelen, NULL );
+	exe[exelen] = 0;
+
+	Host_Shutdown();
+
+	execv( exe, newargs );
+
+	// if execv returned, it's probably an error
+	printf( "execv failed: %s", strerror( errno ));
+
+	for( ; i >= 0; i-- )
+		free( newargs[i] );
+	free( newargs );
+	free( exe );
+
+	return false;
+}