engine: common: add Host_ExitInMain function that will force engine return from Host_Main() function, essentially making engine gracefully exit on Android

In future, this might be used everywhere, so users would be able to add custom
cleanup in some advanced game_launch implementation, on any platform.

engine/common/common.h

@@ -559,6 +559,7 @@ void Host_Error( const char *error, ... ) FORMAT_CHECK( 1 );
 void Host_ValidateEngineFeatures( uint32_t mask, uint32_t features );
 void Host_Frame( double time );
 void Host_Credits( void );
+void Host_ExitInMain( void );
 
 //
 // host_state.c

engine/common/host.c

@@ -43,6 +43,27 @@ GNU General Public License for more details.
 static pfnChangeGame	pChangeGame = NULL;
 host_parm_t		host;	// host parms
 
+#if XASH_ANDROID
+static jmp_buf return_from_main_buf;
+
+/*
+===============
+Host_ExitInMain
+
+On some platforms (e.g. Android) we can't exit with exit(3) as calling it would
+kill wrapper process (e.g. app_process) too early, before all resources would
+be freed, contexts released, files closed, etc, etc...
+
+To fix this, we create jmp_buf in Host_Main function, when jumping into with
+non-zero value will immediately return from it with `error_on_exit`.
+===============
+*/
+void Host_ExitInMain( void )
+{
+	longjmp( return_from_main_buf, 1 );
+}
+#endif // XASH_ANDROID
+
 #ifdef XASH_ENGINE_TESTS
 struct tests_stats_s tests_stats;
 #endif
@@ -1319,6 +1340,11 @@ int EXPORT Host_Main( int argc, char **argv, const char *progname, int bChangeGa
 	// check after all configs were executed
 	HPAK_CheckIntegrity( hpk_custom_file.string );
 
+#if XASH_ANDROID
+	if( setjmp( return_from_main_buf ))
+		return error_on_exit;
+#endif // XASH_ANDROID
+
 	// main window message loop
 	while( !host.crashed )
 	{

engine/common/system.c

@@ -420,6 +420,12 @@ void Sys_Error( const char *error, ... )
 _exit->_Exit->asm._exit->_exit
 As we do not need atexit(), just throw hidden exception
 */
+
+// Hey, you, making an Emscripten port!
+// What if we're not supposed to use exit() on Emscripten and instead we should
+// exit from the main() function? Would this fix this bug? Test this case, pls.
+#error "Read the comment above"
+
 #include <emscripten.h>
 #define exit my_exit
 void my_exit(int ret)
@@ -438,7 +444,11 @@ Sys_Quit
 void Sys_Quit( const char *reason )
 {
 	Host_ShutdownWithReason( reason );
+#if XASH_ANDROID
+	Host_ExitInMain();
+#else
 	exit( error_on_exit );
+#endif
 }
 
 /*