Engine: fix RunDialog command spawning a nested dialog state

This fixes RunDialog script command (or Dialog.Start) spawning a nested dialog state if called within a dialog option script. Something that it should not be doing.
Instead, check if we are inside the dialog, and override a option script's return value, which dialog executor will handle upon receiving control back.

Engine/ac/dialog.cpp

@@ -1425,6 +1425,13 @@ void DialogExec::Run()
 
 void do_conversation(int dlgnum)
 {
+    assert(dialogExec == nullptr);
+    if (dialogExec)
+    {
+        Debug::Printf(kDbgMsg_Error, "ERROR: tried to start a new dialog state while a dialog state is running.");
+        return;
+    }
+
     EndSkippingUntilCharStops();
 
     // AGS 2.x always makes the mouse cursor visible when displaying a dialog.
@@ -1449,6 +1456,21 @@ void do_conversation(int dlgnum)
     dialogExec = {};
 }
 
+bool is_in_dialog()
+{
+    return dialogExec != nullptr;
+}
+
+// NOTE: this is ugly, but I could not come to a better solution at the time...
+void set_dialog_option_result(int dlgopt_result)
+{
+    assert(dialogExec && dialogScriptsInst);
+    if (!dialogExec || !dialogScriptsInst)
+        return;
+
+    dialogScriptsInst->returnValue = dlgopt_result;
+}
+
 bool handle_state_change_in_dialog_request(const char *apiname, int dlgreq_retval, bool expect_dialog_request)
 {
     // Test if we are inside a dialog state AND dialog_request callback

Engine/ac/dialog.h

@@ -30,6 +30,10 @@ void Dialog_Start(ScriptDialog *sd);
 
 // Starts a dialog
 void do_conversation(int dlgnum);
+// Tells if the game is currently running a dialog
+bool is_in_dialog();
+// Assigns a return value to pass to the dialog state after current option's script have finished executing
+void set_dialog_option_result(int dlgopt_result);
 // Displays dialog options, and returns the chosen number, or CHOSE_TEXTPARSER if parser input was activated
 int show_dialog_options(int dlgnum, bool runGameLoopsInBackground);
 // Handles a dialog option, optionally "sais" its text, optionally run corresponding dialog script's entry

Engine/script/script.cpp

@@ -626,7 +626,8 @@ String make_interact_func_name(const String &base, int param, int subd)
     return fname;
 }
 
-void post_script_cleanup() {
+void post_script_cleanup()
+{
     // should do any post-script stuff here, like go to new room
     if (cc_has_error())
         quit(cc_get_error().ErrorString);
@@ -645,7 +646,6 @@ void post_script_cleanup() {
     else {
         curscript = nullptr;
     }
-    //  if (abort_executor) user_disabled_data2=aborted_ip;
 
     int old_room_number = displayed_room;
 
@@ -687,7 +687,14 @@ void post_script_cleanup() {
             load_new_game = thisData;
             return;
         case ePSARunDialog:
-            do_conversation(thisData);
+            if (is_in_dialog())
+            {
+                set_dialog_option_result(thisData);
+            }
+            else
+            {
+                do_conversation(thisData);
+            }
             break;
         case ePSARestartGame:
             cancel_all_scripts();

Engine/script/script.h

@@ -170,8 +170,8 @@ String  GetScriptName(ccInstance *sci);
 // Makes a old-style interaction function name (for interaction list "run script" command)
 String  make_interact_func_name(const String &base, int param, int subd);
 // Performs various updates to the game after script interpreter returns control to the engine.
-// Executes actions and does changes that are not executed immediately at script command, for
-// optimisation and other reasons.
+// Executes scheduled commands and does changes that are not executed immediately during script
+// (either for logical reasons, and for optimization).
 void    post_script_cleanup();
 void    quit_with_script_error(const String &fn_name);
 int     get_nivalue (InteractionCommandList *nic, int idx, int parm);