# Safe and correct way to execute a listing or predicates in the PEngines temporary module?

**URL:** <https://swi-prolog.discourse.group/t/safe-and-correct-way-to-execute-a-listing-or-predicates-in-the-pengines-temporary-module/777>\
**Category:** Help!\
**Created:** [June 6, 2019, 1:35am UTC](https://swi-prolog.discourse.group/t/safe-and-correct-way-to-execute-a-listing-or-predicates-in-the-pengines-temporary-module/777 "2019-06-06T01:35:27Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![prodog](https://avatars.discourse-cdn.com/v4/letter/p/57b2e6/32.png) [@prodog](https://swi-prolog.discourse.group/u/prodog)\
**Post date:** [June 6, 2019, 1:35am UTC](https://swi-prolog.discourse.group/t/safe-and-correct-way-to-execute-a-listing-or-predicates-in-the-pengines-temporary-module/777/1 "2019-06-06T01:35:27Z")

</div>

I’m using: SWI-Prolog version 8.0.2 on Ubuntu Linux 18.04.

I want my PEngines code to have the following control flow:

- NodeJS: Get the user’s Prolog `user_data()` predicates from a standard database (done).
- CREATE PENGINES INSTANCE: Append the `src_text` parameter during the PEngines create call with this data (done). This restores the user data created from the last Prolog query (i.e. - it restores the user’s _state_).
- Execute the current query, which will almost always execute a number of `assert/retract` operations to a predicate named `user_data()`. As I understand it, these operations will only affect a temporary database private to the PEngines thread and that temporary database will be deleted once the PEngines instance is destroyed. (Done).
- Execute a `listing(user_data(_))` query to get the updated state of the `user_data()` database predicate. ( **Can’t do, getting a sandbox permission error** )
- NodeJS: Save the `user_data()` predicate listing from the previous step to the standard database. (Done).
- DESTROY PENGINES INSTANCE: Destroy the PEngines instance now that the current transaction with the user is over. (Done.)

I put the `listing(user_data())` in a dedicated predicate found in the code I pass in the `src_text` parameter:

```prolog
list_user_data :-
    listing(user_data(_)).

```

As stated above, I can’t execute the `list_user_data/0` call from a PEngines query using `ask()` because I get a **sandbox error**.

```prolog
No permission to call sandboxed 
`'$current_module'(_1006,_1008)'
Reachable from:
  '$syspreds':current_module(A)
  '$syspreds':generate_current_predicate(A,B,C:D)
  prolog_listing:current_predicate(A,B:C)
  prolog_listing:list_module(A,B)
  prolog_listing:listing_('d146eeb7-f2cf-42c7-8f3e-acd9a882e022':A,B)
  prolog_listing:listing('d146eeb7-f2cf-42c7-8f3e-acd9a882e022':A,B)
  'd146eeb7-f2cf-42c7-8f3e-acd9a882e022':listing(A)
  'd146eeb7-f2cf-42c7-8f3e-acd9a882e022':list_user_data

```

I was going to move `list_user_data/0` to the main server thread to the code loaded at startup, and then mark that predicate as safe with `safe_primitive/1`, but I am worried that it might list `user_data()` predicates belonging to the a database on another thread (i.e. - list the wrong user’s data). I am also worried that it might list `user_data(_)` predicates that are in the main database on the main server thread since there are a few `user_data()` facts lying around from a few server startup tests that are run when I launch the server.

What is the best and safest way for me to get the list of the `user_data()` predicates stored in the temporary database for the PEngines instance I am currently using?

---

<div class="post-metadata">

**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [June 6, 2019, 7:42am UTC](https://swi-prolog.discourse.group/t/safe-and-correct-way-to-execute-a-listing-or-predicates-in-the-pengines-temporary-module/777/2 "2019-06-06T07:42:30Z")

</div>

> [@prodog](#):
>
> What is the best and safest way for me to get the list of the `user_data()` predicates stored in the temporary database for the PEngines instance I am currently using?

Just call `user_data(X)` from the client? If X are complex terms and you want them as strings, add

```prolog
user_data_string(S) :-
    user_data(D),
    term_string(D, S).

```

---

<div class="post-metadata">

**Author:** ![prodog](https://avatars.discourse-cdn.com/v4/letter/p/57b2e6/32.png) [@prodog](https://swi-prolog.discourse.group/u/prodog)\
**Post date:** [June 6, 2019, 8:30am UTC](https://swi-prolog.discourse.group/t/safe-and-correct-way-to-execute-a-listing-or-predicates-in-the-pengines-temporary-module/777/3 "2019-06-06T08:30:09Z")

</div>

> [@jan](#):
>
> Just call `user_data(X)` from the client?

So is this a case where I use the `chunking` parameter with the `ask()` method to do an ad hoc `findall()`, so that you get all the solutions for `user_data(X)` in an efficient manner? That is a suggestion you made to me on another thread.