Segfault when the query variable should be a list about pyswip

[yuce]

You can pass maxresult=1 to retrieve only a single answer, like:

solu = prolog.query("get_list(3, X)", maxresult=1)

[moqingyan]

But this does not help solve the segfault though. It seems the segfault occurs at some clean up stage:

from pyswip import *

prolog = Prolog()
prolog.consult("test_list.pl")

solu = prolog.query("get_list(3, X)", maxresult=1)
for s in solu:
    print(s)
    break

print(s['X'][1])
print("prog end")

The corresponding output is:

{'X': [3, 2, 1]}
2
prog end
fish: 'python spec_verify.py' terminated by signal SIGSEGV (Address boundary error)

If I change to bash:

{'X': [3, 2, 1]}
2
prog end
Segmentation fault: 11

[yuce]

Can you remove the break in the loop and try again?

[moqingyan]

The program either stuck or quit unexpectedly. The terminal window closed itself when the program quits.

[yuce]

This is the code I've tried:

from pyswip import Prolog                                             
prolog = Prolog()
prolog.consult("test_list.pl")
solu = prolog.query("get_list(3, X)", maxresult=1)
for i in solu:
    print (i)

This is what I get when running it:

$ python test_list.py                       
{'X': [3, 2, 1]}
$

No segfaults, etc. (of course that doesn't rule out a bug in PySwip). This is on Ubuntu 18.04, Python 3.6.8, SWI 7.6.4. All of those using Ubuntu's packages.

Did you try your code using SWI-Prolog? It doesn't terminate after the first solution.

[moqingyan]

Yes, I have tried it with the SWI-Prolog and it worked normally. I think I can try to install the packages again on Ubuntu system and see whether the problem is MacOSX specific. :)

[yuce]

This is what I've tried with SWI-Prolog:

?- [test_list].                                                       
true.                                                                 

?- get_list(3, X).                                                    
X = [3, 2, 1] ;  <-- doesn't terminate after that

What do you get after [3, 2, 1] ?

[fpopineau]

Hi there,
I can confirm that there is definitely some problem here. I encountered a similar situation with a segv on both Windows and MacOSX, with SWI-PL 7.6 and 8.0. I get this result on the test case:

$ python test_list.py
{'X': [3, 2, 1]}
Exception ignored in: <generator object Prolog._QueryWrapper.__call__ at 0x0000019B3385F580>
Traceback (most recent call last):
  File "C:\Local\PyConfig\envs\aienv\lib\site-packages\pyswip\prolog.py", line 130, in __call__
OSError: exception: access violation reading 0x0000000000000110

It seems that an exception occurs inside the finally clause of the call to Prolog, precisely in the call to PL_cut_query(swipl_qid).
I tried to replace this call with PL_close_query(swipl_qid) but the result is the same.

Any idea of what can cause that ?
Regards,

[fpopineau]

More on this issue. I get a constant segfault if I run the query this way:

q = prolog.query("get_list(3, X)", maxresult=1)
solu = next(q)
print(solu['X'])

but everything is perfect if I run it this way:

solu = next(prolog.query("get_list(3, X)", maxresult=1))
print(solu['X'])

The difference is about when the finally clause is called. In the later case, the finally clause is called before printing the term, whereas it is called after printing it int the former case.

And actually the reason lies in a not so well known behavior of Python generators:
https://stackoverflow.com/questions/56062909/try-finally-in-python-3-generator