Here is the output from ps:
143699 pts/6 Sl+ 27:52 /tmp/swi-prolog-devel/src/build/src/swipl -f none --no-packs -q /tmp/swi-prolog-devel/src/swipl-8.3.7/src/test.pl --no-core thread
Heres is the output from gdb:
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Reading symbols from /tmp/swi-prolog-devel/src/build/src/swipl...
(No debugging symbols found in /tmp/swi-prolog-devel/src/build/src/swipl)
Attaching to program: /tmp/swi-prolog-devel/src/build/src/swipl, process 143699
[New LWP 143701]
[New LWP 145376]
[New LWP 145377]
[New LWP 145378]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
0x00007f18f2bda8f0 in __pthread_clockjoin_ex () from /usr/lib/libpthread.so.0
(gdb) bt
#0 0x00007f18f2bda8f0 in __pthread_clockjoin_ex () from /usr/lib/libpthread.so.0
#1 0x00007f18f32a7ee6 in pl_thread_join2_va () from /tmp/swi-prolog-devel/src/build/src/libswipl.so.8
#2 0x00007f18f32e9cae in PL_next_solution () from /tmp/swi-prolog-devel/src/build/src/libswipl.so.8
#3 0x00007f18f3327d8b in query_loop () from /tmp/swi-prolog-devel/src/build/src/libswipl.so.8
#4 0x00007f18f3327c2c in prologToplevel () from /tmp/swi-prolog-devel/src/build/src/libswipl.so.8
#5 0x000055fea96f6059 in main ()
(gdb)
Looks like a thread deadlock problem.