Subject: bug#46796: Cuirass & pointer finalization.

From: Mathieu Othacehe <[email-masked]>
Date: Sat, 27 Feb 2021 13:50:59 +0100

Hey,

Here's a Valgrind backtrace:

--8<---------------cut here---------------start------------->8---
==97844== Thread 17:
==97844== Invalid read of size 4
==97844==    at 0x114465B9: zmq::msg_t::close() (in /gnu/store/zd9lbfqa3170nsfd4177dnr38k1sjbnc-zeromq-4.3.4/lib/libzmq.so.5.2.4)
==97844==    by 0x3A58E98F: ??? 
==97844==    by 0x489AC78: chained_finalizer (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x49A16EE: GC_invoke_finalizers (in /gnu/store/iycnpxxrg8m9wf9w58d6zvp9sdby6m9d-libgc-7.6.12/lib/libgc.so.1.3.6)
==97844==    by 0x489AF08: scm_run_finalizers (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x489AF8C: finalization_thread_proc (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x488BB09: c_body (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x4913148: vm_regular_engine (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x49145B4: scm_call_n (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x4890BB9: scm_call_2 (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x48923B9: scm_c_with_exception_handler (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==    by 0x4909C3C: scm_c_catch (in /gnu/store/q8brh7j5mwy0hbrly6hjb1m3wwndxqc8-guile-3.0.5/lib/libguile-3.0.so.1.3.0)
==97844==  Address 0x7373313569316263 is not stack'd, malloc'd or (recently) free'd
--8<---------------cut here---------------end--------------->8---

It looks like the finalizer is operating on a memory region that has
already been free'd. The documentation associated with the
finalization functions in <gc.h> says:

--8<---------------cut here---------------start------------->8---
        /* When obj is no longer accessible, invoke             */
        /* (*fn)(obj, cd).  If a and b are inaccessible, and    */
        /* a points to b (after disappearing links have been    */
        /* made to disappear), then only a will be              */
--8<---------------cut here---------------end--------------->8---

As far as I understand, OBJ is the wrapped pointer to the bytevector
created in "zmq-msg-init". There's a weak reference between the pointer
and the bytevector that is introduced by "register_weak_reference" in
"bytevector->pointer".

My interrogation is: do I have the guarantee that the pointer and its
references are still readable from within the finalizer? The above
snippet says that FN is invoked when OBJ is unaccessible, but does this
mean its content may have already been collected?

Cc'ing Ludo :)

Thanks,

Mathieu