Re: (guix git) and guile-git finalizers.

[Ludovic Courtès]

Hi!

Mathieu Othacehe <address@hidden> skribis:

> While integrating (guix git) into Cuirass, I noticed unexpected
> segfaults during "latest-repository-commit" evaluation.
>
> Segfaults can be narrowed to the following lines:
>
> (use-modules (git))
> (begin (libgit2-init!)
>        (let ((rep (repository-open "some-git-repo")))
>          (remote-fetch (remote-lookup rep "origin")))
>        (libgit2-shutdown))
>
>
> Note that, it's not 100% reproducible, but re-executing 4 or 5 times the
> above snippet is often enough to observe the segfault.
>
> Here's the gdb backtrace I collected :
>
> #0  0x00007fffef7eb181 in git_mwindow_put_pack () from 
> /gnu/store/ilaffvq27kfy2la235hddkxcalj9r
> #1  0x00007fffef7f2897 in pack_backend(float, long double,...)(...) () from 
> /gnu/store/ilaffvq2
> #2  0x00007fffef7eef72 in git_odb_free () from 
> /gnu/store/ilaffvq27kfy2la235hddkxcalj9r2i7-libg
> #3  0x00007fffef8134ce in git_repository.cleanup () from 
> /gnu/store/ilaffvq27kfy2la235hddkxcalj
> #4  0x00007fffef81351a in git_repository_free () from 
> /gnu/store/ilaffvq27kfy2la235hddkxcalj9r2
> #5  0x00007ffff7b01e37 in chained_finalizer () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs
> #6  0x00007ffff785170f in GC_invoke_finalizers () from 
> /gnu/store/j4p7cnxb6byf1wra3yz6pk8av4g5m
> #7  0x00007ffff7b02099 in scm_run_finalizers () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dx
> #8  0x00007ffff7b020f5 in finalization_thread_proc () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k
> #9  0x00007ffff7af2cba in c_body () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-guile-2.2.
> #10 0x00007ffff7b680c9 in vm_debug_engine () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-g
> #11 0x00007ffff7b752aa in scm_call_n () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-guile-
> #12 0x00007ffff7b644e6 in catch () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-guile-2.2.2
> #13 0x00007ffff7af32a0 in scm_i_with_continuation_barrier () from 
> /gnu/store/5zx29y44nrqj0s8h3j
> #14 0x00007ffff7af3335 in scm_c_with_continuation_barrier () from 
> /gnu/store/5zx29y44nrqj0s8h3j
> #15 0x00007ffff7b6310c in with_guile () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-guile-
> #16 0x00007ffff7858732 in GC_call_with_stack_base () from 
> /gnu/store/j4p7cnxb6byf1wra3yz6pk8av4
> #17 0x00007ffff7b63498 in scm_with_guile () from 
> /gnu/store/5zx29y44nrqj0s8h3jlvlj82k8hj4dxs-gu
> #18 0x00007ffff7625454 in start_thread () from 
> /gnu/store/rmjlycdgiq8pfy5hfi42qhw3k7p6kdav-glib
> #19 0x00007ffff5fed7bf in clone () from 
> /gnu/store/rmjlycdgiq8pfy5hfi42qhw3k7p6kdav-glibc-2.25/
>
> I supposed that what happends is the following race-condition :
> * (libgit2-shutdown) is called
> * the finalizer %repository-free is called in an other thread.
> * because shutdown has already been called the finalizer fails and
> segfaults.

Could be!

I think using ‘libgit2-shutdown’ is risky because as you can see it can
be tricky to implement it correctly.  So my first recommendation would
be to not try to use it.  :-)  Guile cannot really unload modules ATM
anyway.

To fix this, I think ‘libgit2-shutdown’ needs to call
‘scm_run_finalizers’ before calling ‘git_shutdown’, but I’m not sure if
it’s enough.

HTH,
Ludo’.