Re: [outreachy] Walk through the Git history (guix git {authenticate,log})

[Magali]

Hi!

On 18/12/2020 12:34, Ludovic Courtès wrote:
> Hi Magali,
>
> Magali <magalilemes00@gmail.com> skribis:
>
>> scheme@(guix-user)> (let* ((repo (repository-open cache))
>>          (latest-commit
>>           (commit-lookup repo (reference-target (repository-head repo)))))
>>     (let loop ((commit latest-commit)
>>                (res (list latest-commit)))
>>       (match (commit-parents commit)
>>              (() (reverse res))
>>              ((head . tail)
>>               (loop head (cons head res))))))
>> Segmentation fault (core dumped)
> I can reproduce the bug; the C backtrace looks like this:
>
> --8<---------------cut here---------------start------------->8---
> (gdb) bt
> #0  0x00007fabfa9d31ee in git_oidmap_get ()
>    from 
> /gnu/store/zchrrs2zf4l06cszbadqsk18329q78sg-libgit2-1.1.0/lib/libgit2.so
> #1  0x00007fabfa98d516 in cache_get () from 
> /gnu/store/zchrrs2zf4l06cszbadqsk18329q78sg-libgit2-1.1.0/lib/libgit2.so
> #2  0x00007fabfa9cbef7 in git_object_lookup_prefix ()
>    from 
> /gnu/store/zchrrs2zf4l06cszbadqsk18329q78sg-libgit2-1.1.0/lib/libgit2.so
> #3  0x00007fac01b8866d in ffi_call_unix64 ()
>    from /gnu/store/bw15z9kh9c65ycc2vbhl2izwfwfva7p1-libffi-3.3/lib/libffi.so.7
> #4  0x00007fac01b86ac0 in ffi_call_int () from 
> /gnu/store/bw15z9kh9c65ycc2vbhl2izwfwfva7p1-libffi-3.3/lib/libffi.so.7
> #5  0x00007fac01e54f2e in scm_i_foreign_call (cif_scm=<optimized out>, 
> pointer_scm=<optimized out>, 
>     errno_ret=errno_ret@entry=0x7ffe5fc1f95c, argv=0x7fabfe2e4980) at 
> foreign.c:1073
> #6  0x00007fac01ec3a84 in foreign_call (thread=0x7fac014a8d80, cif=<optimized 
> out>, pointer=<optimized out>)
>     at vm.c:1282
> --8<---------------cut here---------------end--------------->8---
>
> Could it be that, if you keep ‘repo’ in a global variable like in the
> example zimoun posted, segfault no longer occurs?

Yes, this seems to work just fine.


> I believe that what happens is a bug in Guile-Git: ‘repo’ is “finalized”
> (freed) before Guile inspects the commit objects to print them, and when
> Guile gets around to printing those commit objects, they now refer to a
> repo that has been freed, hence the crash.
>
> Until the bug is fixed, the workaround is to arrange your code so that
> the repository object outlives commit objects.  We can discuss this
> further here or on IRC if you want.

Nice! Thanks.


> Hope this helps!

It sure did :-)


Magali