Segfault in `git describe`

["Mantas Mikulėnas" <grawity@gmail.com>]

I have a clone of linux.git with various stuff added to it (remotes for
'stable' and 'next', a bunch of local tags, and historical repositories
imported using `git replace`).

Yesterday, I noticed that `git describe`, built from git.git master
(v1.8.3.2-804-g0da7a53, gcc 4.8) would simply crash when run in that
repository, with the following backtrace:

> Program terminated with signal 11, Segmentation fault.
> #0  0x00000000004c39dc in hashcpy (sha_src=0x1c <Address 0x1c out of bounds>, 
>     sha_dst=0x7fffc0b4d610 "\242\271\301\366 \201&\346\337l\002B\214P\037\210ShX\022")
>     at cache.h:694
> 694		memcpy(sha_dst, sha_src, 20);
> (gdb) bt
> #0  0x00000000004c39dc in hashcpy (sha_src=0x1c <Address 0x1c out of bounds>, 
>     sha_dst=0x7fffc0b4d610 "\242\271\301\366 \201&\346\337l\002B\214P\037\210ShX\022")
>     at cache.h:694
> #1  peel_ref (refname=refname@entry=0x1fe2d10 "refs/tags/next-20130607", 
>     sha1=sha1@entry=0x7fffc0b4d610 "\242\271\301\366 \201&\346\337l\002B\214P\037\210ShX\022") at refs.c:1586
> #2  0x0000000000424194 in get_name (path=0x1fe2d10 "refs/tags/next-20130607", 
>     sha1=0x1fe2ce8 "\222V\356\276S5\tk\231Hi\264\r=\336\315\302\225\347\257\300N\376\327\064@\237ZDq[T\246\312\033T\260\314\362\025refs/tags/next-20130607", flag=<optimized out>, 
>     cb_data=<optimized out>) at builtin/describe.c:156
> #3  0x00000000004c1c21 in do_one_ref (entry=0x1fe2ce0, cb_data=0x7fffc0b4d7c0)
>     at refs.c:646
> #4  0x00000000004c318d in do_for_each_entry_in_dir (dir=0x1fe1728, 
>     offset=<optimized out>, fn=0x4c1bc0 <do_one_ref>, cb_data=0x7fffc0b4d7c0)
>     at refs.c:672
> #5  0x00000000004c33d1 in do_for_each_entry_in_dirs (dir1=0x1fdf4d8, dir2=0x1fd6318, 
>     cb_data=0x7fffc0b4d7c0, fn=0x4c1bc0 <do_one_ref>) at refs.c:716
> #6  0x00000000004c33d1 in do_for_each_entry_in_dirs (dir1=0x1fdf1f8, dir2=0x1fd62d8, 
>     cb_data=0x7fffc0b4d7c0, fn=0x4c1bc0 <do_one_ref>) at refs.c:716
> #7  0x00000000004c3540 in do_for_each_entry (refs=refs@entry=0x7a2800 <ref_cache>, 
>     base=base@entry=0x509cc6 "", cb_data=cb_data@entry=0x7fffc0b4d7c0, 
>     fn=0x4c1bc0 <do_one_ref>) at refs.c:1689
> #8  0x00000000004c3ff8 in do_for_each_ref (cb_data=cb_data@entry=0x0, flags=1, trim=0, 
>     fn=fn@entry=0x424120 <get_name>, base=0x509cc6 "", refs=0x7a2800 <ref_cache>)
>     at refs.c:1724
> #9  for_each_rawref (fn=fn@entry=0x424120 <get_name>, cb_data=cb_data@entry=0x0)
>     at refs.c:1873
> #10 0x0000000000424f5b in cmd_describe (argc=0, argv=0x7fffc0b4ddc0, prefix=0x0)
>     at builtin/describe.c:466
> #11 0x000000000040596d in run_builtin (argv=0x7fffc0b4ddc0, argc=1, 
>     p=0x760b40 <commands.21352+576>) at git.c:291
> #12 handle_internal_command (argc=1, argv=0x7fffc0b4ddc0) at git.c:453
> #13 0x0000000000404d6e in run_argv (argv=0x7fffc0b4dc78, argcp=0x7fffc0b4dc5c)
>     at git.c:499
> #14 main (argc=1, av=<optimized out>) at git.c:575
> (gdb) 

According to `git bisect`, the first bad commit is:

commit 9a489f3c17d6c974b18c47cf406404ca2a721c87
Author: Michael Haggerty <mhagger@alum.mit.edu>
Date:   Mon Apr 22 21:52:22 2013 +0200

    refs: extract a function peel_entry()

The crash happens only in repositories that have at least one replaced
object in the branch's history. Running `git --no-replace-objects
describe` avoids the crash.

The crash happens only if there are any tags under .git/refs/tags/ that
do not exist in .git/packed-refs, or if I remove all "peeled" lines from
.git/packed-refs (including the '#' line; /^[#^]/d).

A quick way to reproduce this with git.git master is:

git tag -f test-tag HEAD~10
git replace -f HEAD $(git --no-replace-objects cat-file commit HEAD \
  | sed 's/@/@test/' | git hash-object --stdin -t commit -w)
./git describe

-- 
Mantas Mikulėnas <grawity@gmail.com>