assert failed in submodule edge case

[Dennis Kaarsemaker <dennis@kaarsemaker.net>]

Reported by djanos_ in #git: git add segfaults when you manage to
confuse it with a submodule in the index that is no longer a submodule.

Here's his script to reproduce the segfault:

mkdir segfault
cd segfault

mkdir subrepo
cd subrepo

git init .
echo a > a
git add a
git commit -m "a"

cd ..

git init .
git add .

cd subrepo
rm -rf .git
git add .

This last git add will now barf:
git: pathspec.c:317: prefix_pathspec: Assertion `item->nowildcard_len <= item->len && item->prefix <= item->len' failed.

These all work as I think they are intended to:
$ git -C segfault add subrepo/a
fatal: Pathspec 'subrepo/a' is in submodule 'subrepo'
$ git -C segfault/subrepo add a
fatal: Pathspec 'a' is in submodule 'subrepo'

And this does nothing, it also doesn't crash:

$ git -C segfault add subrepo

GDB backtrace below, this is with next as of e6f3401.

Starting program: /home/dennis/code/git/git -C segfault/subrepo add .
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
git: pathspec.c:317: prefix_pathspec: Assertion `item->nowildcard_len <= item->len && item->prefix <= item->len' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff702ae37 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56	../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  0x00007ffff702ae37 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007ffff702c528 in __GI_abort () at abort.c:89
#2  0x00007ffff7023ce6 in __assert_fail_base (fmt=0x7ffff7173c08 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", 
    assertion=assertion@entry=0x560660 "item->nowildcard_len <= item->len && item->prefix <= item->len", file=file@entry=0x560826 "pathspec.c", line=line@entry=317, 
    function=function@entry=0x560850 <__PRETTY_FUNCTION__.22237> "prefix_pathspec") at assert.c:92
#3  0x00007ffff7023d92 in __GI___assert_fail (assertion=assertion@entry=0x560660 "item->nowildcard_len <= item->len && item->prefix <= item->len", 
    file=file@entry=0x560826 "pathspec.c", line=line@entry=317, function=function@entry=0x560850 <__PRETTY_FUNCTION__.22237> "prefix_pathspec") at assert.c:101
#4  0x00000000004d6a37 in prefix_pathspec (elt=0x7fffffffdaf1 ".", prefixlen=8, prefix=0x7dd09f "subrepo/", flags=50, raw=0x7fffffffd648, 
    p_short_magic=<synthetic pointer>, item=<optimized out>) at pathspec.c:316
#5  parse_pathspec (pathspec=pathspec@entry=0x7fffffffd240, magic_mask=magic_mask@entry=0, flags=flags@entry=50, prefix=prefix@entry=0x7dd09f "subrepo/", 
    argv=argv@entry=0x7fffffffd648) at pathspec.c:417
#6  0x000000000040698c in cmd_add (argc=<optimized out>, argv=0x7fffffffd648, prefix=0x7dd09f "subrepo/") at builtin/add.c:370
#7  0x0000000000406001 in run_builtin (argv=0x7fffffffd640, argc=2, p=0x79d740 <commands>) at git.c:350
#8  handle_builtin (argc=2, argv=0x7fffffffd640) at git.c:532
#9  0x0000000000405151 in run_argv (argv=0x7fffffffd458, argcp=0x7fffffffd43c) at git.c:578
#10 main (argc=2, av=<optimized out>) at git.c:686

I dig a bit into pathspec.c and it looks like the
PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE code in prefix_pathspec is only
stripping out paths inside the submodule, not the submodule itself.

I also guess that it shouldn't as otherwise you can't ever 'git add' a
submodule. So I have no idea how to proceed, or even what the correct
behaviour of 'git add' should be in this case. I do think that failing
an assert() to tell a user he's doing something unexpected/silly/wrong
isn't the right thing to do though :)

-- 
Dennis Kaarsemaker
www.kaarsemaker.net