Re: assert failed in submodule edge case

> 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 :)
>