[BUG] git is segfaulting, was [PATCH v4 04/12] dir: also check directories for matching pathspecs

[Denton Liu <liu.denton@gmail.com>]

Hi Elijah,

I ran into a segfault on MacOS. I managed to bisect it down to
404ebceda0 (dir: also check directories for matching pathspecs,
2019-09-17), which should be the patch in the parent thread. The test
case below works fine without this patch applied but segfaults once it
is applied.

	#!/bin/sh

	git worktree add testdir
	git -C testdir checkout master
	git -C testdir fetch https://github.com/git/git.git todo
	bin-wrappers/git -C testdir checkout FETCH_HEAD # segfault here

Note that the worktree part isn't necessary to reproduce the problem but
I didn't want my files to be constantly refreshed, triggering a rebuild
each time.

I also managed to get this backtrace from running lldb at the segfault
but it is based on the latest "jch" commit, 1cc52d20df (Merge branch
'jt/merge-recursive-symlink-is-not-a-dir-in-way' into jch, 2019-09-20).

	* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x8)
	  * frame #0: 0x00000001000f63a0 git`do_match_pathspec(istate=0x0000000100299940, ps=0x000000010200aa80, name="Gitweb/static/js/lib/", namelen=21, prefix=0, seen=0x0000000000000000, flags=0) at dir.c:420:2 [opt]
		frame #1: 0x00000001000f632c git`match_pathspec(istate=0x0000000100299940, ps=0x0000000000000000, name="Gitweb/static/js/lib/", namelen=21, prefix=0, seen=0x0000000000000000, is_dir=0) at dir.c:490:13 [opt]
		frame #2: 0x00000001000f8315 git`read_directory_recursive(dir=0x00007ffeefbfe278, istate=0x0000000100299940, base=<unavailable>, baselen=17, untracked=<unavailable>, check_only=0, stop_at_first_file=0, pathspec=0x0000000000000000) at dir.c:1990:9 [opt]
		frame #3: 0x00000001000f82e9 git`read_directory_recursive(dir=0x00007ffeefbfe278, istate=0x0000000100299940, base=<unavailable>, baselen=14, untracked=<unavailable>, check_only=0, stop_at_first_file=0, pathspec=0x0000000000000000) at dir.c:1984:5 [opt]
		frame #4: 0x00000001000f82e9 git`read_directory_recursive(dir=0x00007ffeefbfe278, istate=0x0000000100299940, base=<unavailable>, baselen=7, untracked=<unavailable>, check_only=0, stop_at_first_file=0, pathspec=0x0000000000000000) at dir.c:1984:5 [opt]
		frame #5: 0x00000001000f60d1 git`read_directory(dir=0x00007ffeefbfe278, istate=0x0000000100299940, path="Gitweb/", len=7, pathspec=0x0000000000000000) at dir.c:2298:3 [opt]
		frame #6: 0x00000001001bded1 git`verify_clean_subdirectory(ce=<unavailable>, o=0x00007ffeefbfe8c0) at unpack-trees.c:1846:6 [opt]
		frame #7: 0x00000001001bdc1d git`check_ok_to_remove(name="Gitweb", len=6, dtype=4, ce=0x0000000103e70de0, st=0x00007ffeefbfe438, error_type=ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o=0x00007ffeefbfe8c0) at unpack-trees.c:1901:7 [opt]
		frame #8: 0x00000001001bdb01 git`verify_absent_1(ce=<unavailable>, error_type=<unavailable>, o=<unavailable>) at unpack-trees.c:1964:10 [opt]
		frame #9: 0x00000001001bafc0 git`verify_absent(ce=<unavailable>, error_type=<unavailable>, o=<unavailable>) at unpack-trees.c:1052:11 [opt] [artificial]
		frame #10: 0x00000001001bbc3c git`merged_entry(ce=0x0000000100605fb0, old=0x0000000000000000, o=0x00007ffeefbfe8c0) at unpack-trees.c:2013:7 [opt]
		frame #11: 0x00000001001bd2b7 git`call_unpack_fn(src=<unavailable>, o=<unavailable>) at unpack-trees.c:522:12 [opt]
		frame #12: 0x00000001001bca16 git`unpack_nondirectories(n=2, mask=2, dirmask=<unavailable>, src=0x00007ffeefbfe5d0, names=<unavailable>, info=0x00007ffeefbfe718) at unpack-trees.c:1029:12 [opt]
		frame #13: 0x00000001001bad1a git`unpack_callback(n=2, mask=2, dirmask=0, names=0x0000000102007390, info=0x00007ffeefbfe718) at unpack-trees.c:1229:6 [opt]
		frame #14: 0x00000001001b8be2 git`traverse_trees(istate=0x0000000100299940, n=2, t=<unavailable>, info=<unavailable>) at tree-walk.c:497:17 [opt]
		frame #15: 0x00000001001ba80f git`unpack_trees(len=2, t=0x00007ffeefbfebe0, o=0x00007ffeefbfe8c0) at unpack-trees.c:1546:9 [opt]
		frame #16: 0x000000010001a443 git`merge_working_tree(opts=0x00007ffeefbfee38, old_branch_info=0x00007ffeefbfeca0, new_branch_info=0x00007ffeefbfeda0, writeout_error=0x00007ffeefbfeccc) at checkout.c:704:9 [opt]
		frame #17: 0x000000010001a08c git`switch_branches(opts=0x00007ffeefbfee38, new_branch_info=0x00007ffeefbfeda0) at checkout.c:1057:9 [opt]
		frame #18: 0x0000000100018df0 git`checkout_branch(opts=<unavailable>, new_branch_info=<unavailable>) at checkout.c:1426:9 [opt]
		frame #19: 0x0000000100017b90 git`checkout_main(argc=0, argv=0x00007ffeefbff570, prefix=0x0000000000000000, opts=0x00007ffeefbfee38, options=<unavailable>, usagestr=<unavailable>) at checkout.c:1682:10 [opt]
		frame #20: 0x0000000100016f2d git`cmd_checkout(argc=2, argv=0x00007ffeefbff568, prefix=0x0000000000000000) at checkout.c:1731:8 [opt]
		frame #21: 0x00000001000026f6 git`run_builtin(p=0x000000010024c710, argc=2, argv=0x00007ffeefbff568) at git.c:444:11 [opt]
		frame #22: 0x0000000100001a36 git`handle_builtin(argc=2, argv=0x00007ffeefbff568) at git.c:673:3 [opt]
		frame #23: 0x000000010000235c git`run_argv(argcp=0x00007ffeefbff4ec, argv=0x00007ffeefbff4d8) at git.c:740:4 [opt]
		frame #24: 0x0000000100001794 git`cmd_main(argc=2, argv=0x00007ffeefbff568) at git.c:871:19 [opt]
		frame #25: 0x00000001000a4405 git`main(argc=<unavailable>, argv=0x00007ffeefbff560) at common-main.c:52:11 [opt]
		frame #26: 0x00007fff783053d5 libdyld.dylib`start + 1

Sorry for the information dump, I haven't had the time to properly look
into the issue but I just wanted to make sure that you're aware.

Thanks and hope this helps,

Denton