tmp-objdir: skip clean up when handling a signal

tmp-objdir: skip clean up when handling a signal

In the tmp-objdir api, tmp_objdir_create will create a temporary
directory but also register signal handlers responsible for removing
the directory's contents and the directory itself. However, the
function responsible for recursively removing the contents and
directory, remove_dir_recurse() calls opendir(3) and closedir(3).
This can be problematic because these functions allocate and free
memory, which are not async-signal-safe functions. This can lead to
deadlocks.

One place we call tmp_objdir_create() is in git-receive-pack, where
we create a temporary quarantine directory "incoming". Incoming
objects will be written to this directory before they get moved to
the object directory.

We have observed this code leading to a deadlock:

	Thread 1 (Thread 0x7f621ba0b200 (LWP 326305)):
	#0  __lll_lock_wait_private (futex=futex@entry=0x7f621bbf8b80
		<main_arena>) at ./lowlevellock.c:35
	#1  0x00007f621baa635b in __GI___libc_malloc
		(bytes=bytes@entry=32816) at malloc.c:3064
	#2  0x00007f621bae9f49 in __alloc_dir (statp=0x7fff2ea7ed60,
		flags=0, close_fd=true, fd=5)
		at ../sysdeps/posix/opendir.c:118
	#3  opendir_tail (fd=5) at ../sysdeps/posix/opendir.c:69
	#4  __opendir (name=<optimized out>)
		at ../sysdeps/posix/opendir.c:92
	#5  0x0000557c19c77de1 in remove_dir_recurse ()
	git#6  0x0000557c19d81a4f in remove_tmp_objdir_on_signal ()
	#7  <signal handler called>
	git#8  _int_malloc (av=av@entry=0x7f621bbf8b80 <main_arena>,
		bytes=bytes@entry=7160) at malloc.c:4116
	git#9  0x00007f621baa62c9 in __GI___libc_malloc (bytes=7160)
		at malloc.c:3066
	git#10 0x00007f621bd1e987 in inflateInit2_ ()
		from /opt/gitlab/embedded/lib/libz.so.1
	git#11 0x0000557c19dbe5f4 in git_inflate_init ()
	git#12 0x0000557c19cee02a in unpack_compressed_entry ()
	git#13 0x0000557c19cf08cb in unpack_entry ()
	git#14 0x0000557c19cf0f32 in packed_object_info ()
	git#15 0x0000557c19cd68cd in do_oid_object_info_extended ()
	git#16 0x0000557c19cd6e2b in read_object_file_extended ()
	git#17 0x0000557c19cdec2f in parse_object ()
	git#18 0x0000557c19c34977 in lookup_commit_reference_gently ()
	git#19 0x0000557c19d69309 in mark_uninteresting ()
	git#20 0x0000557c19d2d180 in do_for_each_repo_ref_iterator ()
	git#21 0x0000557c19d21678 in for_each_ref ()
	git#22 0x0000557c19d6a94f in assign_shallow_commits_to_refs ()
	git#23 0x0000557c19bc02b2 in cmd_receive_pack ()
	git#24 0x0000557c19b29fdd in handle_builtin ()
	git#25 0x0000557c19b2a526 in cmd_main ()
	git#26 0x0000557c19b28ea2 in main ()

Since we can't do the cleanup in a portable and signal-safe way, skip
the cleanup when we're handling a signal.

This means that when signal handling, the temporary directory may not
get cleaned up properly. This is mitigated by b3cecf49ea (tmp-objdir: new
API for creating temporary writable databases, 2021-12-06) which changed
the default name and allows gc to clean up these temporary directories.

In the event of a normal exit, we should still be cleaning up via the
atexit() handler.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>