lockfile.c: store absolute path

Locked paths can be saved in a linked list so that if something wrong
happens, *.lock are removed. For relative paths, this works fine if we
keep cwd the same, which is true 99% of time except:

- update-index and read-tree hold the lock on $GIT_DIR/index really
  early, then later on may call setup_work_tree() to move cwd.

- Suppose a lock is being held (e.g. by "git add") then somewhere
  down the line, somebody calls real_path (e.g. "link_alt_odb_entry"),
  which temporarily moves cwd away and back.

During that time when cwd is moved (either permanently or temporarily)
and we decide to die(), attempts to remove relative *.lock will fail,
and the next operation will complain that some files are still locked.

Avoid this case by turning relative paths to absolute before storing
the path in "filename" field.

Reported-by: Yue Lin Ho <yuelinho777@gmail.com>
Helped-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Adapted-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -128,9 +128,17 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 		    path);
 	}
 
-	strbuf_add(&lk->filename, path, pathlen);
-	if (!(flags & LOCK_NO_DEREF))
-		resolve_symlink(&lk->filename);
+	if (flags & LOCK_NO_DEREF) {
+		strbuf_add_absolute_path(&lk->filename, path);
+	} else {
+		struct strbuf resolved_path = STRBUF_INIT;
+
+		strbuf_add(&resolved_path, path, pathlen);
+		resolve_symlink(&resolved_path);
+		strbuf_add_absolute_path(&lk->filename, resolved_path.buf);
+		strbuf_release(&resolved_path);
+	}
+
 	strbuf_addstr(&lk->filename, LOCK_SUFFIX);
 	lk->fd = open(lk->filename.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
 	if (lk->fd < 0) {

t/t2107-update-index-basic.sh

@@ -65,4 +65,19 @@ test_expect_success '--cacheinfo mode,sha1,path (new syntax)' '
 	test_cmp expect actual
 '
 
+test_expect_success '.lock files cleaned up' '
+	mkdir cleanup &&
+	(
+	cd cleanup &&
+	mkdir worktree &&
+	git init repo &&
+	cd repo &&
+	git config core.worktree ../../worktree &&
+	# --refresh triggers late setup_work_tree,
+	# active_cache_changed is zero, rollback_lock_file fails
+	git update-index --refresh &&
+	! test -f .git/index.lock
+	)
+'
+
 test_done