[PATCH v2 2/2] check-ignore.c, dir.c: fix segfault with '.' argument from repo root

[Adam Spiers <git@adamspiers.org>]

Fix a corner case where check-ignore would segfault when run with the
'.' argument from the top level of a repository, due to prefix_path()
converting '.' into the empty string.  It doesn't make much sense to
call check-ignore from the top level with '.' as a parameter, since
the top-level directory would never typically be ignored, but of
course it should not segfault in this case.  The existing code
attempted to check for this case but failed due to using the wrong
variable.  Instead we move the check to last_exclude_matching_path(),
in case other callers present or future have a similar issue.

Signed-off-by: Adam Spiers <git@adamspiers.org>
---
 builtin/check-ignore.c | 2 +-
 dir.c                  | 8 ++++++++
 t/t0008-ignores.sh     | 5 +++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/builtin/check-ignore.c b/builtin/check-ignore.c
index 709535c..0240f99 100644
--- a/builtin/check-ignore.c
+++ b/builtin/check-ignore.c
@@ -89,7 +89,7 @@ static int check_ignore(const char *prefix, const char **pathspec)
 					? strlen(prefix) : 0, path);
 		full_path = check_path_for_gitlink(full_path);
 		die_if_path_beyond_symlink(full_path, prefix);
-		if (!seen[i] && path[0]) {
+		if (!seen[i]) {
 			exclude = last_exclude_matching_path(&check, full_path,
 							     -1, &dtype);
 			if (exclude) {
diff --git a/dir.c b/dir.c
index 57394e4..1ae0b90 100644
--- a/dir.c
+++ b/dir.c
@@ -828,6 +828,14 @@ struct exclude *last_exclude_matching_path(struct path_exclude_check *check,
 	struct exclude *exclude;
 
 	/*
+	 * name could be the empty string, e.g. if check-ignore was
+	 * invoked from the top level with '.', prefix_path() will
+	 * convert it into "".
+	 */
+	if (!*name)
+		return NULL;
+
+	/*
 	 * we allow the caller to pass namelen as an optimization; it
 	 * must match the length of the name, as we eventually call
 	 * is_excluded() on the whole name string.
diff --git a/t/t0008-ignores.sh b/t/t0008-ignores.sh
index ebe7c70..9c1bde1 100755
--- a/t/t0008-ignores.sh
+++ b/t/t0008-ignores.sh
@@ -138,6 +138,7 @@ test_expect_success 'setup' '
 	cat <<-\EOF >.gitignore &&
 		one
 		ignored-*
+		top-level-dir/
 	EOF
 	for dir in . a
 	do
@@ -177,6 +178,10 @@ test_expect_success 'setup' '
 #
 # test invalid inputs
 
+test_expect_success_multi '. corner-case' '' '
+	test_check_ignore . 1
+'
+
 test_expect_success_multi 'empty command line' '' '
 	test_check_ignore "" 128 &&
 	stderr_contains "fatal: no path specified"
-- 
1.8.2.rc0.18.g543d1e4