git-checkout: be careful about untracked symlinks

This fixes the case where an untracked symlink that points at a directory
with tracked paths confuses the checkout logic, demostrated in t6035.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -468,6 +468,9 @@ extern int index_fd(unsigned char *sha1, int fd, struct stat *st, int write_obje
 extern int index_path(unsigned char *sha1, const char *path, struct stat *st, int write_object);
 extern void fill_stat_cache_info(struct cache_entry *ce, struct stat *st);
 
+/* "careful lstat()" */
+extern int check_path(const char *path, int len, struct stat *st);
+
 #define REFRESH_REALLY		0x0001	/* ignore_valid */
 #define REFRESH_UNMERGED	0x0002	/* allow unmerged */
 #define REFRESH_QUIET		0x0004	/* be quiet about it */

entry.c

@@ -175,6 +175,19 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
 	return 0;
 }
 
+/*
+ * This is like 'lstat()', except it refuses to follow symlinks
+ * in the path.
+ */
+int check_path(const char *path, int len, struct stat *st)
+{
+	if (has_symlink_leading_path(path, len)) {
+		errno = ENOENT;
+		return -1;
+	}
+	return lstat(path, st);
+}
+
 int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *topath)
 {
 	static char path[PATH_MAX + 1];
@@ -188,7 +201,7 @@ int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *t
 	strcpy(path + len, ce->name);
 	len += ce_namelen(ce);
 
-	if (!lstat(path, &st)) {
+	if (!check_path(path, len, &st)) {
 		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID);
 		if (!changed)
 			return 0;

t/t6035-merge-dir-to-symlink.sh

@@ -17,7 +17,7 @@ test_expect_success 'create a commit where dir a/b changed to symlink' '
 	git commit -m "dir to symlink"
 '
 
-test_expect_failure 'keep a/b-2/c/d across checkout' '
+test_expect_success 'keep a/b-2/c/d across checkout' '
 	git checkout HEAD^0 &&
 	git reset --hard master &&
 	git rm --cached a/b &&