mv: handle lstat() failure correctly

When moving a directory onto another with `git mv` various checks are
performed. One of of these validates that the destination is not existing.

When calling `lstat` on the destination path and it fails as the path
doesn't exist, some environments seem to overwrite the passed  in
`stat` memory nonetheless (I observed this issue on debian 12 of x86_64,
running on OrbStack on ARM, emulated with Rosetta).

This would affect the code that followed as it would still acccess a now
modified `st` structure, which now seems to contain uninitialized memory.
`S_ISDIR(st_dir_mode)` would then typically return false causing the code
to run into a bad case.

The fix avoids overwriting the existing `st` structure, providing an
alternative that exists only for that purpose.

Note that this patch minimizes complexity instead of stack-frame size.

Signed-off-by: Sebastian Thiel <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Byron

builtin/mv.c

@@ -183,7 +183,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 	int src_dir_nr = 0, src_dir_alloc = 0;
 	struct strbuf a_src_dir = STRBUF_INIT;
 	enum update_mode *modes, dst_mode = 0;
-	struct stat st;
+	struct stat st, dest_st;
 	struct string_list src_for_dst = STRING_LIST_INIT_NODUP;
 	struct lock_file lock_file = LOCK_INIT;
 	struct cache_entry *ce;
@@ -303,7 +303,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 			goto act_on_entry;
 		}
 		if (S_ISDIR(st.st_mode)
-		    && lstat(dst, &st) == 0) {
+		    && lstat(dst, &dest_st) == 0) {
 			bad = _("cannot move directory over file");
 			goto act_on_entry;
 		}

t/t7001-mv.sh

@@ -174,6 +174,13 @@ test_expect_success 'do not move directory over existing directory' '
 	test_must_fail git mv path2 path0
 '
 
+test_expect_success 'rename directory to non-existing directory' '
+	mkdir dir-a &&
+	>dir-a/f &&
+	git add dir-a &&
+	git mv dir-a non-existing-dir
+'
+
 test_expect_success 'move into "."' '
 	git mv path1/path2/ .
 '