extmod/vfs: Retain previous working directory if chdir fails.

Fixes issue #6069.

extmod/vfs.c

@@ -322,7 +322,6 @@ MP_DEFINE_CONST_FUN_OBJ_KW(mp_vfs_open_obj, 0, mp_vfs_open);
 mp_obj_t mp_vfs_chdir(mp_obj_t path_in) {
     mp_obj_t path_out;
     mp_vfs_mount_t *vfs = lookup_path(path_in, &path_out);
-    MP_STATE_VM(vfs_cur) = vfs;
     if (vfs == MP_VFS_ROOT) {
         // If we change to the root dir and a VFS is mounted at the root then
         // we must change that VFS's current dir to the root dir so that any
@@ -334,9 +333,11 @@ mp_obj_t mp_vfs_chdir(mp_obj_t path_in) {
                 break;
             }
         }
+        vfs = MP_VFS_ROOT;
     } else {
         mp_vfs_proxy_call(vfs, MP_QSTR_chdir, 1, &path_out);
     }
+    MP_STATE_VM(vfs_cur) = vfs;
     return mp_const_none;
 }
 MP_DEFINE_CONST_FUN_OBJ_1(mp_vfs_chdir_obj, mp_vfs_chdir);

tests/extmod/vfs_basic.py

@@ -10,8 +10,9 @@ except (ImportError, AttributeError):
 
 
 class Filesystem:
-    def __init__(self, id):
+    def __init__(self, id, fail=0):
         self.id = id
+        self.fail = fail
 
     def mount(self, readonly, mkfs):
         print(self.id, "mount", readonly, mkfs)
@@ -25,6 +26,8 @@ class Filesystem:
 
     def chdir(self, dir):
         print(self.id, "chdir", dir)
+        if self.fail:
+            raise OSError(self.fail)
 
     def getcwd(self):
         print(self.id, "getcwd")
@@ -158,3 +161,18 @@ uos.chdir("/")
 uos.umount("/")
 print(uos.listdir("/"))
 uos.umount("/mnt")
+
+# chdir to a non-existent mount point (current directory should remain unchanged)
+try:
+    uos.chdir("/foo")
+except OSError:
+    print("OSError")
+print(uos.getcwd())
+
+# chdir to a non-existent subdirectory in a mounted filesystem
+uos.mount(Filesystem(5, 1), "/mnt")
+try:
+    uos.chdir("/mnt/subdir")
+except OSError:
+    print("OSError")
+print(uos.getcwd())

tests/extmod/vfs_basic.py.exp

@@ -58,3 +58,9 @@ OSError
 3 umount
 ['mnt']
 4 umount
+OSError
+/
+5 mount False False
+5 chdir /subdir
+OSError
+/