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

Fixes issue #6069.

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
+/