extmod/vfs_fat: Implement POSIX behaviour of rename, allow to overwrite.

If the destination of os.rename() exists then it will be overwritten if it
is a file.  This is the POSIX behaviour, which is also the CPython
behaviour, and so we follow suit.

See issue #2598 for discussion.

extmod/vfs_fat.c

@@ -121,6 +121,12 @@ STATIC mp_obj_t fat_vfs_rename(mp_obj_t vfs_in, mp_obj_t path_in, mp_obj_t path_
     const char *old_path = mp_obj_str_get_str(path_in);
     const char *new_path = mp_obj_str_get_str(path_out);
     FRESULT res = f_rename(old_path, new_path);
+    if (res == FR_EXIST) {
+        // if new_path exists then try removing it (but only if it's a file)
+        fat_vfs_remove_internal(path_out, 0); // 0 == file attribute
+        // try to rename again
+        res = f_rename(old_path, new_path);
+    }
     if (res == FR_OK) {
         return mp_const_none;
     } else {

tests/extmod/vfs_fat_fileio.py

@@ -140,6 +140,14 @@ print(vfs.listdir("foo_dir"))
 vfs.rename("foo_dir/file.txt", "moved-to-root.txt")
 print(vfs.listdir())
 
+# check that renaming to existing file will overwrite it
+with vfs.open("temp", "w") as f:
+    f.write("new text")
+vfs.rename("temp", "moved-to-root.txt")
+print(vfs.listdir())
+with vfs.open("moved-to-root.txt") as f:
+    print(f.read())
+
 # valid removes
 vfs.remove("foo_dir/sub_file.txt")
 vfs.remove("foo_file.txt")

tests/extmod/vfs_fat_fileio.py.exp

@@ -18,5 +18,7 @@ b'data in file'
 True
 ['sub_file.txt', 'file.txt']
 ['foo_file.txt', 'foo_dir', 'moved-to-root.txt']
+['foo_file.txt', 'foo_dir', 'moved-to-root.txt']
+new text
 ['moved-to-root.txt']
 ENOSPC: True