Because of limitations in how littlefs manages attributes on disk,
littlefs views zero-length attributes and missing attributes as the same
thing. The simpliest implementation of attributes mirrors this behaviour
transparently for the user.
Now that littlefs has been rebuilt almost from the ground up with the
intention to support custom attributes, adding in custom attribute
support is relatively easy.
The highest bit in the 9-bit type structure indicates that an attribute
is a user-specified custom attribute. The user then has a full 8-bits to
specify the attribute type. Other than that, custom attributes are
treated the same as system-level attributes.
Also made some tweaks to custom attributes:
- Adopted the opencfg for file-level attributes provided by dpgeorge
- Changed setattrs/getattrs to the simpler setattr/getattr functions
users will probably be more familiar with. Note that multiple
attributes can still be committed atomically with files, though not
with directories.
- Changed LFS_ATTRS_MAX -> LFS_ATTR_MAX since there's no longer a global
limit on the sum of attribute sizes, which was rather confusing.
Though they are still limited by what can fit in a metadata-pair.
