Ada: implement extended attributes

[onnozweers]

The dCache API supports extended attrbutes. These can probably be used to store metadata with files. We can implement them in Ada.

The /namespace API call allows setting and removing them:

If action is ‘rm-xattr’ then extended attributes of a file or directory are removed as given by the ‘names’ item. The ‘names’ value is either a string or an array of strings.
If action is ‘set-xattr’ then extended attributes are created or modified. The optional ‘mode’ item controls whether to create a new attribute (CREATE), to modify an existing attribute (MODIFY), or to assign the value by either creating a new attribute or modifying an existing attribute (EITHER). EITHER is the default mode. The ‘attributes’ item value is a JSON Object with the new attributes,where the JSON Object’s key is the attribute name and the corresponding JSON Object’s value is this attribute’s value.

[onnozweers]

Testing with the /namespace set-xattr and rm-xattr operation. These are working examples:

{"action" : "set-xattr" ; "attributes" : {"a" : "1"; "b" : "2"} }
{"action" : "rm-xattr" ; "names" : "a" }

But then I found some examples of NetCDF headers: they don't seem to be in a straight key-value format, but have some hierarchy.

Testing if that works in dCache:

{"action" : "set-xattr" ; "attributes" : {"a" : "1", "b" : { "b1" : "2"} }

This is refused with a "bad request" message:

{
  "detail": "Expected a ',' or '}' at 74 [character 75 line 1]",
  "title": "Bad Request",
  "status": "400"
}

OK, looks like we'll have to stick to a straight key-value input. Fair enough. If users have more complicated metadata, they'll have to find their own solution.