tuf-client-interfaces.py

import abc
from typing import BinaryIO, TextIO, Any, Dict

JsonDict = Dict[str, Any]

#
# Simple interfaces for abstracting away the storage and network requirements
# of a TUF client.
# Interface shape shamelessly stolen from go-tuf:
# https://github.com/theupdateframework/go-tuf/blob/9d8af573a771f7069ef7a24da942ad9262704534/client/client.go#L23
#
# Pros:
# * Class-based/OO is easier for most to reason about
# * Clients with existing download logic can wrap that in a thin
#   implementation of a two method interface
# * Testing of the core client logic can be simplified with lightweight
#   implementations of the interfaces i.e. MemoryStorer(Storer) and 
#   LocalFile(Fetcher)
#
# Cons:
# * EAFP style isn't fully compatible with interfaces, we can't encode the
#    expected Exceptions
#    * Better to return multiple values with an error type?

class Fetcher(metaclass=abc.ABCMeta):

    # Returns a file object
    # This leaves deciphering the file format (i.e. JSON) to the client logic,
    # whereas it might be nicer to return a more structured datatype here
    # (JsonDict?) and have the Fetcher implementation do translation from the wire
    # format to the format the client can operate on
    @abc.abstractmethod
    def getMeta(self, path: str) -> JsonDict:
        pass
    
    # Returns a file object
    @abc.abstractmethod
    def getTarget(self, path: str) -> TemporaryFile:
        pass

class Storer(metaclass=abc.ABCMeta):

    # FIXME: parameters? go-tuf just loads all metadata, returning
    # Dict[str, JsonDict] mapping role filenames -> JSON metadata
    @abc.abstractmethod
    def loadMeta(role: str) -> JsonDict:
        pass

    @abc.abstractmethod
    def saveMeta(name: str, meta: JsonDict):
        pass


class Updater(fetch: Fetcher, store: Storer):
    # ...