Client: implement state downloading

[vpulim]

In order to perform a fast sync, we must implement the ability to recursively download a complete recent state trie. We can do this by querying peers for trie nodes and their children starting from the state root using the GetNodeData message of the ETH wire protocol.

[aunyks]

Has someone claimed this?

[vpulim]

@aunyks I was planning on taking this on next, but wanted to do some re-factoring of the sync/fetcher classes in preparation. State download will be much easier after the refactor. If you’d like to claim this, would you mind waiting until next week after I committed the refactor changes?

[aunyks]

Of course!

[vpulim]

Cool, thanks

[vpulim]

@aunyks Thanks for your patience. The refactor is done (ethereumjs/ethereumjs-client#85) so feel free to work on the state downloader. You should be able to subclass either Fetcher or BlockFetcher to implement a new StateFetcher class. Since state fetching will need to walk the state tree when fetching, I think the best approach is to add a single task containing the root node in the tasks() method and to make a call to enqueue() in the process() method to enqueue each of the children once a node is downloaded.

[aunyks]

@vpulim School just started for me recently so it could be a while before I get a lot of work on this. Since the priority is high, I'd recommend someone else to work on it as well. If that's not very efficient I can also give up the task if needed.

[holgerd77]

Superseeded by #1848 (implementation not yet done), will close.