Add initial Storage Iterator implementation

[ixje]

What current issue(s) does this address, or what feature is it adding?
Fixes #211

This adds the new Storage Iterator API for smart contracts. Note that we should wait with merging until the C# side is fixed (see the issue linked in #211). I wanted to get the feedback loop started while the C# side sorts out their implementation.

How did you solve this problem?
Port code

How did you make sure your solution works?
Compiled a contract in C#, invoked using neo-python. Default tests currently fail because it requires updating the fixtures (see reason below) which I'm waiting with until the C# side is finalised + feedback here is finalised.

Are there any special changes in the code that we should be aware of?
Yes, the leveldb keys for the Storage Put/Get/Delete calls have been altered to make prefix searching possible (discussed with @localhuman) which requires rebuilding the chain + recreating the fixtures.

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[localhuman]

This is a great start! Hopefully things get fixed on the C# side ...

[ixje]

So it seems like the C# side is fixed with this commit; neo-project/neo@6d5cf63

Most of the changes are just specific to their database wrapper so to speak. One thing I was 'worried' about for a moment are the changes to the serialize and deserialize methods of StorageKey. A quick reference search doesn't really show them to be used. I also did a caveman approach by putting a breakpoint on both routines and then sync up my testnet from 930K to ~980. No breakpoint was hit so I don't think it's used anywhere, but it might be good if the reader can also give it some thought if it could be used somewhere now (or possibly in the future).

If after this all you still think that the serialize/deserialize changes don't affect us, please let me know so I can go and update the fixtures and write tests

[localhuman]

I don't think we'll need or use Serialize or Deserialize for storage keys. We could make the StorageKey not inherit SerializableMixin and remove those methods so that nobody is tempted to use them in the future.

[ixje]

@localhuman has to upload the new fixtures. If the build fails because of fixtures_v6.tar.gz not found, please manually see if there file is already there and restart the build if you notice it before me :)

[coveralls]

Coverage decreased (-0.2%) to 79.174% when pulling 7563a65 on ixje:sc_storage_iterator into 0678cc2 on CityOfZion:development.

[ixje]

Hooray build passed! :)

So there's 3 things left

1. somehow inform users that they'll have to resync their chain once they're on a version that includes these changes. How did we do that previous times?
2. add a bootstrap file. Do you already hate me @localhuman 😆
3. add neo-boa support so automated tests can be added. I currently have manually tested it by compiling and invoking the C# contract mentioned here: Add StorageIterator neo-project/neo-devpack-dotnet#19 (comment)

Now, for the boa support I can add a Find(context, needle) call to the Storage class, but that's not enough. Take something like this:

from boa.interop.Neo.Runtime import Notify
from boa.interop.Neo.Storage import GetContext, Put, Find

def Main(args):
    ctx = GetContext()
    Put(ctx, 'my_prefixA', "1")
    Put(ctx, 'my_prefixB', "2")
    Put(ctx, 'something', "3")

    result_iterator = Find(ctx, 'my_prefix')
    while result_iterator.Next():
        Notify(result_iterator.Key()) 

The compiler will complain it can't find Next() and Key(). I gave it a quick try but I just kept getting new errors so I think it's best if someone that understands the neo-boa architecture well would do it. cough that would be you @localhuman 😆 cough

[localhuman]

Sounds like a plan!

[localhuman]

Note, i've created a new branch here https://github.com/CityOfZion/neo-python/tree/feature-storage-iterator which have these changes. I needed to change a few things to get it working.

Also, do you have the compiled avm from C# which I can test with 😀

[localhuman]

Also, for reference see CityOfZion/neo-boa#70

[localhuman]

closing in favor of #406