Implement State object

[pipermerriam]

What is wrong?

Currently, the VM class is part of the constructor for the Computation object. Here are the places where it is currently used.

evm/logic/block.py:7:    block_hash = computation.vm.get_ancestor_hash(block_number)
evm/logic/block.py:13:    computation.stack.push(computation.vm.block.header.coinbase)
evm/logic/block.py:17:    computation.stack.push(computation.vm.block.header.timestamp)
evm/logic/block.py:21:    computation.stack.push(computation.vm.block.header.block_number)
evm/logic/block.py:25:    computation.stack.push(computation.vm.block.header.difficulty)
evm/logic/block.py:29:    computation.stack.push(computation.vm.block.header.gas_limit)
evm/logic/system.py:87:    computation.vm.logger.debug(
evm/logic/system.py:141:            computation.vm.logger.debug(
evm/vm/forks/spurious_dragon/utils.py:22:        yield computation.vm.block.header.coinbase
evm/vm/computation.py:76:        self.vm = vm
evm/vm/computation.py:197:            child_computation = self.vm.apply_create_message(child_msg)
evm/vm/computation.py:199:            child_computation = self.vm.apply_message(child_msg)
evm/vm/computation.py:280:        with self.vm.state_db(read_only, self.msg.access_list) as state_db:

Giving computation direct access to the VM is problematic for a number of reasons.

• poor isolation for what data and APIs are available during EVM execution.
• makes implementation of stateless clients difficult.
• makes VM execution inherently non-pure.

How can it be fixed

To fix this, lets create a new State object. Similar to how the Transaction class is specified for each VM, the State object should be similarly overrideable.

• VM.get_state_class()
• VM.get_state()
• VM.state (this could be a convenience property that just delegates to VM.get_state().

The State object will need the following things.

• Access to the current block header information (coinbase/timestamp/block_number/difficulty/gas_limit.
• The ancestry hashes for the current block number.
• Access to the StateDB

The State object will also need the following APIs available to it.

• VM.apply_message
• VM.apply_create_message

I think the best way to accomplish this is to relocate the following APIs to live directly on the State object itself, removing them from the VM object.

• VM.apply_message
• VM.apply_create_message
• VM.apply_computation

Note that VM.apply_computation needs access to VM.get_opcode_fn() and VM.precompiles. This indicates we'll need to supply the State object with a way to get at this data. The simplest solution that comes to mind is to add them as extra arguments:

class State:
    def apply_computation(self, message, opcodes, precompiles):
        ...

[pipermerriam]

@hwwhww I was thinking you (or someone on the sharding team) would be a good fit for implementing this since you'll have a better idea of what's needs to be built on top of it. Otherwise, I'm happy to take this on.

[hwwhww]

@pipermerriam
It's really helpful and clear, thank you!
I will start this task first (in master).

[hwwhww]

Worklog

In /tests/json-fixtures/test_virtual_machine.py, it is configured to not actually apply messages.

py-evm/tests/json-fixtures/test_virtual_machine.py

Lines 112 to 113 in 288e65b

	apply_message=apply_create_message_for_testing,
	apply_create_message=apply_create_message_for_testing,

But these two function were moved from VM in to State.

1. Because we use VM.get_state() to get state instance, simply monkey patching on HomesteadVMForTesting doesn't works.
2. Set configuration for State object via VM may work, but looks hassle if it's only for tests....?

[pipermerriam]

With the idea that the State object is configurable for each VM, I think it's reasonable to do the following for those tests.

• Create a specific State object subclass for the given VM that has these methods overridden with the testing versions.
• Create a specific VM object subclass for the geven VM that has the augmented State object configured as it's default state object.

Thoughts? Not sure if I missed something. I'm open to alternate solutions.

[hwwhww]

@pipermerriam Your solution works well and clean! I apply it by replacing _state_class:

HomesteadStateForTesting = HomesteadState.configure(
    name='HomesteadStateForTesting',
    apply_message=apply_message_for_testing,
    apply_create_message=apply_create_message_for_testing,
    get_ancestor_hash=get_block_hash_for_testing,
)
HomesteadVMForTesting = HomesteadVM.configure(
    name='HomesteadVMForTesting',
    _state_class=HomesteadStateForTesting,
)

Thank you so much.

[hwwhww]

close via #247