VM: state_db is accessible only via a contextmanager now

evm/chain.py

@@ -96,12 +96,6 @@ def configure(cls, name, vm_configuration, **overrides):
     #
     # Convenience and Helpers
     #
-    def get_state_db(self):
-        """
-        Passthrough helper to the current VM class.
-        """
-        return self.get_vm().state_db
-
     def get_block(self):
         """
         Passthrough helper to the current VM class.

evm/logic/call.py

@@ -56,9 +56,8 @@ def __call__(self, computation):
         computation.gas_meter.consume_gas(child_msg_gas_fee, reason=self.mnemonic)
 
         # Pre-call checks
-        sender_balance = computation.state_db.get_balance(
-            computation.msg.storage_address,
-        )
+        with computation.vm.state_db(read_only=True) as state_db:
+            sender_balance = state_db.get_balance(computation.msg.storage_address)
         insufficient_funds = should_transfer_value and sender_balance < value
         stack_too_deep = computation.msg.depth + 1 > constants.STACK_DEPTH_LIMIT
 
@@ -82,10 +81,11 @@ def __call__(self, computation):
             computation.gas_meter.return_gas(child_msg_gas)
             computation.stack.push(0)
         else:
-            if code_address:
-                code = computation.state_db.get_code(code_address)
-            else:
-                code = computation.state_db.get_code(to)
+            with computation.vm.state_db(read_only=True) as state_db:
+                if code_address:
+                    code = state_db.get_code(code_address)
+                else:
+                    code = state_db.get_code(to)
 
             child_msg_kwargs = {
                 'gas': child_msg_gas,
@@ -123,7 +123,8 @@ def __call__(self, computation):
 
 class Call(BaseCall):
     def compute_msg_extra_gas(self, computation, gas, to, value):
-        account_exists = computation.state_db.account_exists(to)
+        with computation.vm.state_db(read_only=True) as state_db:
+            account_exists = state_db.account_exists(to)
         transfer_gas_fee = constants.GAS_CALLVALUE if value else 0
         create_gas_fee = constants.GAS_NEWACCOUNT if not account_exists else 0
         return transfer_gas_fee + create_gas_fee

evm/logic/context.py

@@ -13,7 +13,8 @@
 
 def balance(computation):
     addr = force_bytes_to_address(computation.stack.pop(type_hint=constants.BYTES))
-    balance = computation.state_db.get_balance(addr)
+    with computation.vm.state_db(read_only=True) as state_db:
+        balance = state_db.get_balance(addr)
     computation.stack.push(balance)
 
 
@@ -107,7 +108,8 @@ def gasprice(computation):
 
 def extcodesize(computation):
     account = force_bytes_to_address(computation.stack.pop(type_hint=constants.BYTES))
-    code_size = len(computation.state_db.get_code(account))
+    with computation.vm.state_db(read_only=True) as state_db:
+        code_size = len(state_db.get_code(account))
 
     computation.stack.push(code_size)
 
@@ -130,7 +132,8 @@ def extcodecopy(computation):
         reason='EXTCODECOPY: word gas cost',
     )
 
-    code = computation.state_db.get_code(account)
+    with computation.vm.state_db(read_only=True) as state_db:
+        code = state_db.get_code(account)
     code_bytes = code[code_start_position:code_start_position + size]
     padded_code_bytes = pad_right(code_bytes, size, b'\x00')
 

evm/logic/storage.py

@@ -8,10 +8,11 @@
 def sstore(computation):
     slot, value = computation.stack.pop(num_items=2, type_hint=constants.UINT256)
 
-    current_value = computation.state_db.get_storage(
-        address=computation.msg.storage_address,
-        slot=slot,
-    )
+    with computation.vm.state_db(read_only=True) as state_db:
+        current_value = state_db.get_storage(
+            address=computation.msg.storage_address,
+            slot=slot,
+        )
 
     is_currently_empty = not bool(current_value)
     is_going_to_be_empty = not bool(value)
@@ -42,18 +43,20 @@ def sstore(computation):
     if gas_refund:
         computation.gas_meter.refund_gas(gas_refund)
 
-    computation.state_db.set_storage(
-        address=computation.msg.storage_address,
-        slot=slot,
-        value=value,
-    )
+    with computation.vm.state_db() as state_db:
+        state_db.set_storage(
+            address=computation.msg.storage_address,
+            slot=slot,
+            value=value,
+        )
 
 
 def sload(computation):
     slot = computation.stack.pop(type_hint=constants.UINT256)
 
-    value = computation.state_db.get_storage(
-        address=computation.msg.storage_address,
-        slot=slot,
-    )
+    with computation.vm.state_db(read_only=True) as state_db:
+        value = state_db.get_storage(
+            address=computation.msg.storage_address,
+            slot=slot,
+        )
     computation.stack.push(value)

evm/logic/system.py

@@ -28,25 +28,24 @@ def suicide(computation):
 
 def suicide_eip150(computation):
     beneficiary = force_bytes_to_address(computation.stack.pop(type_hint=constants.BYTES))
-    if not computation.state_db.account_exists(beneficiary):
-        computation.gas_meter.consume_gas(
-            constants.GAS_SUICIDE_NEWACCOUNT, reason=mnemonics.SUICIDE)
+    with computation.vm.state_db(read_only=True) as state_db:
+        if not state_db.account_exists(beneficiary):
+            computation.gas_meter.consume_gas(
+                constants.GAS_SUICIDE_NEWACCOUNT, reason=mnemonics.SUICIDE)
     _suicide(computation, beneficiary)
 
 
 def _suicide(computation, beneficiary):
-    local_balance = computation.state_db.get_balance(computation.msg.storage_address)
-    beneficiary_balance = computation.state_db.get_balance(beneficiary)
-
-    # 1st: Transfer to beneficiary
-    computation.state_db.set_balance(
-        beneficiary,
-        local_balance + beneficiary_balance,
-    )
-    # 2nd: Zero the balance of the address being deleted (must come after
-    # sending to beneficiary in case the contract named itself as the
-    # beneficiary.
-    computation.state_db.set_balance(computation.msg.storage_address, 0)
+    with computation.vm.state_db() as state_db:
+        local_balance = state_db.get_balance(computation.msg.storage_address)
+        beneficiary_balance = state_db.get_balance(beneficiary)
+
+        # 1st: Transfer to beneficiary
+        state_db.set_balance(beneficiary, local_balance + beneficiary_balance)
+        # 2nd: Zero the balance of the address being deleted (must come after
+        # sending to beneficiary in case the contract named itself as the
+        # beneficiary.
+        state_db.set_balance(computation.msg.storage_address, 0)
 
     # 3rd: Register the account to be deleted
     computation.register_account_for_deletion(computation.msg.storage_address)
@@ -66,8 +65,9 @@ def __call__(self, computation):
 
         computation.extend_memory(start_position, size)
 
-        insufficient_funds = computation.state_db.get_balance(
-            computation.msg.storage_address) < value
+        with computation.vm.state_db(read_only=True) as state_db:
+            insufficient_funds = state_db.get_balance(
+                computation.msg.storage_address) < value
         stack_too_deep = computation.msg.depth + 1 > constants.STACK_DEPTH_LIMIT
 
         if insufficient_funds or stack_too_deep:
@@ -80,8 +80,8 @@ def __call__(self, computation):
             computation.gas_meter.gas_remaining)
         computation.gas_meter.consume_gas(create_msg_gas, reason="CREATE")
 
-        creation_nonce = computation.state_db.get_nonce(
-            computation.msg.storage_address)
+        with computation.vm.state_db(read_only=True) as state_db:
+            creation_nonce = state_db.get_nonce(computation.msg.storage_address)
         contract_address = generate_contract_address(
             computation.msg.storage_address, creation_nonce)
 

evm/utils/fixture_tests.py

@@ -372,7 +372,6 @@ def setup_state_db(desired_state, state_db):
         state_db.set_nonce(account, nonce)
         state_db.set_code(account, code)
         state_db.set_balance(account, balance)
-    return state_db
 
 
 def verify_state_db(expected_state, state_db):

evm/vm/base.py

@@ -1,5 +1,6 @@
 from __future__ import absolute_import
 
+from contextlib import contextmanager
 import logging
 
 from evm.constants import (
@@ -56,19 +57,17 @@ def configure(cls,
                 )
         return type(name, (cls,), overrides)
 
-    _block = None
-    state_db = None
-
-    @property
-    def block(self):
-        if self._block is None:
-            raise AttributeError("No block property set")
-        return self._block
-
-    @block.setter
-    def block(self, value):
-        self._block = value
-        self.state_db = State(db=self.db, root_hash=value.header.state_root)
+    @contextmanager
+    def state_db(self, read_only=False):
+        state = State(db=self.db, root_hash=self.block.header.state_root)
+        yield state
+        if read_only:
+            # TODO: This is a bit of a hack; ideally we should raise an error whenever the
+            # callsite tries to call a State method that modifies it.
+            assert state.root_hash == self.block.header.state_root
+        elif self.block.header.state_root != state.root_hash:
+            self.logger.debug("Updating block's state_root to %s", state.root_hash)
+            self.block.header.state_root = state.root_hash
 
     #
     # Logging
@@ -85,12 +84,7 @@ def apply_transaction(self, transaction):
         Apply the transaction to the vm in the current block.
         """
         computation = self.execute_transaction(transaction)
-        # NOTE: mutation. Needed in order to update self.state_db, so we should be able to get rid
-        # of this once we fix https://github.com/pipermerriam/py-evm/issues/67
-        self.block = self.block.add_transaction(
-            transaction=transaction,
-            computation=computation,
-        )
+        self.block.add_transaction(transaction, computation)
         return computation
 
     def execute_transaction(self, transaction):
@@ -151,12 +145,15 @@ def mine_block(self, *args, **kwargs):
         block = self.block
         block.mine(*args, **kwargs)
 
-        if block.number > 0:
-            block_reward = self.get_block_reward(block.number) + (
-                len(block.uncles) * self.get_nephew_reward(block.number)
-            )
+        if block.number == 0:
+            return block
+
+        block_reward = self.get_block_reward(block.number) + (
+            len(block.uncles) * self.get_nephew_reward(block.number)
+        )
 
-            self.state_db.delta_balance(block.header.coinbase, block_reward)
+        with self.state_db() as state_db:
+            state_db.delta_balance(block.header.coinbase, block_reward)
             self.logger.debug(
                 "BLOCK REWARD: %s -> %s",
                 block_reward,
@@ -167,17 +164,13 @@ def mine_block(self, *args, **kwargs):
                 uncle_reward = BLOCK_REWARD * (
                     UNCLE_DEPTH_PENALTY_FACTOR + uncle.block_number - block.number
                 ) // UNCLE_DEPTH_PENALTY_FACTOR
-                self.state_db.delta_balance(uncle.coinbase, uncle_reward)
+                state_db.delta_balance(uncle.coinbase, uncle_reward)
                 self.logger.debug(
                     "UNCLE REWARD REWARD: %s -> %s",
                     uncle_reward,
                     uncle.coinbase,
                 )
 
-            self.logger.debug('BEFORE ROOT: %s', block.header.state_root)
-            block.header.state_root = self.state_db.root_hash
-            self.logger.debug('STATE_ROOT: %s', block.header.state_root)
-
         return block
 
     #
@@ -264,15 +257,17 @@ def snapshot(self):
 
         TODO: This needs to do more than just snapshot the state_db but this is a start.
         """
-        return self.state_db.snapshot()
+        with self.state_db(read_only=True) as state_db:
+            return state_db.snapshot()
 
     def revert(self, snapshot):
         """
         Revert the VM to the state
 
         TODO: This needs to do more than just snapshot the state_db but this is a start.
         """
-        return self.state_db.revert(snapshot)
+        with self.state_db() as state_db:
+            return state_db.revert(snapshot)
 
     #
     # Opcode API

evm/vm/computation.py

@@ -95,13 +95,6 @@ def is_origin_computation(self):
         """
         return self.msg.is_origin
 
-    @property
-    def state_db(self):
-        """
-        Convenience access to the state database
-        """
-        return self.vm.state_db
-
     #
     # Execution
     #