Transaction: Move contract address generation outside the EVM

The current EVM generates its own new contract addresses, and this is why there
are separate `msg.contractAddress` and `msg.codeAddress` fields in the
computation start message.

In EVMC, account updates are only allowed on the host side, including contract
generation, and the start message has one destination field, `msg.destination`.
The EVM cannot select addresses, only use them.  It's a sensible design.

The difference makes the current EVM incompatible with EVMC and its message
format, so this patch corrects the difference.  It moves contract address
generation to the host side.  This simplifies the EVM and its API a little.

(As an API change, this is incompatible with vm2, so it's guarded under
`evmc_enabled` to allow vm2 to continue to build and run at this time.  This is
also why there are fewer deletions than would otherwise be expected.)

Signed-off-by: Jamie Lokier <jamie@shareable.org>

nimbus/transaction/call_common.nim

@@ -9,7 +9,7 @@
 import
   eth/common/eth_types, stint, options, stew/ranges/ptr_arith,
   ".."/[vm_types, vm_types2, vm_state, vm_computation, vm_state_transactions],
-  ".."/[db/accounts_cache, vm_precompiles, vm_gas_costs],
+  ".."/[db/accounts_cache, utils, vm_precompiles, vm_gas_costs],
   ".."/vm_internals,
   ./host_types
 
@@ -136,15 +136,44 @@ proc setupHost(call: CallParams): TransactionHost =
     # All other defaults in `TransactionHost` are fine.
   )
 
-  if call.input.len > 0:
-    host.msg.input_size = call.input.len.csize_t
-    # Must copy the data so the `host.msg.input_data` pointer
-    # remains valid after the end of `call` lifetime.
-    host.input = call.input
-    host.msg.input_data = host.input[0].addr
+  # Generate new contract address, prepare code, and update message `destination`
+  # with the contract address.  This differs from the previous Nimbus EVM API.
+  # Guarded under `evmc_enabled` for now so it doesn't break vm2.
+  when defined(evmc_enabled):
+    var code: seq[byte]
+    if call.isCreate:
+      let sender = call.sender
+      let contractAddress =
+        generateAddress(sender, call.vmState.readOnlyStateDB.getNonce(sender))
+      host.msg.destination = contractAddress.toEvmc
+      host.msg.input_size = 0
+      host.msg.input_data = nil
+      code = call.input
+    else:
+      # TODO: Share the underlying data, but only after checking this does not
+      # cause problems with the database.
+      code = host.vmState.readOnlyStateDB.getCode(host.msg.destination.fromEvmc)
+      if call.input.len > 0:
+        host.msg.input_size = call.input.len.csize_t
+        # Must copy the data so the `host.msg.input_data` pointer
+        # remains valid after the end of `call` lifetime.
+        host.input = call.input
+        host.msg.input_data = host.input[0].addr
+
+    let cMsg = hostToComputationMessage(host.msg)
+    host.computation = newComputation(vmState, cMsg, code)
+
+  else:
+    if call.input.len > 0:
+      host.msg.input_size = call.input.len.csize_t
+      # Must copy the data so the `host.msg.input_data` pointer
+      # remains valid after the end of `call` lifetime.
+      host.input = call.input
+      host.msg.input_data = host.input[0].addr
+
+    let cMsg = hostToComputationMessage(host.msg)
+    host.computation = newComputation(vmState, cMsg)
 
-  let cMsg = hostToComputationMessage(host.msg)
-  host.computation = newComputation(vmState, cMsg)
   return host
 
 proc runComputation*(call: CallParams): CallResult =

nimbus/vm/computation.nim

@@ -163,6 +163,24 @@ proc newComputation*(vmState: BaseVMState, message: Message, salt= 0.u256): Comp
       cast[evmc_host_context](result)
     )
 
+proc newComputation*(vmState: BaseVMState, message: Message, code: seq[byte]): Computation =
+  new result
+  result.vmState = vmState
+  result.msg = message
+  result.memory = Memory()
+  result.stack = newStack()
+  result.returnStack = @[]
+  result.gasMeter.init(message.gas)
+  result.touchedAccounts = initHashSet[EthAddress]()
+  result.selfDestructs = initHashSet[EthAddress]()
+  result.code = newCodeStream(code)
+
+  when evmc_enabled:
+    result.host.init(
+      nim_host_get_interface(),
+      cast[evmc_host_context](result)
+    )
+
 template gasCosts*(c: Computation): untyped =
   c.vmState.gasCosts