RPC: Prepare rpcSetupComputation to support estimateGas

The RPC `estimateGas` behaves differently from RPC `call` in a number of ways.

These differences may be bugs due to `rpcEstimateGas` calling the EVM in a very
different way than `rpcDoCall`, or they may be intentional.  To be sure, we'll
need to test behaviour with Geth, Infura etc to find out (their documentation
isn't enough.)  For now, though, we'll keep the same behaviour as we always had.

`rpcEstimateGas` cannot use `rpcSetupComputation` as it is, because
`estimateGas` accounts for "intrinsic gas", and `call` does not.

This patch changes `rpcSetupComputation` to accomodate both behaviours.

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

nimbus/transaction/call_evm.nim

@@ -8,7 +8,7 @@
 
 import
   eth/common/eth_types, stint, options, stew/byteutils,
-  ".."/[vm_types, vm_types2, vm_state, vm_computation],
+  ".."/[vm_types, vm_types2, vm_state, vm_computation, utils],
   ".."/[db/db_chain, config, vm_state_transactions, rpc/hexstrings],
   ".."/[db/accounts_cache, p2p/executor], eth/trie/db
 
@@ -22,23 +22,28 @@ type
     data*: seq[byte]
     contractCreation*: bool
 
-proc rpcSetupComputation*(vmState: BaseVMState, call: RpcCallData, fork: Fork): Computation =
+proc rpcSetupComputation*(vmState: BaseVMState, call: RpcCallData,
+                          fork: Fork, gasLimit: GasInt): Computation =
   vmState.setupTxContext(
     origin = call.source,
     gasPrice = call.gasPrice,
     forkOverride = some(fork)
   )
 
-  let msg = Message(
-    kind: evmcCall,
+  var msg = Message(
+    kind: if call.contractCreation: evmcCreate else: evmcCall,
     depth: 0,
-    gas: call.gas,
+    gas: gasLimit,
     sender: call.source,
-    contractAddress: call.to,
+    contractAddress:
+      if not call.contractCreation:
+        call.to
+      else:
+        generateAddress(call.source, vmState.readOnlyStateDB.getNonce(call.source)),
     codeAddress: call.to,
     value: call.value,
     data: call.data
-    )
+  )
 
   return newComputation(vmState, msg)
 
@@ -50,7 +55,7 @@ proc rpcDoCall*(call: RpcCallData, header: BlockHeader, chain: BaseChainDB): Hex
     # which use previous block stateRoot
     vmState = newBaseVMState(header.stateRoot, header, chain)
     fork    = toFork(chain.config, header.blockNumber)
-    comp    = rpcSetupComputation(vmState, call, fork)
+    comp    = rpcSetupComputation(vmState, call, fork, call.gas)
 
   comp.execComputation()
   result = hexDataStr(comp.output)
@@ -62,7 +67,7 @@ proc rpcMakeCall*(call: RpcCallData, header: BlockHeader, chain: BaseChainDB): (
     # which use previous block stateRoot
     vmState = newBaseVMState(header.stateRoot, header, chain)
     fork    = toFork(chain.config, header.blockNumber)
-    comp    = rpcSetupComputation(vmState, call, fork)
+    comp    = rpcSetupComputation(vmState, call, fork, call.gas)
 
   let gas = comp.gasMeter.gasRemaining
   comp.execComputation()