Transaction: Merge entry point for `*...SetupComputation` functions

There are currently six entry points to running an EVM computation, all with
slightly different parameters, and expecting slightly different EVM behaviours.

First step in merging them is a common `setupComputation` that replaces all
the different `*...SetupComputation` functions.

This uses the `TransactionHost` type because it's a step towards using that
type for all EVM calls using only EVMC.  For now an EVMC message is created
then translated to EVM-internal `Message`.  It is done this way to build up
the new interface in stages where all tests pass at each stage.

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

nimbus/transaction/call_evm.nim

@@ -7,11 +7,76 @@
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
 import
-  eth/common/eth_types, stint, options, stew/byteutils,
+  eth/common/eth_types, stint, options, stew/byteutils, stew/ranges/ptr_arith,
   ".."/[vm_types, vm_types2, vm_state, vm_computation, utils],
   ".."/[db/db_chain, config, vm_state_transactions, rpc/hexstrings],
   ".."/[db/accounts_cache, transaction, vm_precompiles, vm_gas_costs], eth/trie/db,
-  ".."/vm_internals
+  ".."/vm_internals,
+  ./host_types
+
+proc hostToComputationMessage(msg: EvmcMessage): Message =
+  Message(
+    kind:            CallKind(msg.kind),
+    depth:           msg.depth,
+    gas:             msg.gas,
+    sender:          msg.sender.fromEvmc,
+    contractAddress: msg.destination.fromEvmc,
+    codeAddress:     msg.destination.fromEvmc,
+    value:           msg.value.fromEvmc,
+    # When input size is zero, input data pointer may be null.
+    data:            if msg.input_size <= 0: @[]
+                     else: @(makeOpenArray(msg.input_data, msg.input_size.int)),
+    flags:           if msg.isStatic: emvcStatic else: emvcNoFlags
+  )
+
+type
+  CallParams = object
+    vmState:      BaseVMState
+    forkOverride: Option[Fork]
+    origin:       Option[HostAddress]
+    gasPrice:     GasInt
+    gasLimit:     GasInt
+    sender:       HostAddress
+    to:           HostAddress
+    isCreate:     bool
+    value:        HostValue
+    input:        seq[byte]
+
+proc setupCall(call: CallParams): TransactionHost =
+  let vmState = call.vmState
+  vmState.setupTxContext(
+    origin       = call.origin.get(call.sender),
+    gasPrice     = call.gasPrice,
+    forkOverride = call.forkOverride
+  )
+
+  let host = TransactionHost(
+    vmState:       vmState,
+    msg: EvmcMessage(
+      kind:        if call.isCreate: EVMC_CREATE else: EVMC_CALL,
+      # Default: flags:       {},
+      # Default: depth:       0,
+      gas:         call.gasLimit,
+      destination: call.to.toEvmc,
+      sender:      call.sender.toEvmc,
+      value:       call.value.toEvmc,
+    )
+    # 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
+
+  let cMsg = hostToComputationMessage(host.msg)
+  host.computation = newComputation(vmState, cMsg)
+  return host
+
+proc setupComputation(call: CallParams): Computation =
+  return setupCall(call).computation
 
 type
   RpcCallData* = object