Precompiles: Change precompile tests to use fixtureCallEvm

Move the EVM setup and call in precompile tests to `fixtureCallEvm` in
`call_evm`.  Extra return values needed for testing are returned specially, and
the convention for reporting gas used is changed to match `asmCallEvm`.

Although the precompile tests used `execPrecompiles` before, `executeOpcodes`
does perfectly well as a substitute, allowing `fixtureCallEvm` to be shared.

_Significantly, this patch also makes `Computation` more or less an internal
type of the EVM now._

Nothing outside the EVM (except `call_evm`) needs access any more to
`Computation`, `execComputation`, `executeOpcodes` or `execPrecompiles`.
Many imports can be trimmed, some files removed, and EVMC is much closer.

(As a bonus, the functions in `call_evm` reveal what capabilities parts of the
program have needed over time, makes certain bugs and inconsistencies clearer,
and suggests how to refactor into a more useful shared entry point.)

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

nimbus/transaction/call_evm.nim

@@ -302,7 +302,7 @@ type
   FixtureResult* = object
     isError*:         bool
     error*:           Error
-    gasRemaining*:    GasInt
+    gasUsed*:         GasInt
     output*:          seq[byte]
     vmState*:         BaseVMState
     logEntries*:      seq[Log]
@@ -310,6 +310,7 @@ type
 proc fixtureCallEvm*(vmState: BaseVMState, call: RpcCallData,
                      origin: EthAddress, forkOverride = none(Fork)): FixtureResult =
   var c = fixtureSetupComputation(vmState, call, origin, forkOverride)
+  let gas = c.gasMeter.gasRemaining
 
   # Next line differs from all the other EVM calls.  With `execComputation`,
   # most "vm json tests" fail with either `balanceDiff` or `nonceDiff` errors.
@@ -319,7 +320,7 @@ proc fixtureCallEvm*(vmState: BaseVMState, call: RpcCallData,
   # computation doesn't return.  We'll have to obtain them outside EVMC.
   result.isError         = c.isError
   result.error           = c.error
-  result.gasRemaining    = c.gasMeter.gasRemaining
+  result.gasUsed         = gas - c.gasMeter.gasRemaining
   result.output          = c.output
   result.vmState         = c.vmState
   shallowCopy(result.logEntries, c.logEntries)

tests/test_precompiles.nim

@@ -9,7 +9,8 @@ import
   unittest2, ../nimbus/vm_precompiles, json, stew/byteutils, test_helpers, os, tables,
   strformat, strutils, eth/trie/db, eth/common, ../nimbus/db/db_chain,
   ../nimbus/[vm_computation, vm_types, vm_state, vm_types2], macros,
-  test_allowed_to_fail
+  test_allowed_to_fail,
+  ../nimbus/transaction/call_evm, options
 
 proc initAddress(i: byte): EthAddress = result[19] = i
 
@@ -21,46 +22,32 @@ template doTest(fixture: JsonNode, fork: Fork, address: PrecompileAddresses): un
       expectedErr = test.hasKey("ExpectedError")
       expected = if test.hasKey("Expected"): hexToSeqByte(test["Expected"].getStr) else: @[]
       dataStr = test["Input"].getStr
-      data = if dataStr.len > 0: dataStr.hexToSeqByte else: @[]
       vmState = newBaseVMState(header.stateRoot, header, newBaseChainDB(newMemoryDb()))
-      gas = 1_000_000_000.GasInt
-      gasPrice = 1.GasInt
-      sender = initAddress(0x00)
-      toAddress = initAddress(address.byte)
-      gasCost = if test.hasKey("Gas"): test["Gas"].getInt else: -1
+      gasExpected = if test.hasKey("Gas"): test["Gas"].getInt else: -1
 
-    vmState.setupTxContext(
-      origin = sender,
-      gasPrice = gasPrice
-    )
+    var call: RpcCallData
+    call.source = initAddress(0x00)
+    call.to = initAddress(address.byte)
+    call.gas = 1_000_000_000.GasInt
+    call.gasPrice = 1.GasInt
+    call.value = 0.u256
+    call.data = if dataStr.len > 0: dataStr.hexToSeqByte else: @[]
+    call.contractCreation = false
 
-    var
-      message = Message(
-        kind: evmcCall,
-        gas: gas,
-        sender: sender,
-        contractAddress: toAddress,
-        codeAddress: toAddress,
-        value: 0.u256,
-        data: data
-        )
-      comp = newComputation(vmState, message)
-
-    let initialGas = comp.gasMeter.gasRemaining
-    discard execPrecompiles(comp, fork)
+    let fixtureResult = fixtureCallEvm(vmState, call, call.source, some(fork))
 
     if expectedErr:
-      check comp.isError
+      check fixtureResult.isError
     else:
-      let c = comp.output == expected
-      if not c: echo "Output  : " & comp.output.toHex & "\nExpected: " & expected.toHex
+      check not fixtureResult.isError
+      let c = fixtureResult.output == expected
+      if not c: echo "Output  : " & fixtureResult.output.toHex & "\nExpected: " & expected.toHex
       check c
 
-      if gasCost >= 0:
-        let gasFee = initialGas - comp.gasMeter.gasRemaining
-        if gasFee != gasCost:
-          debugEcho "GAS: ", gasFee, " ", gasCost
-        check gasFee == gasCost
+      if gasExpected >= 0:
+        if fixtureResult.gasUsed != gasExpected:
+          debugEcho "GAS: ", fixtureResult.gasUsed, " ", gasExpected
+        check fixtureResult.gasUsed == gasExpected
 
 proc testFixture(fixtures: JsonNode, testStatusIMPL: var TestStatus) =
   let

tests/test_vm_json.nim

@@ -80,7 +80,7 @@ proc testFixture(fixtures: JsonNode, testStatusIMPL: var TestStatus) =
     check(fixtureResult.output.bytesToHex == expectedOutput)
 
     let expectedGasRemaining = fixture{"gas"}.getHexadecimalInt
-    let actualGasRemaining = fixtureResult.gasRemaining
+    let actualGasRemaining = call.gas - fixtureResult.gasUsed
     checkpoint(&"Remaining: {actualGasRemaining} - Expected: {expectedGasRemaining}")
     check(actualGasRemaining == expectedGasRemaining)