Transaction: Calculate EIP-1283/2200/2929 gas refund in `setStorage`

Make the host service `setStorage` calculate the gas refund itself, instead of
depending on EVM internals.

In EVMC, host `setStorage` is responsible for adding the gas refund using the
rules of EIP-1283 (Constantinople), EIP-2200 (Istanbul) or EIP-2929 (Berlin).

It is not obvious that the host must do it from EVMC documentation, but it's
how it has to be.  Note, this is very different from the gas _cost_, which the
host does not calculate.

Gas refund doesn't affect computation.  It is applied after the whole
transaction is complete, so it can be tracked on the host or EVM side.  But
`setStorage` doesn't return enough information for the EVM to calculate the
refund, so the host must do it when `setStorage` is used.

For now, this continues using Nimbus `Computation` just to hold the gas refund,
to fit around existing structures and get new EVMC working.  But the host can't
keep using `Computation`, so gas refund will be moved out of it in due course.

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

nimbus/transaction/host_services.nim

@@ -87,8 +87,40 @@ proc accountExists(host: TransactionHost, address: HostAddress): bool {.show.} =
 proc getStorage(host: TransactionHost, address: HostAddress, key: HostKey): HostValue {.show.} =
   host.vmState.readOnlyStateDB.getStorage(address, key)
 
-proc setStorage1(host: TransactionHost, address: HostAddress,
-                 key: HostKey, value: HostValue): EvmcStorageStatus =
+const
+  # EIP-1283
+  SLOAD_GAS_CONSTANTINOPLE = 200
+  # EIP-2200
+  SSTORE_SET_GAS           = 20000
+  SSTORE_RESET_GAS         = 5000
+  SSTORE_CLEARS_SCHEDULE   = 15000
+  SLOAD_GAS_ISTANBUL       = 800
+  # EIP-2929
+  WARM_STORAGE_READ_COST   = 100
+  COLD_SLOAD_COST          = 2100
+  COLD_ACCOUNT_ACCESS_COST = 2600
+
+func storageModifiedAgainRefund(originalValue, oldValue, value: HostValue,
+                                fork: Fork): int {.inline.} =
+  # Calculate `SSTORE` refund according to EIP-2929 (Berlin),
+  # EIP-2200 (Istanbul) or EIP-1283 (Constantinople only).
+  result = 0
+  if value == originalValue:
+    result = if value.isZero: SSTORE_SET_GAS
+             elif fork >= FkBerlin: SSTORE_RESET_GAS - COLD_SLOAD_COST
+             else: SSTORE_RESET_GAS
+    let SLOAD_GAS = if fork >= FkBerlin: WARM_STORAGE_READ_COST
+                    elif fork >= FkIstanbul: SLOAD_GAS_ISTANBUL
+                    else: SLOAD_GAS_CONSTANTINOPLE
+    result -= SLOAD_GAS
+  if not originalValue.isZero:
+    if value.isZero:
+      result += SSTORE_CLEARS_SCHEDULE
+    elif oldValue.isZero:
+      result -= SSTORE_CLEARS_SCHEDULE
+
+proc setStorage(host: TransactionHost, address: HostAddress,
+                key: HostKey, value: HostValue): EvmcStorageStatus {.show.} =
   let db = host.vmState.readOnlyStateDB
   let oldValue = db.getStorage(address, key)
 
@@ -101,26 +133,24 @@ proc setStorage1(host: TransactionHost, address: HostAddress,
   if host.vmState.fork >= FkIstanbul or host.vmState.fork == FkConstantinople:
     let originalValue = db.getCommittedStorage(address, key)
     if oldValue != originalValue:
+      # Gas refund for `MODIFIED_AGAIN` (EIP-1283/2200/2929 only).
+      let refund = storageModifiedAgainRefund(originalValue, oldValue, value,
+                                              host.vmState.fork)
+      # TODO: Refund depends on `Computation` at the moment.
+      if refund != 0:
+        host.computation.gasMeter.refundGas(refund)
       return EVMC_STORAGE_MODIFIED_AGAIN
 
   if oldValue.isZero:
     return EVMC_STORAGE_ADDED
   elif value.isZero:
+    # Gas refund for `DELETED` (all forks).
+    # TODO: Refund depends on `Computation` at the moment.
+    host.computation.gasMeter.refundGas(SSTORE_CLEARS_SCHEDULE)
     return EVMC_STORAGE_DELETED
   else:
     return EVMC_STORAGE_MODIFIED
 
-proc setStorage(host: TransactionHost, address: HostAddress,
-                key: HostKey, value: HostValue): EvmcStorageStatus {.show.} =
-  let status = setStorage1(host, address, key, value)
-  let gasParam = GasParams(kind: Op.Sstore,
-                           s_status: status,
-                           s_currentValue: currValue,
-                           s_originalValue: origValue)
-  gasRefund = ctx.gasCosts[Sstore].c_handler(newValue, gasParam)[1]
-  if gasRefund != 0:
-    host.computation.gasMeter.refundGas(gasRefund)
-
 proc getBalance(host: TransactionHost, address: HostAddress): HostBalance {.show.} =
   host.vmState.readOnlyStateDB.getBalance(address)