Fix broken verifier gassing in Pact 5

This is a tricky thing to get right. The tests betrayed that we had it
wrong, but the correct thing to do here was hard to find in the
presence of both saturating unsigned arithmetic (which doesn't seem to
fully work, I got overflow errors when subtraction went below 0) and
milligas/gas conversion/rounding.

Change-Id: Id00000006858d16ed26102355a3fe9b80e6c8b3a

src/Chainweb/Pact5/TransactionExec.hs

@@ -223,8 +223,19 @@ runVerifiers txCtx cmd = do
           let verifierGasRemaining = fromIntegral @Int64 @SatWord pact4VerifierGasRemaining
           -- NB: this is not nice.
           -- TODO: better gas info here
-          chargeGas noInfo $ GAConstant $ gasToMilliGas $ Gas $
-            verifierGasRemaining - min (_gas (milliGasToGas initGasRemaining)) verifierGasRemaining
+          -- Explanation by cases:
+          -- Case 1:
+          -- gasToMilliGas verifierGasRemaining is less than initGasRemaining,
+          -- in which case the verifier charges gas.
+          -- In that case we can subtract it from initGasRemaining and charge that safely.
+          -- Case 2:
+          -- gasToMilliGas verifierGasRemaining is greater than or equal to initGasRemaining,
+          -- in which case the verifier has not charged gas, or has charged less than
+          -- rounding error.
+          -- In that case we do not charge gas at all.
+          --
+          when (gasToMilliGas (Gas verifierGasRemaining) < initGasRemaining) $
+            chargeGas noInfo $ GAConstant $ MilliGas $ coerce initGasRemaining - coerce (gasToMilliGas (Gas verifierGasRemaining))
 
 applyLocal
     :: (Logger logger)

test/unit/Chainweb/Test/Pact5/TransactionExecTest.hs

@@ -534,12 +534,12 @@ applyCmdVerifierSpec rdb = readFromAfterGenesis v rdb $
                     [ P.fun _crEvents ? P.list
                         [ event
                             (P.equals "TRANSFER")
-                            (P.equals [PString "sender00", PString "NoMiner", PDecimal 162])
+                            (P.equals [PString "sender00", PString "NoMiner", PDecimal 362])
                             (P.equals coinModuleName)
                         ]
                     , P.fun _crResult ? P.equals ? PactResultOk (PInteger 1)
                     -- reflects buyGas gas usage, as well as that of the payload
-                    , P.fun _crGas ? P.equals ? Gas 81
+                    , P.fun _crGas ? P.equals ? Gas 181
                     , P.fun _crContinuation ? P.equals ? Nothing
                     , P.fun _crMetaData ? P.equals ? Nothing
                     ]