Fix div by zero in concat gas model

docs/en/pact-functions.md

@@ -1928,7 +1928,7 @@ Retreive any accumulated events and optionally clear event state. Object returne
  *→* `[string]`
 
 
-Queries, or with arguments, sets execution config flags. Valid flags: ["AllowReadInLocal","DisableHistoryInTransactionalMode","DisableInlineMemCheck","DisableModuleInstall","DisableNewTrans","DisablePact40","DisablePact420","DisablePact43","DisablePact431","DisablePact44","DisablePact45","DisablePact46","DisablePact47","DisablePact48","DisablePact49","DisablePactEvents","DisableRuntimeReturnTypeChecking","EnforceKeyFormats","OldReadOnlyBehavior","PreserveModuleIfacesBug","PreserveModuleNameBug","PreserveNsModuleInstallBug","PreserveShowDefs"]
+Queries, or with arguments, sets execution config flags. Valid flags: ["AllowReadInLocal","DisableHistoryInTransactionalMode","DisableInlineMemCheck","DisableModuleInstall","DisableNewTrans","DisablePact40","DisablePact410","DisablePact420","DisablePact43","DisablePact431","DisablePact44","DisablePact45","DisablePact46","DisablePact47","DisablePact48","DisablePact49","DisablePactEvents","DisableRuntimeReturnTypeChecking","EnforceKeyFormats","OldReadOnlyBehavior","PreserveModuleIfacesBug","PreserveModuleNameBug","PreserveNsModuleInstallBug","PreserveShowDefs"]
 ```lisp
 pact> (env-exec-config ['DisableHistoryInTransactionalMode]) (env-exec-config)
 ["DisableHistoryInTransactionalMode"]

src/Pact/Gas/Table.hs

@@ -260,14 +260,17 @@ tableGasModel gasConfig =
           gasToMilliGas $ fromIntegral n * _gasCostConfig_sortFactor gasConfig
         GConcatenation i j -> gasToMilliGas $
           fromIntegral (i + j) * _gasCostConfig_concatenationFactor gasConfig
-        GTextConcatenation nChars nStrings ->
+        GTextConcatenation nChars nStrings fixupDivByZero ->
           let
             MilliGas offsetCost = _gasCostConfig_textConcatenationFactorOffset gasConfig
             MilliGas charCost = _gasCostConfig_textConcatenationFactorStringLen gasConfig
             MilliGas stringCost = _gasCostConfig_textConcatenationFactorStrings gasConfig
 
+            avgDenom | fixupDivByZero = max (fromIntegral nStrings) 1
+                     | otherwise = fromIntegral nStrings
+
             costForAverageStringLength =
-              (fromIntegral nChars  * charCost) `div` fromIntegral nStrings
+              (fromIntegral nChars  * charCost) `div` avgDenom
             costForNumberOfStrings =
               fromIntegral nStrings * stringCost
           in

src/Pact/Native.hs

@@ -1285,6 +1285,7 @@ concat' :: GasRNativeFun e
 concat' i [TList ls _ _] = do
 
   disablePact48 <- isExecutionFlagSet FlagDisablePact48
+  fixupDivByZero <- not <$> isExecutionFlagSet FlagDisablePact410
   let concatGasCost =
         if disablePact48
         then
@@ -1301,7 +1302,7 @@ concat' i [TList ls _ _] = do
                     _ -> 0
               nStrings = V.length ls
           in
-          GTextConcatenation nChars nStrings
+          GTextConcatenation nChars nStrings fixupDivByZero
   computeGas' i concatGasCost $
     let
       ls' = V.toList ls

src/Pact/Types/Gas.hs

@@ -140,9 +140,11 @@ data GasArgs
   -- ^ Cost of sorting by lookup fields
   | GConcatenation !Int !Int
   -- ^ Cost of concatenating two strings before pact 4.8, lists, and objects
-  | GTextConcatenation !Int !Int
-  -- ^ Cost of concatenating a list of strings with the given total character
-  -- count and list length after pact 4.8
+  | GTextConcatenation !Int !Int !Bool
+  -- ^ Cost of concatenating a list of strings in pact ≥4.8, given
+  -- 1. total character count,
+  -- 2. list length, and
+  -- 3. whether to fixup division by zero (pact ≥4.10).
   | GUnreduced ![Term Ref]
   -- ^ Cost of using a native function
   | GPostRead !ReadValue
@@ -224,7 +226,7 @@ instance Pretty GasArgs where
     GSelect {} -> "GSelect"
     GSortFieldLookup i -> "GSortFieldLookup:" <> pretty i
     GConcatenation i j -> "GConcatenation:" <> pretty i <> colon <> pretty j
-    GTextConcatenation nChars nStrings -> "GTextConcatenation:" <> pretty nChars <> colon <> pretty nStrings
+    GTextConcatenation nChars nStrings fixupDiv -> "GTextConcatenation:" <> pretty nChars <> colon <> pretty nStrings <> colon <> pretty fixupDiv
     GUnreduced {} -> "GUnreduced"
     GPostRead rv -> "GPostRead:" <> pretty rv
     GPreWrite wv szVer -> "GWrite:" <> pretty wv <> colon <> pretty szVer

src/Pact/Types/Runtime.hs

@@ -200,6 +200,8 @@ data ExecutionFlag
   | FlagDisablePact48
   -- | Disable Pact 4.9 Features
   | FlagDisablePact49
+  -- | Disable Pact 4.10 Features
+  | FlagDisablePact410
   deriving (Eq,Ord,Show,Enum,Bounded)
 
 -- | Flag string representation

tests/pact/gas.repl

@@ -826,6 +826,12 @@ d.G3
 (env-gas 0)
 (concat strings_1000_2000)
 (expect "calling (concat strings_1000_2000)" 151 (env-gas))
+
+;; Test issue #1316
+(expect "concat gassing works on empty list" "" (concat []))
+
+(env-exec-config ["DisablePact410"])
+(expect-failure "concat gassing replicates 4.8 behaviour on empty list" "divide by zero" (concat []))
 (commit-tx)
 
 (begin-tx)