fix: round down tokens thawing when slashing (TRST-H04)

Author: Maikol

packages/horizon/contracts/staking/HorizonStaking.sol

@@ -461,8 +461,8 @@ contract HorizonStaking is HorizonStakingBase, IHorizonStakingMain {
             _graphToken().burnTokens(providerTokensSlashed - tokensVerifier);
 
             // Provision accounting
-            // TODO check for rounding issues
-            uint256 provisionFractionSlashed = (providerTokensSlashed * FIXED_POINT_PRECISION) / prov.tokens;
+            uint256 provisionFractionSlashed = (providerTokensSlashed * FIXED_POINT_PRECISION + prov.tokens - 1) /
+                prov.tokens;
             prov.tokensThawing =
                 (prov.tokensThawing * (FIXED_POINT_PRECISION - provisionFractionSlashed)) /
                 (FIXED_POINT_PRECISION);
@@ -497,7 +497,8 @@ contract HorizonStaking is HorizonStakingBase, IHorizonStakingMain {
                 _graphToken().burnTokens(tokensToSlash);
 
                 // Delegation pool accounting
-                uint256 delegationFractionSlashed = (tokensToSlash * FIXED_POINT_PRECISION) / pool.tokens;
+                uint256 delegationFractionSlashed = (tokensToSlash * FIXED_POINT_PRECISION + pool.tokens - 1) /
+                    pool.tokens;
                 pool.tokens = pool.tokens - tokensToSlash;
                 pool.tokensThawing =
                     (pool.tokensThawing * (FIXED_POINT_PRECISION - delegationFractionSlashed)) /

packages/horizon/test/shared/horizon-staking/HorizonStakingShared.t.sol

@@ -1488,7 +1488,7 @@ abstract contract HorizonStakingSharedTest is GraphBaseTest {
             uint256 tokensSlashed = calcValues.providerTokensSlashed +
                 (isDelegationSlashingEnabled ? calcValues.delegationTokensSlashed : 0);
             uint256 provisionThawingTokens = (before.provision.tokensThawing *
-                (1e18 - ((calcValues.providerTokensSlashed * 1e18) / before.provision.tokens))) / (1e18);
+                (1e18 - ((calcValues.providerTokensSlashed * 1e18 + before.provision.tokens - 1) / before.provision.tokens))) / (1e18);
 
             // assert
             assertEq(afterProvision.tokens + calcValues.providerTokensSlashed, before.provision.tokens);
@@ -1506,7 +1506,7 @@ abstract contract HorizonStakingSharedTest is GraphBaseTest {
                 (before.provision.sharesThawing != 0 && afterProvision.sharesThawing == 0) ? before.provision.thawingNonce + 1 : before.provision.thawingNonce);
             if (isDelegationSlashingEnabled) {
                 uint256 poolThawingTokens = (before.pool.tokensThawing *
-                    (1e18 - ((calcValues.delegationTokensSlashed * 1e18) / before.pool.tokens))) / (1e18);
+                    (1e18 - ((calcValues.delegationTokensSlashed * 1e18 + before.pool.tokens - 1) / before.pool.tokens))) / (1e18);
                 assertEq(afterPool.tokens + calcValues.delegationTokensSlashed, before.pool.tokens);
                 assertEq(afterPool.shares, before.pool.shares);
                 assertEq(afterPool.tokensThawing, poolThawingTokens);

packages/horizon/test/staking/slash/slash.t.sol

@@ -140,4 +140,46 @@ contract HorizonStakingSlashTest is HorizonStakingTest {
         vm.startPrank(subgraphDataServiceAddress);
         _slash(users.indexer, subgraphDataServiceAddress, tokens + delegationTokens, 0);
     }
+
+    function testSlash_RoundDown_TokensThawing_Provision() public useIndexer {
+        uint256 tokens = 1 ether + 1;
+        _useProvision(subgraphDataServiceAddress, tokens, MAX_PPM, MAX_THAWING_PERIOD);
+
+        _thaw(users.indexer, subgraphDataServiceAddress, tokens);
+
+        resetPrank(subgraphDataServiceAddress);
+        _slash(users.indexer, subgraphDataServiceAddress, 1, 0);
+
+        resetPrank(users.indexer);
+        Provision memory provision = staking.getProvision(users.indexer, subgraphDataServiceAddress);
+        assertEq(provision.tokens, tokens - 1);
+        // Tokens thawing should be rounded down
+        assertEq(provision.tokensThawing, tokens - 2);
+    }
+
+    function testSlash_RoundDown_TokensThawing_Delegation(
+        uint256 tokens
+    ) public useIndexer useProvision(tokens, MAX_PPM, 0) useDelegationSlashing {
+        resetPrank(users.delegator);
+        uint256 delegationTokens = 1 ether + 1;
+        _delegate(users.indexer, subgraphDataServiceAddress, delegationTokens, 0);
+
+        DelegationInternal memory delegation = _getStorage_Delegation(
+            users.indexer,
+            subgraphDataServiceAddress,
+            users.delegator,
+            false
+        );
+        _undelegate(users.indexer, subgraphDataServiceAddress, delegation.shares);
+
+        resetPrank(subgraphDataServiceAddress);
+        // Slash 1 token from delegation
+        _slash(users.indexer, subgraphDataServiceAddress, tokens + 1, 0);
+
+        resetPrank(users.delegator);
+        DelegationPool memory pool = staking.getDelegationPool(users.indexer, subgraphDataServiceAddress);
+        assertEq(pool.tokens, delegationTokens - 1);
+        // Tokens thawing should be rounded down
+        assertEq(pool.tokensThawing, delegationTokens - 2);
+    }
 }