DRY common exit/liquidation validation code

The signature verification code in these two functions is virtually
identical now (there was some slight optimization that had been applied
to one of these but not the other, but is functionally identical).

contracts/ServiceNodeRewards.sol

@@ -542,26 +542,9 @@ contract ServiceNodeRewards is Initializable, Ownable2StepUpgradeable, PausableU
         BLSSignatureParams calldata blsSignature,
         uint64[] memory ids
     ) external whenNotPaused whenStarted hasEnoughSigners(ids.length) {
-        bytes memory pubkeyBytes = BN256G1.getKeyForG1Point(blsPubkey);
-        uint64 serviceNodeID = serviceNodeIDs[pubkeyBytes];
-        if (serviceNodeID == 0)
-            revert BLSPubkeyDoesNotExist(blsPubkey);
-        if (signatureTimestampHasExpired(timestamp)) {
-            revert SignatureExpired(serviceNodeID, timestamp, block.timestamp);
-        }
 
-        if (
-            blsPubkey.X != _serviceNodes[serviceNodeID].blsPubkey.X || blsPubkey.Y != _serviceNodes[serviceNodeID].blsPubkey.Y
-        ) revert BLSPubkeyDoesNotMatch(serviceNodeID, blsPubkey);
-        if (block.timestamp < node.addedTimestamp + MINIMUM_EXIT_AGE)
-            revert ExitTooEarly(serviceNodeID, node.addedTimestamp, block.timestamp);
-
-        // NOTE: Validate signature
-        {
-            bytes memory encodedMessage = abi.encodePacked(exitTag, blsPubkey.X, blsPubkey.Y, timestamp);
-            BN256G2.G2Point memory Hm = BN256G2.hashToG2(encodedMessage, hashToG2Tag);
-            validateSignatureOrRevert(ids, blsSignature, Hm);
-        }
+        (uint64 serviceNodeID,) = _validateBLSExitWithSignature(
+            blsPubkey, timestamp, blsSignature, exitTag, ids);
 
         _exitBLSPublicKey(serviceNodeID, _serviceNodes[serviceNodeID].deposit);
     }
@@ -599,49 +582,68 @@ contract ServiceNodeRewards is Initializable, Ownable2StepUpgradeable, PausableU
         emit ServiceNodeExit(serviceNodeID, operator, returnedAmount, pubkey);
     }
 
-    /// @notice Exits a service node by liquidating their node from the
-    /// network rewarding the caller for maintaining the list.
-    ///
-    /// This function can be called by anyone, but requires the network to
-    /// approve the liquidation by aggregating a valid BLS signature. The nodes
-    /// will only provide this signature if the consensus rules permit the node
-    /// to be forcibly exited (e.g. the node was deregistered by consensus in
-    /// Oxen's state-chain, or does not exist on the Oxen chain).
+    /// @dev Internal function to handle common liquidate/exit checks when
+    /// exiting/liquidating with a service node network signature.
     ///
-    /// @param blsPubkey 64 byte BLS public key for the service node to be
-    /// exited.
-    /// @param timestamp The signature creation time.
-    /// @param blsSignature 128 byte BLS signature that affirms that the
-    /// `blsPubkey` is to be liquidated.
-    /// @param ids Array of service node IDs that didn't sign the signature
-    /// and are to be excluded from verification.
-    function liquidateBLSPublicKeyWithSignature(
+    /// @return serviceNodeID the ID of the service node containing `blsPubkey`
+    /// @return node the ServiceNode info for the node
+    function _validateBLSExitWithSignature(
         BN256G1.G1Point calldata blsPubkey,
         uint256 timestamp,
         BLSSignatureParams calldata blsSignature,
+        bytes32 signatureTag,
         uint64[] memory ids
-    ) external whenNotPaused whenStarted hasEnoughSigners(ids.length) {
+    ) internal returns (uint64 serviceNodeID, ServiceNode memory node) {
+
         bytes memory pubkeyBytes = BN256G1.getKeyForG1Point(blsPubkey);
-        uint64 serviceNodeID = serviceNodeIDs[pubkeyBytes];
+        serviceNodeID = serviceNodeIDs[pubkeyBytes];
         if (serviceNodeID == 0)
             revert BLSPubkeyDoesNotExist(blsPubkey);
         if (signatureTimestampHasExpired(timestamp)) {
             revert SignatureExpired(serviceNodeID, timestamp, block.timestamp);
         }
 
-        ServiceNode memory node = _serviceNodes[serviceNodeID];
+        node = _serviceNodes[serviceNodeID];
         if (blsPubkey.X != node.blsPubkey.X || blsPubkey.Y != node.blsPubkey.Y) {
             revert BLSPubkeyDoesNotMatch(serviceNodeID, blsPubkey);
         }
+
         if (block.timestamp < node.addedTimestamp + MINIMUM_EXIT_AGE)
             revert ExitTooEarly(serviceNodeID, node.addedTimestamp, block.timestamp);
 
         // NOTE: Validate signature
         {
-            bytes memory encodedMessage = abi.encodePacked(liquidateTag, blsPubkey.X, blsPubkey.Y, timestamp);
+            bytes memory encodedMessage = abi.encodePacked(signatureTag, blsPubkey.X, blsPubkey.Y, timestamp);
             BN256G2.G2Point memory Hm = BN256G2.hashToG2(encodedMessage, hashToG2Tag);
             validateSignatureOrRevert(ids, blsSignature, Hm);
         }
+    }
+
+    /// @notice Exits a service node by liquidating their node from the
+    /// network rewarding the caller for maintaining the list.
+    ///
+    /// This function can be called by anyone, but requires the network to
+    /// approve the liquidation by aggregating a valid BLS signature. The nodes
+    /// will only provide this signature if the consensus rules permit the node
+    /// to be forcibly exited (e.g. the node was deregistered by consensus in
+    /// Oxen's state-chain, or does not exist on the Oxen chain).
+    ///
+    /// @param blsPubkey 64 byte BLS public key for the service node to be
+    /// exited.
+    /// @param timestamp The signature creation time.
+    /// @param blsSignature 128 byte BLS signature that affirms that the
+    /// `blsPubkey` is to be liquidated.
+    /// @param ids Array of service node IDs that didn't sign the signature
+    /// and are to be excluded from verification.
+    function liquidateBLSPublicKeyWithSignature(
+        BN256G1.G1Point calldata blsPubkey,
+        uint256 timestamp,
+        BLSSignatureParams calldata blsSignature,
+        uint64[] memory ids
+    ) external whenNotPaused whenStarted hasEnoughSigners(ids.length) {
+
+        (uint64 serviceNodeID, ServiceNode memory node) = _validateBLSExitWithSignature(
+            blsPubkey, timestamp, blsSignature, liquidateTag, ids);
 
         // Calculating how much liquidator is paid out
         emit ServiceNodeLiquidated(serviceNodeID, node.operator, node.blsPubkey);