Support free gas networks when using London fee market (#4003)

If baseFeePerGas is configured to "0x0" in the genesis file, bypass the need for a transaction to have a gas price of >= 7 Wei

Ensure baseFeeFloor is either 0 or >= 7 to avoid integer arithmetic issues.

Signed-off-by: Simon Dudley <simon.dudley@consensys.net>

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 ### Bug Fixes
 - Fixed a snapsync issue that can sometimes block the healing step [#3920](https://github.com/hyperledger/besu/pull/3920)
+- Support free gas networks in the London fee market [#4003](https://github.com/hyperledger/besu/pull/4003)
 
 ## 22.4.3
 

ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/feemarket/LondonFeeMarket.java

@@ -32,11 +32,14 @@ public class LondonFeeMarket implements BaseFeeMarket {
       GenesisConfigFile.BASEFEE_AT_GENESIS_DEFAULT_VALUE;
   static final long DEFAULT_BASEFEE_MAX_CHANGE_DENOMINATOR = 8L;
   static final long DEFAULT_SLACK_COEFFICIENT = 2L;
+  // required for integer arithmetic to work when baseFee > 0
+  private static final Wei DEFAULT_BASEFEE_FLOOR = Wei.of(7L);
   private static final Logger LOG = LoggerFactory.getLogger(LondonFeeMarket.class);
 
   private final Wei baseFeeInitialValue;
   private final long londonForkBlockNumber;
   private final TransactionPriceCalculator txPriceCalculator;
+  private Wei baseFeeFloor = DEFAULT_BASEFEE_FLOOR;
 
   public LondonFeeMarket(final long londonForkBlockNumber) {
     this(londonForkBlockNumber, Optional.empty());
@@ -47,6 +50,14 @@ public class LondonFeeMarket implements BaseFeeMarket {
     this.txPriceCalculator = TransactionPriceCalculator.eip1559();
     this.londonForkBlockNumber = londonForkBlockNumber;
     this.baseFeeInitialValue = baseFeePerGasOverride.orElse(DEFAULT_BASEFEE_INITIAL_VALUE);
+    if (baseFeeInitialValue.isZero()) {
+      baseFeeFloor = Wei.ZERO;
+    } else if (baseFeeInitialValue.lessThan(DEFAULT_BASEFEE_FLOOR)) {
+      throw new IllegalStateException(
+          String.format(
+              "baseFee must be either 0 or > %s wei to avoid integer arithmetic issues",
+              DEFAULT_BASEFEE_FLOOR));
+    }
   }
 
   @Override
@@ -81,12 +92,11 @@ public class LondonFeeMarket implements BaseFeeMarket {
 
   @Override
   public boolean satisfiesFloorTxCost(final Transaction txn) {
-    // London fee market arithmetic never allows for a base fee below 7 wei
-    // ensure effective baseFee is at least 7 wei
+    // ensure effective baseFee is at least above floor
     return txn.getGasPrice()
         .map(Optional::of)
         .orElse(txn.getMaxFeePerGas())
-        .filter(fee -> fee.greaterOrEqualThan(Wei.of(7L)))
+        .filter(fee -> fee.greaterOrEqualThan(baseFeeFloor))
         .isPresent();
   }
 

ethereum/core/src/test/java/org/hyperledger/besu/ethereum/core/feemarket/TransactionPriceCalculatorTest.java

@@ -72,6 +72,8 @@ public class TransactionPriceCalculatorTest {
         new Object[][] {
           // legacy transaction must return gas price
           {FRONTIER_CALCULATOR, FRONTIER, Wei.of(578L), null, null, Optional.empty(), Wei.of(578L)},
+          // legacy transaction zero price
+          {FRONTIER_CALCULATOR, FRONTIER, Wei.ZERO, null, null, Optional.empty(), Wei.ZERO},
           // ACCESSLIST transaction must return gas price
           {
             FRONTIER_CALCULATOR,
@@ -92,6 +94,8 @@ public class TransactionPriceCalculatorTest {
             Optional.of(Wei.of(150L)),
             Wei.of(578L)
           },
+          // london legacy transaction zero price
+          {EIP_1559_CALCULATOR, FRONTIER, Wei.ZERO, null, null, Optional.of(Wei.ZERO), Wei.ZERO},
           // ACCESSLIST transaction must return gas price
           {
             EIP_1559_CALCULATOR,
@@ -121,7 +125,9 @@ public class TransactionPriceCalculatorTest {
             Wei.of(300L),
             Optional.of(Wei.of(250L)),
             Wei.of(300L)
-          }
+          },
+          // EIP-1559 transaction zero price
+          {EIP_1559_CALCULATOR, EIP1559, null, Wei.ZERO, Wei.ZERO, Optional.of(Wei.ZERO), Wei.ZERO}
         });
   }
 

ethereum/core/src/test/java/org/hyperledger/besu/ethereum/mainnet/MainnetTransactionValidatorTest.java

@@ -395,6 +395,29 @@ public class MainnetTransactionValidatorTest {
         .isEqualTo(ValidationResult.invalid(INVALID_TRANSACTION_FORMAT));
   }
 
+  @Test
+  public void shouldAcceptZeroGasPriceTransactionIfBaseFeeIsZero() {
+    final Optional<Wei> zeroBaseFee = Optional.of(Wei.ZERO);
+    final MainnetTransactionValidator validator =
+        new MainnetTransactionValidator(
+            gasCalculator,
+            FeeMarket.london(0L, zeroBaseFee),
+            false,
+            Optional.of(BigInteger.ONE),
+            Set.of(TransactionType.FRONTIER, TransactionType.EIP1559),
+            defaultGoQuorumCompatibilityMode);
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.EIP1559)
+            .maxPriorityFeePerGas(Optional.of(Wei.ZERO))
+            .maxFeePerGas(Optional.of(Wei.ZERO))
+            .chainId(Optional.of(BigInteger.ONE))
+            .createTransaction(senderKeys);
+
+    assertThat(validator.validate(transaction, zeroBaseFee, transactionValidationParams))
+        .isEqualTo(ValidationResult.valid());
+  }
+
   @Test
   public void shouldAcceptValidEIP1559() {
     final MainnetTransactionValidator validator =

ethereum/core/src/test/java/org/hyperledger/besu/ethereum/mainnet/feemarket/LondonFeeMarketTest.java

@@ -0,0 +1,81 @@
+/*
+ * Copyright Hyperledger Besu Contributors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package org.hyperledger.besu.ethereum.mainnet.feemarket;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.hyperledger.besu.crypto.KeyPair;
+import org.hyperledger.besu.crypto.SignatureAlgorithmFactory;
+import org.hyperledger.besu.datatypes.Wei;
+import org.hyperledger.besu.ethereum.core.Transaction;
+import org.hyperledger.besu.ethereum.core.TransactionTestFixture;
+import org.hyperledger.besu.plugin.data.TransactionType;
+
+import java.util.Optional;
+
+import org.junit.Test;
+
+public class LondonFeeMarketTest {
+
+  private static final KeyPair KEY_PAIR1 =
+      SignatureAlgorithmFactory.getInstance().generateKeyPair();
+
+  @Test
+  public void satisfiesFloorTxCost() {
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.FRONTIER)
+            .gasPrice(Wei.of(7))
+            .createTransaction(KEY_PAIR1);
+
+    final LondonFeeMarket londonFeeMarket = new LondonFeeMarket(0);
+    assertThat(londonFeeMarket.satisfiesFloorTxCost(transaction)).isTrue();
+  }
+
+  @Test
+  public void maxFeePerGasLessThanMinimumBaseFee() {
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.EIP1559)
+            .maxFeePerGas(Optional.of(Wei.of(6)))
+            .maxPriorityFeePerGas(Optional.of(Wei.of(0)))
+            .gasPrice(null)
+            .createTransaction(KEY_PAIR1);
+
+    final LondonFeeMarket londonFeeMarket = new LondonFeeMarket(0);
+    assertThat(londonFeeMarket.satisfiesFloorTxCost(transaction)).isFalse();
+  }
+
+  @Test
+  public void satisfiesFloorTxCostWhenBaseFeeInitialValueIsZero() {
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.EIP1559)
+            .maxFeePerGas(Optional.of(Wei.ZERO))
+            .maxPriorityFeePerGas(Optional.of(Wei.ZERO))
+            .gasPrice(null)
+            .createTransaction(KEY_PAIR1);
+
+    final LondonFeeMarket londonFeeMarket = new LondonFeeMarket(0, Optional.of(Wei.ZERO));
+    assertThat(londonFeeMarket.satisfiesFloorTxCost(transaction)).isTrue();
+  }
+
+  @Test
+  public void throwsWhenBaseFeeOverrideIsNonZeroAndBelowFloor() {
+    assertThatThrownBy(() -> new LondonFeeMarket(0, Optional.of(Wei.of(6))))
+        .isInstanceOf(IllegalStateException.class);
+  }
+}

ethereum/eth/src/test/java/org/hyperledger/besu/ethereum/eth/transactions/TransactionPoolTest.java

@@ -45,6 +45,7 @@ import org.hyperledger.besu.ethereum.chain.MutableBlockchain;
 import org.hyperledger.besu.ethereum.core.Block;
 import org.hyperledger.besu.ethereum.core.BlockBody;
 import org.hyperledger.besu.ethereum.core.BlockHeader;
+import org.hyperledger.besu.ethereum.core.BlockHeaderBuilder;
 import org.hyperledger.besu.ethereum.core.BlockHeaderTestFixture;
 import org.hyperledger.besu.ethereum.core.Difficulty;
 import org.hyperledger.besu.ethereum.core.ExecutionContextTestFixture;
@@ -61,6 +62,7 @@ import org.hyperledger.besu.ethereum.eth.manager.EthScheduler;
 import org.hyperledger.besu.ethereum.eth.manager.RespondingEthPeer;
 import org.hyperledger.besu.ethereum.eth.messages.EthPV65;
 import org.hyperledger.besu.ethereum.eth.transactions.sorter.GasPricePendingTransactionsSorter;
+import org.hyperledger.besu.ethereum.mainnet.MainnetBlockHeaderFunctions;
 import org.hyperledger.besu.ethereum.mainnet.MainnetTransactionValidator;
 import org.hyperledger.besu.ethereum.mainnet.ProtocolSchedule;
 import org.hyperledger.besu.ethereum.mainnet.ProtocolSpec;
@@ -999,6 +1001,78 @@ public class TransactionPoolTest {
     assertThat(result.getInvalidReason()).isEqualTo(TransactionInvalidReason.GAS_PRICE_TOO_LOW);
   }
 
+  @Test
+  public void shouldAcceptZeroGasPriceFrontierTxsWhenMinGasPriceIsZero() {
+    when(miningParameters.getMinTransactionGasPrice()).thenReturn(Wei.ZERO);
+
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.FRONTIER)
+            .gasPrice(Wei.ZERO)
+            .value(Wei.ONE)
+            .chainId(Optional.of(BigInteger.ONE))
+            .createTransaction(KEY_PAIR1);
+
+    givenTransactionIsValid(transaction);
+
+    final ValidationResult<TransactionInvalidReason> result =
+        transactionPool.addLocalTransaction(transaction);
+
+    assertThat(result).isEqualTo(ValidationResult.valid());
+  }
+
+  @Test
+  public void shouldAcceptZeroGasPriceFrontierTxsWhenMinGasPriceIsZeroAndLondonWithZeroBaseFee() {
+    when(miningParameters.getMinTransactionGasPrice()).thenReturn(Wei.ZERO);
+    when(protocolSpec.getFeeMarket()).thenReturn(FeeMarket.london(0, Optional.of(Wei.ZERO)));
+    whenBlockBaseFeeIsZero();
+
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.FRONTIER)
+            .gasPrice(Wei.ZERO)
+            .createTransaction(KEY_PAIR1);
+
+    givenTransactionIsValid(transaction);
+
+    final ValidationResult<TransactionInvalidReason> result =
+        transactionPool.addLocalTransaction(transaction);
+
+    assertThat(result).isEqualTo(ValidationResult.valid());
+  }
+
+  private void whenBlockBaseFeeIsZero() {
+    final BlockHeader header =
+        BlockHeaderBuilder.fromHeader(blockchain.getChainHeadHeader())
+            .baseFee(Wei.ZERO)
+            .blockHeaderFunctions(new MainnetBlockHeaderFunctions())
+            .parentHash(blockchain.getChainHeadHash())
+            .buildBlockHeader();
+    blockchain.appendBlock(new Block(header, BlockBody.empty()), emptyList());
+  }
+
+  @Test
+  public void shouldAcceptZeroGasPrice1559TxsWhenMinGasPriceIsZeroAndLondonWithZeroBaseFee() {
+    when(miningParameters.getMinTransactionGasPrice()).thenReturn(Wei.ZERO);
+    when(protocolSpec.getFeeMarket()).thenReturn(FeeMarket.london(0, Optional.of(Wei.ZERO)));
+    whenBlockBaseFeeIsZero();
+
+    final Transaction transaction =
+        new TransactionTestFixture()
+            .type(TransactionType.EIP1559)
+            .gasPrice(null)
+            .maxFeePerGas(Optional.of(Wei.ZERO))
+            .maxPriorityFeePerGas(Optional.of(Wei.ZERO))
+            .createTransaction(KEY_PAIR1);
+
+    givenTransactionIsValid(transaction);
+
+    final ValidationResult<TransactionInvalidReason> result =
+        transactionPool.addLocalTransaction(transaction);
+
+    assertThat(result).isEqualTo(ValidationResult.valid());
+  }
+
   private void assertTransactionPending(final Transaction t) {
     assertThat(transactions.getTransactionByHash(t.getHash())).contains(t);
   }