feat: add a way to read memory without altering the word capacity

CHANGELOG.md

@@ -18,6 +18,7 @@
 
 ### Additions and Improvements
 - Ethereum Classic Spiral network upgrade [#6078](https://github.com/hyperledger/besu/pull/6078)
+- Add a method to read from a `Memory` instance without altering its inner state [#6073](https://github.com/hyperledger/besu/pull/6073)
 
 ### Bug fixes
 

ethereum/core/src/test/java/org/hyperledger/besu/ethereum/vm/MemoryTest.java

@@ -65,6 +65,22 @@ public void shouldSetMemoryWhenLengthGreaterThanSourceLength() {
     assertThat(memory.getWord(64)).isEqualTo(Bytes32.ZERO);
   }
 
+  @Test
+  public void shouldNotIncreaseActiveWordsIfGetBytesWithoutGrowth() {
+    final Bytes value = Bytes.concatenate(WORD1, WORD2);
+    memory.setBytes(0, value.size(), value);
+    final int initialActiveWords = memory.getActiveWords();
+
+    assertThat(memory.getBytesWithoutGrowth(64, Bytes32.SIZE)).isEqualTo((Bytes32.ZERO));
+    assertThat(memory.getActiveWords()).isEqualTo(initialActiveWords);
+
+    assertThat(memory.getBytes(32, Bytes32.SIZE)).isEqualTo((WORD2));
+    assertThat(memory.getActiveWords()).isEqualTo(initialActiveWords);
+
+    assertThat(memory.getBytes(64, Bytes32.SIZE)).isEqualTo((Bytes32.ZERO));
+    assertThat(memory.getActiveWords()).isEqualTo(initialActiveWords + 1);
+  }
+
   @Test
   public void shouldClearMemoryAfterSourceDataWhenLengthGreaterThanSourceLength() {
     memory.setWord(64, WORD3);

evm/src/main/java/org/hyperledger/besu/evm/frame/Memory.java

@@ -18,6 +18,7 @@
 
 import java.util.Arrays;
 
+import com.google.common.annotations.VisibleForTesting;
 import org.apache.tuweni.bytes.Bytes;
 import org.apache.tuweni.bytes.Bytes32;
 import org.apache.tuweni.bytes.MutableBytes;
@@ -179,7 +180,8 @@ int getActiveBytes() {
    *
    * @return The current number of active words stored in memory.
    */
-  int getActiveWords() {
+  @VisibleForTesting
+  public int getActiveWords() {
     return activeWords;
   }
 
@@ -202,11 +204,40 @@ public Bytes getBytes(final long location, final long numBytes) {
     }
 
     final int start = asByteIndex(location);
-
     ensureCapacityForBytes(start, length);
     return Bytes.wrap(Arrays.copyOfRange(memBytes, start, start + length));
   }
 
+  /**
+   * Returns a copy of bytes by peeking into memory without expanding the active words.
+   *
+   * @param location The location in memory to start with.
+   * @param numBytes The number of bytes to get.
+   * @return A fresh copy of the bytes from memory starting at {@code location} and extending {@code
+   *     numBytes}.
+   */
+  public Bytes getBytesWithoutGrowth(final long location, final long numBytes) {
+    // Note: if length == 0, we don't require any memory expansion, whatever location is. So
+    // we must call asByteIndex(location) after this check so as it doesn't throw if the location
+    // is too big but the length is 0 (which is somewhat nonsensical, but is exercise by some
+    // tests).
+    final int length = asByteLength(numBytes);
+    if (length == 0) {
+      return Bytes.EMPTY;
+    }
+
+    final int start = asByteIndex(location);
+
+    // Arrays.copyOfRange would throw if start > memBytes.length, so just return the expected
+    // number of zeros without expanding the memory.
+    // Otherwise, just follow the happy path.
+    if (start > memBytes.length) {
+      return Bytes.wrap(new byte[(int) numBytes]);
+    } else {
+      return Bytes.wrap(Arrays.copyOfRange(memBytes, start, start + length));
+    }
+  }
+
   /**
    * Returns a copy of bytes from memory.
    *

evm/src/main/java/org/hyperledger/besu/evm/frame/MessageFrame.java

@@ -654,6 +654,17 @@ public MutableBytes readMutableMemory(final long offset, final long length) {
     return readMutableMemory(offset, length, false);
   }
 
+  /**
+   * Read bytes in memory without expanding the word capacity.
+   *
+   * @param offset The offset in memory
+   * @param length The length of the bytes to read
+   * @return The bytes in the specified range
+   */
+  public Bytes shadowReadMemory(final long offset, final long length) {
+    return memory.getBytesWithoutGrowth(offset, length);
+  }
+
   /**
    * Read bytes in memory .
    *

evm/src/test/java/org/hyperledger/besu/evm/frame/MessageFrameTest.java

@@ -0,0 +1,91 @@
+/*
+ * 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.evm.frame;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.hyperledger.besu.datatypes.Address;
+import org.hyperledger.besu.datatypes.Hash;
+import org.hyperledger.besu.datatypes.Wei;
+import org.hyperledger.besu.evm.code.CodeV0;
+import org.hyperledger.besu.evm.toy.ToyBlockValues;
+import org.hyperledger.besu.evm.toy.ToyWorld;
+
+import org.apache.tuweni.bytes.Bytes;
+import org.apache.tuweni.bytes.Bytes32;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class MessageFrameTest {
+
+  private static final Bytes32 WORD1 = Bytes32.fromHexString(Long.toString(1).repeat(64));
+  private static final Bytes32 WORD2 = Bytes32.fromHexString(Long.toString(2).repeat(64));
+
+  private MessageFrame.Builder messageFrameBuilder;
+
+  @BeforeEach
+  void setUp() {
+    messageFrameBuilder =
+        MessageFrame.builder()
+            .worldUpdater(new ToyWorld())
+            .originator(Address.ZERO)
+            .gasPrice(Wei.ONE)
+            .blobGasPrice(Wei.ONE)
+            .blockValues(new ToyBlockValues())
+            .miningBeneficiary(Address.ZERO)
+            .blockHashLookup((l) -> Hash.ZERO)
+            .type(MessageFrame.Type.MESSAGE_CALL)
+            .initialGas(1)
+            .address(Address.ZERO)
+            .contract(Address.ZERO)
+            .inputData(Bytes32.ZERO)
+            .sender(Address.ZERO)
+            .value(Wei.ZERO)
+            .apparentValue(Wei.ZERO)
+            .code(CodeV0.EMPTY_CODE)
+            .completer(messageFrame -> {});
+  }
+
+  @Test
+  void shouldNotExpandMemory() {
+    final MessageFrame messageFrame = messageFrameBuilder.build();
+
+    final Bytes value = Bytes.concatenate(WORD1, WORD2);
+    messageFrame.writeMemory(0, value.size(), value);
+    int initialActiveWords = messageFrame.memoryWordSize();
+
+    // Fully in bounds read
+    assertThat(messageFrame.shadowReadMemory(64, Bytes32.SIZE)).isEqualTo(Bytes32.ZERO);
+    assertThat(messageFrame.memoryWordSize()).isEqualTo(initialActiveWords);
+
+    // Straddling read
+    final Bytes straddlingRead = messageFrame.shadowReadMemory(50, Bytes32.SIZE);
+    assertThat(messageFrame.memoryWordSize()).isEqualTo(initialActiveWords);
+    assertThat(straddlingRead.get(0)).isEqualTo((byte) 0x22); // Still in WORD2
+    assertThat(straddlingRead.get(13)).isEqualTo((byte) 0x22); // Just before uninitialized memory
+    assertThat(straddlingRead.get(14)).isEqualTo((byte) 0); // Just in uninitialized memory
+    assertThat(straddlingRead.get(20)).isEqualTo((byte) 0); // In uninitialized memory
+
+    // Fully out of bounds read
+    assertThat(messageFrame.shadowReadMemory(64, Bytes32.SIZE)).isEqualTo(Bytes32.ZERO);
+    assertThat(messageFrame.memoryWordSize()).isEqualTo(initialActiveWords);
+
+    assertThat(messageFrame.shadowReadMemory(32, Bytes32.SIZE)).isEqualTo(WORD2);
+    assertThat(messageFrame.memoryWordSize()).isEqualTo(initialActiveWords);
+  }
+}