Handle invalid snap getTrieNode requests with empty paths gracefully (#7221)

Signed-off-by: Jason Frame <jason.frame@consensys.net>
Co-authored-by: Sally MacFarlane <macfarla.github@gmail.com>

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/manager/snap/SnapServer.java

@@ -519,6 +519,12 @@ class SnapServer implements BesuEvents.InitialSyncCompletionListener {
                     }
 
                   } else {
+                    // There must be at least one element in the path otherwise it is invalid
+                    if (triePath.isEmpty()) {
+                      LOGGER.debug("returned empty trie nodes message due to invalid path");
+                      return EMPTY_TRIE_NODES_MESSAGE;
+                    }
+
                     // otherwise the first element should be account hash, and subsequent paths
                     // are compact encoded account storage paths
 

ethereum/eth/src/test/java/org/hyperledger/besu/ethereum/eth/manager/snap/SnapServerTest.java

@@ -368,6 +368,18 @@ public class SnapServerTest {
     assertThat(trieNodes.size()).isEqualTo(2);
   }
 
+  @Test
+  public void assertAccountTrieRequest_invalidEmptyPath() {
+    insertTestAccounts(acct1);
+    var partialPathToAcct1 = Bytes.fromHexString("0x01"); // first nibble is 1
+    var trieNodeRequest =
+        requestTrieNodes(
+            storageTrie.getRootHash(), List.of(List.of(), List.of(partialPathToAcct1)));
+    assertThat(trieNodeRequest).isNotNull();
+    List<Bytes> trieNodes = trieNodeRequest.nodes(false);
+    assertThat(trieNodes.isEmpty()).isTrue();
+  }
+
   @Test
   public void assertAccountTrieLimitRequest() {
     insertTestAccounts(acct1, acct2, acct3, acct4);