Add debug logging to see why we stop downloading from peer (hyperledg…

…er#6885)

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

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/fastsync/FastSyncDownloadPipelineFactory.java

@@ -47,7 +47,12 @@
 
 import java.util.concurrent.CompletionStage;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 public class FastSyncDownloadPipelineFactory implements DownloadPipelineFactory {
+  private static final Logger LOG = LoggerFactory.getLogger(FastSyncDownloadPipelineFactory.class);
+
   protected final SynchronizerConfiguration syncConfig;
   protected final ProtocolSchedule protocolSchedule;
   protected final ProtocolContext protocolContext;
@@ -172,6 +177,18 @@ protected BlockHeader getCommonAncestor(final SyncTarget syncTarget) {
   protected boolean shouldContinueDownloadingFromPeer(
       final EthPeer peer, final BlockHeader lastRoundHeader) {
     final BlockHeader pivotBlockHeader = fastSyncState.getPivotBlockHeader().get();
-    return !peer.isDisconnected() && lastRoundHeader.getNumber() < pivotBlockHeader.getNumber();
+    final boolean shouldContinue =
+        !peer.isDisconnected() && lastRoundHeader.getNumber() < pivotBlockHeader.getNumber();
+
+    if (!shouldContinue && peer.isDisconnected()) {
+      LOG.debug("Stopping chain download due to disconnected peer {}", peer);
+    } else if (!shouldContinue && lastRoundHeader.getNumber() >= pivotBlockHeader.getNumber()) {
+      LOG.debug(
+          "Stopping chain download as lastRoundHeader={} is not less than pivotBlockHeader={} for peer {}",
+          lastRoundHeader.getNumber(),
+          pivotBlockHeader.getNumber(),
+          peer);
+    }
+    return shouldContinue;
   }
 }