eth/downloader: only roll back light sync if not fully validating

eth/downloader/downloader.go

@@ -1501,13 +1501,14 @@ func (d *Downloader) processHeaders(origin uint64, pivot uint64, td *big.Int) er
 						rollbackErr = err
 
 						// If some headers were inserted, track them as uncertain
-						if n > 0 && rollback == 0 {
+						if (mode == FastSync || frequency > 1) && n > 0 && rollback == 0 {
 							rollback = chunk[0].Number.Uint64()
 						}
-						log.Debug("Invalid header encountered", "number", chunk[n].Number, "hash", chunk[n].Hash(), "parent", chunk[n].ParentHash, "err", err)
+						log.Warn("Invalid header encountered", "number", chunk[n].Number, "hash", chunk[n].Hash(), "parent", chunk[n].ParentHash, "err", err)
 						return fmt.Errorf("%w: %v", errInvalidChain, err)
 					}
 					// All verifications passed, track all headers within the alloted limits
+					if mode == FastSync {
 						head := chunk[len(chunk)-1].Number.Uint64()
 						if head-rollback > uint64(fsHeaderSafetyNet) {
 							rollback = head - uint64(fsHeaderSafetyNet)
@@ -1515,6 +1516,7 @@ func (d *Downloader) processHeaders(origin uint64, pivot uint64, td *big.Int) er
 							rollback = 1
 						}
 					}
+				}
 				// Unless we're doing light chains, schedule the headers for associated content retrieval
 				if mode == FullSync || mode == FastSync {
 					// If we've reached the allowed number of pending headers, stall a bit

eth/downloader/downloader_test.go

@@ -1023,7 +1023,6 @@ func testShiftedHeaderAttack(t *testing.T, protocol int, mode SyncMode) {
 func TestInvalidHeaderRollback63Fast(t *testing.T) { testInvalidHeaderRollback(t, 63, FastSync) }
 func TestInvalidHeaderRollback64Fast(t *testing.T) { testInvalidHeaderRollback(t, 64, FastSync) }
 func TestInvalidHeaderRollback65Fast(t *testing.T) { testInvalidHeaderRollback(t, 65, FastSync) }
-func TestInvalidHeaderRollback65Light(t *testing.T) { testInvalidHeaderRollback(t, 65, LightSync) }
 
 func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 	t.Parallel()