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

eth/downloader/downloader.go

@@ -1501,18 +1501,20 @@ 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
-					head := chunk[len(chunk)-1].Number.Uint64()
-					if head-rollback > uint64(fsHeaderSafetyNet) {
-						rollback = head - uint64(fsHeaderSafetyNet)
-					} else {
-						rollback = 1
+					if mode == FastSync {
+						head := chunk[len(chunk)-1].Number.Uint64()
+						if head-rollback > uint64(fsHeaderSafetyNet) {
+							rollback = head - uint64(fsHeaderSafetyNet)
+						} else {
+							rollback = 1
+						}
 					}
 				}
 				// Unless we're doing light chains, schedule the headers for associated content retrieval

eth/downloader/downloader_test.go

@@ -1020,10 +1020,9 @@ func testShiftedHeaderAttack(t *testing.T, protocol int, mode SyncMode) {
 // Tests that upon detecting an invalid header, the recent ones are rolled back
 // for various failure scenarios. Afterwards a full sync is attempted to make
 // sure no state was corrupted.
-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 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 testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 	t.Parallel()