eth/downloader/whitelist: skip future chain validation (#796)

* eth/downloader/whitelist: skip future chain validation

* eth/downloader/whitelist: fix tests for future chain import

eth/downloader/whitelist/service.go

@@ -100,18 +100,20 @@ func (w *Service) IsValidChain(currentHeader *types.Header, chain []*types.Heade
 	}
 
 	// Split the chain into past and future chain
-	pastChain, futureChain := splitChain(current, chain)
+	pastChain, _ := splitChain(current, chain)
+
+	// Note: Do not act on future chain and allow importing all kinds of future chains.
 
 	// Add an offset to future chain if it's not in continuity
-	offset := 0
-	if len(futureChain) != 0 {
-		offset += int(futureChain[0].Number.Uint64()-currentHeader.Number.Uint64()) - 1
-	}
+	// offset := 0
+	// if len(futureChain) != 0 {
+	// 	offset += int(futureChain[0].Number.Uint64()-currentHeader.Number.Uint64()) - 1
+	// }
 
 	// Don't accept future chain of unacceptable length (from current block)
-	if len(futureChain)+offset > int(w.checkpointInterval) {
-		return false, ErrLongFutureChain
-	}
+	// if len(futureChain)+offset > int(w.checkpointInterval) {
+	// 	return false, ErrLongFutureChain
+	// }
 
 	// Iterate over the chain and validate against the last checkpoint
 	// It will handle all cases where the incoming chain has atleast one checkpoint

eth/downloader/whitelist/service_test.go

@@ -166,18 +166,21 @@ func TestIsValidChain(t *testing.T) {
 	// create a future chain to be imported of length <= `checkpointInterval`
 	chainB := createMockChain(21, 30) // B21->B22...B29->B30
 
-	// case5: Try importing a future chain of acceptable length
+	// case5: Try importing a future chain (1)
 	res, err = s.IsValidChain(chainA[len(chainA)-1], chainB)
 	require.Equal(t, res, true, "expected chain to be valid")
 	require.Equal(t, err, nil, "expected error to be nil")
 
 	// create a future chain to be imported of length > `checkpointInterval`
 	chainB = createMockChain(21, 40) // C21->C22...C39->C40
 
-	// case5: Try importing a future chain of unacceptable length
+	// Note: Earlier, it used to reject future chains longer than some threshold.
+	// That check is removed for now.
+
+	// case6: Try importing a future chain (2)
 	res, err = s.IsValidChain(chainA[len(chainA)-1], chainB)
-	require.Equal(t, res, false, "expected chain to be invalid")
-	require.Equal(t, err, ErrLongFutureChain, "expected error")
+	require.Equal(t, res, true, "expected chain to be valid")
+	require.Equal(t, err, nil, "expected error to be nil")
 }
 
 func TestSplitChain(t *testing.T) {