les: implement ultralight client (#16904)

For more information about this light client mode, read
https://hackmd.io/s/HJy7jjZpm

1 files changed, 10 insertions, 6 deletions

diff --git a/core/headerchain.go b/core/headerchain.go
index d2093113c..8904dd887 100644
--- a/core/headerchain.go
+++ b/core/headerchain.go
@@ -219,14 +219,18 @@ func (hc *HeaderChain) ValidateHeaderChain(chain []*types.Header, checkFreq int)
 
 	// Generate the list of seal verification requests, and start the parallel verifier
 	seals := make([]bool, len(chain))
-	for i := 0; i < len(seals)/checkFreq; i++ {
-		index := i*checkFreq + hc.rand.Intn(checkFreq)
-		if index >= len(seals) {
-			index = len(seals) - 1
+	if checkFreq != 0 {
+		// In case of checkFreq == 0 all seals are left false.
+		for i := 0; i < len(seals)/checkFreq; i++ {
+			index := i*checkFreq + hc.rand.Intn(checkFreq)
+			if index >= len(seals) {
+				index = len(seals) - 1
+			}
+			seals[index] = true
 		}
-		seals[index] = true
+		// Last should always be verified to avoid junk.
+		seals[len(seals)-1] = true
 	}
-	seals[len(seals)-1] = true // Last should always be verified to avoid junk
 
 	abort, results := hc.engine.VerifyHeaders(hc, chain, seals)
 	defer close(abort)