consensus/parlia: support recovery when snapshot of parlia is gone in…

… disk

consensus/parlia/parlia.go

@@ -53,7 +53,7 @@ const (
 	inMemoryHeaders    = 86400 // Number of recent headers to keep in memory for double sign detection,
 
 	checkpointInterval = 1024        // Number of blocks after which to save the snapshot to the database
-	defaultEpochLength = uint64(100) // Default number of blocks of checkpoint to update validatorSet from contract
+	defaultEpochLength = uint64(200) // Default number of blocks of checkpoint to update validatorSet from contract
 
 	extraVanity      = 32 // Fixed number of extra-data prefix bytes reserved for signer vanity
 	extraSeal        = 65 // Fixed number of extra-data suffix bytes reserved for signer seal
@@ -713,25 +713,38 @@ func (p *Parlia) snapshot(chain consensus.ChainHeaderReader, number uint64, hash
 			}
 		}
 
-		// If we're at the genesis, snapshot the initial state.
-		if number == 0 {
-			checkpoint := chain.GetHeaderByNumber(number)
-			if checkpoint != nil {
-				// get checkpoint data
-				hash := checkpoint.Hash()
-
+		// If we're at the genesis, snapshot the initial state. Alternatively if we have
+		// piled up more headers than allowed to be reorged (chain reinit from a freezer),
+		// consider the checkpoint trusted and snapshot it.
+		// even BEP-341 enabled, an offset `defaultEpochLength/2` can ensure getting the right validators.
+		if number == 0 || (number%p.config.Epoch == defaultEpochLength/2 && (len(headers) > int(params.FullImmutabilityThreshold)/10)) {
+			var (
+				checkpoint *types.Header
+				blockHash  common.Hash
+			)
+			if number == 0 {
+				checkpoint := chain.GetHeaderByNumber(0)
+				blockHash = checkpoint.Hash()
+			} else {
+				checkpoint = chain.GetHeaderByNumber(number - defaultEpochLength/2)
+				blockHeader := chain.GetHeaderByNumber(number)
+				if blockHeader != nil {
+					blockHash = blockHeader.Hash()
+				}
+			}
+			if checkpoint != nil && blockHash != (common.Hash{}) {
 				// get validators from headers
 				validators, voteAddrs, err := parseValidators(checkpoint, p.chainConfig, p.config)
 				if err != nil {
 					return nil, err
 				}
 
 				// new snapshot
-				snap = newSnapshot(p.config, p.signatures, number, hash, validators, voteAddrs, p.ethAPI)
+				snap = newSnapshot(p.config, p.signatures, number, blockHash, validators, voteAddrs, p.ethAPI)
 				if err := snap.store(p.db); err != nil {
 					return nil, err
 				}
-				log.Info("Stored checkpoint snapshot to disk", "number", number, "hash", hash)
+				log.Info("Stored checkpoint snapshot to disk", "number", number, "hash", blockHash)
 				break
 			}
 		}