make indexer handle null block hashes (#473)

Before the future release 0.2.0 the block_hash is not stored in
database. In future releases we're using the block_hash to check for
reorgs.

There's a rare corner case where the indexer tries to retrieve a block
hash that happens to be null, in that case we should skip it and check
for reorgs later in the chain.

In the case when a reorg happens later in the chain, and the reorg
handler algorithm determines the reorg happened in a range we don't have
any data about, then the indexer should simply start indexing from 0, as
it's the safest option.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Enhanced reorganization detection by adding safeguards to handle
scenarios with missing block data.
	- Improved error handling to prevent issues during blockchain updates.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

pkg/indexer/indexer.go

@@ -304,7 +304,8 @@ func indexLogs(
 				zap.Uint64("blockNumber", reorgCheckAt),
 			)
 
-			if !bytes.Equal(storedBlockHash, onchainBlock.Hash().Bytes()) {
+			if storedBlockHash != nil &&
+				!bytes.Equal(storedBlockHash, onchainBlock.Hash().Bytes()) {
 				logger.Warn("blockchain reorg detected",
 					zap.Uint64("storedBlockNumber", storedBlockNumber),
 					zap.String("storedBlockHash", hex.EncodeToString(storedBlockHash)),

pkg/indexer/reorgHandler.go

@@ -72,6 +72,26 @@ func (r *ReorgHandler) FindReorgPoint(detectedAt uint64) (uint64, []byte, error)
 		}
 
 		oldestBlock := storedBlocks[0]
+
+		if oldestBlock.BlockHash == nil {
+			startAtZero, err := r.client.BlockByNumber(
+				r.ctx,
+				big.NewInt(0),
+			)
+			if err != nil {
+				return 0, nil, fmt.Errorf("%w %d: %v", ErrGetBlock, oldestBlock.BlockNumber, err)
+			}
+
+			if err := r.queries.UpdateBlocksCanonicalityInRange(r.ctx, queries.UpdateBlocksCanonicalityInRangeParams{
+				StartBlockNumber: 0,
+				EndBlockNumber:   detectedAt,
+			}); err != nil {
+				return 0, nil, fmt.Errorf("failed to update block range canonicality: %w", err)
+			}
+
+			return 0, startAtZero.Hash().Bytes(), nil
+		}
+
 		chainBlock, err := r.client.BlockByNumber(r.ctx, big.NewInt(int64(oldestBlock.BlockNumber)))
 		if err != nil {
 			return 0, nil, fmt.Errorf("%w %d: %v", ErrGetBlock, oldestBlock.BlockNumber, err)