fix: resolve indexer infinite loop (#82)

on very slow drives or when run with limited resources, a node can have
a delay between the block existing & being saved and the block_results
getting saved. if the block exists, but the block_results do not, an
infinite loop occurs. the indexer will repeatedly request the block and
block_results until they both exist. the lack of delay can further
constrain the node's resources and result in many calls for block_results
before they are committed.

this commit updates the condition for waiting to include whenever an error
occurred during indexing. if the indexer fails to find the block_results
it will bombard the node with requests for it without backing off. this
change causes errors to trigger a wait. after waiting for either a new
block or for the timeout, the block results are more likely to exist.

server/indexer_service.go

@@ -23,6 +23,7 @@ import (
 	"github.com/cenkalti/backoff/v4"
 	"github.com/cometbft/cometbft/libs/service"
 	rpcclient "github.com/cometbft/cometbft/rpc/client"
+	coretypes "github.com/cometbft/cometbft/rpc/core/types"
 	"github.com/cometbft/cometbft/types"
 
 	ethermint "github.com/evmos/ethermint/types"
@@ -111,24 +112,33 @@ func (eis *EVMIndexerService) OnStart() error {
 	if lastBlock == -1 {
 		lastBlock = latestBlock
 	}
+	// blockErr indicates an error fetching an expected block or its results
+	var blockErr error
 	for {
-		if latestBlock <= lastBlock {
-			// nothing to index. wait for signal of new block
+		var block *coretypes.ResultBlock
+		var blockResult *coretypes.ResultBlockResults
+		if latestBlock <= lastBlock || blockErr != nil {
+			// two cases:
+			// 1. nothing to index (indexer is caught up). wait for signal of new block.
+			// 2. previous attempt to index errored (failed to fetch the Block or BlockResults).
+			//    in this case, wait before retrying the data fetching, rather than infinite looping
+			//    a failing fetch. this can occur due to drive latency between the block existing and its
+			//    block_results getting saved.
 			select {
 			case <-newBlockSignal:
 			case <-time.After(NewBlockWaitTimeout):
 			}
 			continue
 		}
 		for i := lastBlock + 1; i <= latestBlock; i++ {
-			block, err := eis.client.Block(ctx, &i)
-			if err != nil {
-				eis.Logger.Error("failed to fetch block", "height", i, "err", err)
+			block, blockErr = eis.client.Block(ctx, &i)
+			if blockErr != nil {
+				eis.Logger.Error("failed to fetch block", "height", i, "err", blockErr)
 				break
 			}
-			blockResult, err := eis.client.BlockResults(ctx, &i)
-			if err != nil {
-				eis.Logger.Error("failed to fetch block result", "height", i, "err", err)
+			blockResult, blockErr = eis.client.BlockResults(ctx, &i)
+			if blockErr != nil {
+				eis.Logger.Error("failed to fetch block result", "height", i, "err", blockErr)
 				break
 			}
 			if err := eis.txIdxr.IndexBlock(block.Block, blockResult.TxsResults); err != nil {