Add validation for start block number in tier1 service

Added checks in blocks method of Tier1 service to ensure that the requested start block number is equal to or greater than the first streamable block of the chain. This prevents errors caused by invalid block number requests.

docs/release-notes/change-log.md

@@ -4,6 +4,14 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Bug fixes
+
+* If the initial block or start block is less than the first block in the chain, the substreams will now start from the
+  first block in the chain. Previously, setting the initial block to a block before the first block in the chain would
+  cause the substreams to hang.
+
 ## v1.1.12
 
 ### Highlights

service/tier1.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/streamingfast/bstream"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -284,6 +285,13 @@ func (s *Tier1Service) writePackage(ctx context.Context, request *pbsubstreamsrp
 var IsValidCacheTag = regexp.MustCompile(`^[a-zA-Z0-9_-]+$`).MatchString
 
 func (s *Tier1Service) blocks(ctx context.Context, request *pbsubstreamsrpc.Request, outputGraph *outputmodules.Graph, respFunc substreams.ResponseFunc) error {
+	chainFirstStreamableBlock := bstream.GetProtocolFirstStreamableBlock
+	if request.StartBlockNum != 0 && request.StartBlockNum < int64(chainFirstStreamableBlock) {
+		return stream.NewErrInvalidArg("invalid start block %d, must be >= %d (the first streamable block of the chain)", request.StartBlockNum, chainFirstStreamableBlock)
+	} else if request.StartBlockNum == 0 {
+		request.StartBlockNum = int64(chainFirstStreamableBlock)
+	}
+
 	logger := reqctx.Logger(ctx)
 
 	requestDetails, undoSignal, err := pipeline.BuildRequestDetails(ctx, request, s.getRecentFinalBlock, s.resolveCursor, s.getHeadBlock)