Fixed panic of .idx during merge of historical .idxes (#12133)

eth/stagedsync/stage_snapshots.go

@@ -74,7 +74,14 @@ import (
 	"github.com/erigontech/erigon/turbo/snapshotsync/freezeblocks"
 )
 
-const pruneMarkerSafeThreshold = 1000 // Keep 1000 blocks of markers in the DB below snapshot available blocks
+const (
+	/*
+		we strive to read indexes from snapshots instead to db... this means that there can be sometimes (e.g when we merged past indexes),
+		a situation when we need to read indexes and we choose to read them from either a corrupt index or an incomplete index.
+		so we need to extend the threshold to > max_merge_segment_size.
+	*/
+	pruneMarkerSafeThreshold = snaptype.Erigon2MergeLimit * 1.5 // 1.5x the merge limit
+)
 
 type SnapshotsCfg struct {
 	db          kv.RwDB