fix nil-ptr in DumpBeaconBlocks (#19532)

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1b469aa]
goroutine 13874 [running]:
github.com/erigontech/erigon/db/snapcfg.MergeLimitFromCfg(...)
	github.com/erigontech/erigon/db/snapcfg/util.go:456
github.com/erigontech/erigon/db/snapshotsync/freezeblocks.chooseSegmentEnd(0x1959420, 0x1960950, 0x0?, 0x0?)
	github.com/erigontech/erigon/db/snapshotsync/freezeblocks/block_snapshots.go:132 +0x2a
github.com/erigontech/erigon/db/snapshotsync/freezeblocks.DumpBeaconBlocks({_, _}, {_, _}, _, _, _, {{0xc001a16f50, 0xc}, {0xc001a16f50, ...}, ...}, ...)
	github.com/erigontech/erigon/db/snapshotsync/freezeblocks/caplin_snapshots.go:600 +0x179
github.com/erigontech/erigon/cl/antiquary.(*Antiquary).antiquate(0xc0f4c34488)
	github.com/erigontech/erigon/cl/antiquary/antiquary.go:315 +0x3a8
github.com/erigontech/erigon/cl/antiquary.(*Antiquary).Loop(0xc0f4c34488)
	github.com/erigontech/erigon/cl/antiquary/antiquary.go:257 +0x13b8
github.com/erigontech/erigon/cmd/caplin/caplin1.RunCaplinService.func3()
	github.com/erigontech/erigon/cmd/caplin/caplin1/run.go:450 +0xe7
created by github.com/erigontech/erigon/cmd/caplin/caplin1.RunCaplinService in goroutine 779
	github.com/erigontech/erigon/cmd/caplin/caplin1/run.go:443 +0x31ae
```

Author: AskAlexSharov

cmd/downloader/main.go

@@ -363,7 +363,7 @@ func Downloader(cmd *cobra.Command, logger log.Logger) error {
 	d.InitBackgroundLogger(true)
 	if seedbox {
 		var downloadItems []*downloaderproto.DownloadItem
-		snapCfg, _ := snapcfg.KnownCfg(chain)
+		snapCfg := snapcfg.KnownCfgOrDevnet(chain)
 		for _, it := range snapCfg.Preverified.Items {
 			downloadItems = append(downloadItems, &downloaderproto.DownloadItem{
 				Path:        it.Name,

db/downloader/downloadercfg/downloadercfg.go

@@ -308,7 +308,8 @@ func New(
 // LoadSnapshotsHashes checks local preverified.toml. If file exists, used local hashes.
 // If there are no such file, try to fetch hashes from the web and create local file.
 func LoadSnapshotsHashes(ctx context.Context, dirs datadir.Dirs, chainName string) error {
-	if _, known := snapcfg.KnownCfg(chainName); !known {
+	cfg, known := snapcfg.KnownCfg(chainName)
+	if !known {
 		log.Root().Warn("No snapshot hashes for chain", "chain", chainName)
 		return nil
 	}
@@ -333,7 +334,6 @@ func LoadSnapshotsHashes(ctx context.Context, dirs datadir.Dirs, chainName strin
 			return fmt.Errorf("failed to fetch remote snapshot hashes for chain %s", chainName)
 		}
 	}
-	cfg, _ := snapcfg.KnownCfg(chainName)
 	cfg.Local = exists
 	return nil
 }

db/downloader/util.go

@@ -79,7 +79,7 @@ func seedableSegmentFiles(dir string, chainName string, skipSeedableCheck bool)
 		return nil, err
 	}
 
-	segConfig, _ := snapcfg.KnownCfg(chainName)
+	segConfig := snapcfg.KnownCfgOrDevnet(chainName)
 
 	res := make([]string, 0, len(files))
 	for _, fPath := range files {

db/snapcfg/util.go

@@ -444,32 +444,10 @@ func RegisterKnownTypes(networkName string, types []snaptype.Type) {
 
 var knownTypes = map[string][]snaptype.Type{}
 
-func Seedable(networkName string, info snaptype.FileInfo) bool {
-	if networkName == "" {
-		return false
-	}
-	snapCfg, _ := KnownCfg(networkName)
-	return snapCfg.Seedable(info)
-}
-
 func MergeLimitFromCfg(cfg *Cfg, snapType snaptype.Enum, fromBlock uint64) uint64 {
 	return cfg.MergeLimit(snapType, fromBlock)
 }
 
-func MaxSeedableSegment(chain string, dir string) uint64 {
-	var _max uint64
-	segConfig, _ := KnownCfg(chain)
-	if list, err := snaptype.Segments(dir); err == nil {
-		for _, info := range list {
-			if segConfig.Seedable(info) && info.Type.Enum() == snaptype.MinCoreEnum && info.To > _max {
-				_max = info.To
-			}
-		}
-	}
-
-	return _max
-}
-
 var oldMergeSteps = append([]uint64{snaptype.Erigon2OldMergeLimit}, snaptype.MergeSteps...)
 
 func MergeStepsFromCfg(cfg *Cfg, snapType snaptype.Enum, fromBlock uint64) []uint64 {

db/snapshotsync/freezeblocks/caplin_snapshots.go

@@ -590,14 +590,14 @@ func DumpBlobSidecarsRange(ctx context.Context, db kv.RoDB, storage blob_storage
 }
 
 func DumpBeaconBlocks(ctx context.Context, db kv.RoDB, fromSlot, toSlot uint64, salt uint32, dirs datadir.Dirs, workers int, lvl log.Lvl, logger log.Logger) error {
-	cfg, _ := snapcfg.KnownCfg("")
-	for i := fromSlot; i < toSlot; i = chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BeaconBlocks, nil) {
+	cfg := snapcfg.KnownCfgOrDevnet("")
+	for i := fromSlot; i < toSlot; i = chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BeaconBlocks, cfg) {
 		blocksPerFile := snapcfg.MergeLimitFromCfg(cfg, snaptype.CaplinEnums.BeaconBlocks, i)
 
 		if toSlot-i < blocksPerFile {
 			break
 		}
-		to := chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BeaconBlocks, nil)
+		to := chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BeaconBlocks, cfg)
 		logger.Log(lvl, "Dumping beacon blocks", "from", i, "to", to)
 		if err := dumpBeaconBlocksRange(ctx, db, i, to, salt, dirs, workers, lvl, logger); err != nil {
 			return err
@@ -609,14 +609,14 @@ func DumpBeaconBlocks(ctx context.Context, db kv.RoDB, fromSlot, toSlot uint64,
 type BlobCountBySlotFn func(slot uint64) (uint64, error)
 
 func DumpBlobsSidecar(ctx context.Context, blobStorage blob_storage.BlobStorage, db kv.RoDB, fromSlot, toSlot uint64, salt uint32, dirs datadir.Dirs, compressWorkers int, blobCountFn BlobCountBySlotFn, lvl log.Lvl, logger log.Logger) error {
-	cfg, _ := snapcfg.KnownCfg("")
-	for i := fromSlot; i < toSlot; i = chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BlobSidecars, nil) {
+	cfg := snapcfg.KnownCfgOrDevnet("")
+	for i := fromSlot; i < toSlot; i = chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BlobSidecars, cfg) {
 		blocksPerFile := snapcfg.MergeLimitFromCfg(cfg, snaptype.CaplinEnums.BlobSidecars, i)
 
 		if toSlot-i < blocksPerFile {
 			break
 		}
-		to := chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BlobSidecars, nil)
+		to := chooseSegmentEnd(i, toSlot, snaptype.CaplinEnums.BlobSidecars, cfg)
 		logger.Log(lvl, "Dumping blobs sidecars", "from", i, "to", to)
 		if err := DumpBlobSidecarsRange(ctx, db, blobStorage, i, to, salt, dirs, compressWorkers, blobCountFn, lvl, logger); err != nil {
 			return err

db/snapshotsync/freezeblocks/caplin_snapshots_test.go

@@ -0,0 +1,46 @@
+// Copyright 2024 The Erigon Authors
+// This file is part of Erigon.
+//
+// Erigon is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Erigon is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with Erigon. If not, see <http://www.gnu.org/licenses/>.
+
+package freezeblocks
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/erigontech/erigon/common/log/v3"
+	"github.com/erigontech/erigon/db/datadir"
+	"github.com/erigontech/erigon/db/kv/dbcfg"
+	"github.com/erigontech/erigon/db/kv/memdb"
+	"github.com/erigontech/erigon/db/snaptype"
+)
+
+// TestDumpBeaconBlocksNoPanic is a higher-level regression test: the production
+// panic occurred inside DumpBeaconBlocks when chooseSegmentEnd was called with
+// nil snCfg. This test uses toSlot=CaplinMergeLimit so the loop body executes
+// (doesn't break early), exercising the exact code path that panicked.
+func TestDumpBeaconBlocksNoPanic(t *testing.T) {
+	db := memdb.NewTestDB(t, dbcfg.ChainDB)
+	dirs := datadir.New(t.TempDir())
+
+	// toSlot == CaplinMergeLimit: toSlot-fromSlot is not < blocksPerFile,
+	// so the loop reaches chooseSegmentEnd and dumpBeaconBlocksRange.
+	// Before the fix this panicked; now it returns an error (no snap dir / empty db).
+	require.NotPanics(t, func() {
+		_ = DumpBeaconBlocks(context.Background(), db, 0, snaptype.CaplinMergeLimit, 0, dirs, 1, log.LvlDebug, log.New())
+	})
+}