cmd/utils, eth: disallow invalid snap sync / snapshot flag combos (#28657)

* eth: prevent startup in snap mode without snapshots

* cmd/utils: try to fix bad flag combos wrt snap sync and snapshot generation

cmd/utils/flags.go

@@ -1677,10 +1677,16 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *ethconfig.Config) {
 	if ctx.IsSet(CacheLogSizeFlag.Name) {
 		cfg.FilterLogCacheSize = ctx.Int(CacheLogSizeFlag.Name)
 	}
-	if !ctx.Bool(SnapshotFlag.Name) {
+	if !ctx.Bool(SnapshotFlag.Name) || cfg.SnapshotCache == 0 {
 		// If snap-sync is requested, this flag is also required
 		if cfg.SyncMode == downloader.SnapSync {
-			log.Info("Snap sync requested, enabling --snapshot")
+			if !ctx.Bool(SnapshotFlag.Name) {
+				log.Warn("Snap sync requested, enabling --snapshot")
+			}
+			if cfg.SnapshotCache == 0 {
+				log.Warn("Snap sync requested, resetting --cache.snapshot")
+				cfg.SnapshotCache = ctx.Int(CacheFlag.Name) * CacheSnapshotFlag.Value / 100
+			}
 		} else {
 			cfg.TrieCleanCache += cfg.SnapshotCache
 			cfg.SnapshotCache = 0 // Disabled

eth/handler.go

@@ -178,6 +178,10 @@ func newHandler(config *handlerConfig) (*handler, error) {
 			log.Info("Enabled snap sync", "head", head.Number, "hash", head.Hash())
 		}
 	}
+	// If snap sync is requested but snapshots are disabled, fail loudly
+	if h.snapSync.Load() && config.Chain.Snapshots() == nil {
+		return nil, errors.New("snap sync not supported with snapshots disabled")
+	}
 	// Construct the downloader (long sync)
 	h.downloader = downloader.New(config.Database, h.eventMux, h.chain, nil, h.removePeer, h.enableSyncedFeatures)
 	if ttd := h.chain.Config().TerminalTotalDifficulty; ttd != nil {