rpcsrv: set minimal default SessionExpirationTime to 5s, fix #3509

Signed-off-by: Roman Khimov <roman@nspcc.ru>

docs/node-configuration.md

@@ -250,8 +250,8 @@ where:
   enable `SessionBackedByMPT`, see `SessionBackedByMPT` documentation for more
   details.
 - `SessionExpirationTime` is a lifetime of iterator session in seconds. It is set
-  to `TimePerBlock` seconds by default and is relevant only if `SessionEnabled`
-  is set to `true`.
+  to `TimePerBlock` seconds (but not less than 5s) by default and is relevant
+  only if `SessionEnabled` is set to `true`.
 - `SessionBackedByMPT` is a flag forcing JSON-RPC server into using MPT-backed
   storage for delayed iterator traversal. If `true`, then iterator resources got
   after `invoke*` calls will be released immediately. Further iterator traversing

pkg/services/rpcsrv/server.go

@@ -274,6 +274,9 @@ func New(chain Ledger, conf config.RPC, coreServer *network.Server,
 	if conf.SessionEnabled {
 		if conf.SessionExpirationTime <= 0 {
 			conf.SessionExpirationTime = int(protoCfg.TimePerBlock / time.Second)
+			if conf.SessionExpirationTime < 5 {
+				conf.SessionExpirationTime = 5
+			}
 			log.Info("SessionExpirationTime is not set or wrong, setting default value", zap.Int("SessionExpirationTime", conf.SessionExpirationTime))
 		}
 		if conf.SessionPoolSize <= 0 {