diff -Nurp net-snmp-5.9.1-orig/agent/mibgroup/agentx/subagent.c net-snmp-5.9.1/agent/mibgroup/agentx/subagent.c
--- net-snmp-5.9.1-orig/agent/mibgroup/agentx/subagent.c	2021-05-26 00:19:35.000000000 +0200
+++ net-snmp-5.9.1/agent/mibgroup/agentx/subagent.c	2022-03-09 16:44:30.609053225 +0100
@@ -80,6 +80,7 @@ typedef struct _net_snmpsubagent_magic_s
 struct agent_netsnmp_set_info {
     int             transID;
     int             mode;
+    int             req_pending;
     int             errstat;
     time_t          uptime;
     netsnmp_session *sess;
@@ -190,6 +191,7 @@ save_set_vars(netsnmp_session * ss, nets
     ptr->sess = ss;
     ptr->mode = SNMP_MSG_INTERNAL_SET_RESERVE1;
     ptr->uptime = netsnmp_get_agent_uptime();
+    ptr->req_pending = 0;
 
     ptr->var_list = snmp_clone_varbind(pdu->variables);
     if (ptr->var_list == NULL) {
@@ -204,6 +206,18 @@ save_set_vars(netsnmp_session * ss, nets
 }
 
 struct agent_netsnmp_set_info *
+pending_trans_set_info(netsnmp_session * sess, netsnmp_pdu *pdu)
+{
+    struct agent_netsnmp_set_info *ptr;
+
+    for (ptr = Sets; ptr != NULL; ptr = ptr->next)
+        if (ptr->sess == sess && ptr->transID == pdu->transid)
+            break;
+
+    return ptr;
+}
+
+struct agent_netsnmp_set_info *
 restore_set_vars(netsnmp_session * sess, netsnmp_pdu *pdu)
 {
     struct agent_netsnmp_set_info *ptr;
@@ -413,6 +427,14 @@ handle_agentx_packet(int operation, nets
          * XXXWWW we have to map this twice to both RESERVE1 and RESERVE2 
          */
         DEBUGMSGTL(("agentx/subagent", "  -> testset\n"));
+        asi = pending_trans_set_info(session, pdu);
+        if (asi) {
+            DEBUGMSGTL(("agentx/subagent",
+                        "dropping testset retry for transid 0x%x in mode %d\n",
+                        (unsigned)pdu->transid, asi->mode));
+            //SNMP_FREE(retmagic); XXX necessary?
+            return 1;
+        }
         asi = save_set_vars(session, pdu);
         if (asi == NULL) {
             SNMP_FREE(smagic);
@@ -434,6 +456,13 @@ handle_agentx_packet(int operation, nets
             send_agentx_error(session, pdu, AGENTX_ERR_PROCESSING_ERROR, 0);
             return 1;
         }
+        if (asi->req_pending) {
+            DEBUGMSGTL(("agentx/subagent",
+                        "dropping commitset - request pending for transid 0x%x in mode %d\n",
+                        (unsigned)pdu->transid, asi->mode));
+            //SNMP_FREE(retmagic); XXX necessary?
+            return 1;
+        }
         if (asi->mode != SNMP_MSG_INTERNAL_SET_RESERVE2) {
             SNMP_FREE(smagic);
             snmp_log(LOG_WARNING,
@@ -456,6 +485,13 @@ handle_agentx_packet(int operation, nets
             send_agentx_error(session, pdu, AGENTX_ERR_PROCESSING_ERROR, 0);
             return 1;
         }
+        if (asi->req_pending) {
+            DEBUGMSGTL(("agentx/subagent",
+                        "dropping cleanupset - request pending for transid 0x%x in mode %d\n",
+                        (unsigned)pdu->transid, asi->mode));
+            //SNMP_FREE(retmagic); XXX necessary?
+            return 1;
+        }
         if (asi->mode == SNMP_MSG_INTERNAL_SET_RESERVE1 ||
             asi->mode == SNMP_MSG_INTERNAL_SET_RESERVE2) {
             asi->mode = pdu->command = SNMP_MSG_INTERNAL_SET_FREE;
@@ -481,6 +517,13 @@ handle_agentx_packet(int operation, nets
             send_agentx_error(session, pdu, AGENTX_ERR_PROCESSING_ERROR, 0);
             return 1;
         }
+        if (asi->req_pending) {
+            DEBUGMSGTL(("agentx/subagent",
+                        "dropping undoset - request pending for transid 0x%x in mode %d\n",
+                        (unsigned)pdu->transid, asi->mode));
+            //SNMP_FREE(retmagic); XXX necessary?
+            return 1;
+        }
         asi->mode = pdu->command = SNMP_MSG_INTERNAL_SET_UNDO;
         mycallback = handle_subagent_set_response;
         retmagic = asi;
@@ -516,6 +559,8 @@ handle_agentx_packet(int operation, nets
                     retmagic);
     if (result == 0) {
         snmp_free_pdu( internal_pdu );
+    } else if (asi) {
+        asi->req_pending = 1;
     }
     return 1;
 }
@@ -637,6 +682,7 @@ handle_subagent_set_response(int op, net
 {
     netsnmp_session *retsess;
     struct agent_netsnmp_set_info *asi;
+    int new_mode;
     int result;
 
     if (op != NETSNMP_CALLBACK_OP_RECEIVED_MESSAGE || magic == NULL) {
@@ -645,15 +691,63 @@ handle_subagent_set_response(int op, net
 
     DEBUGMSGTL(("agentx/subagent",
                 "handling agentx subagent set response (mode=%d,req=0x%x,"
-                "trans=0x%x,sess=0x%x)\n",
+                "trans=0x%x,sess=0x%x,magic=%p)\n",
                 (unsigned)pdu->command, (unsigned)pdu->reqid,
-		(unsigned)pdu->transid, (unsigned)pdu->sessid));
+               (unsigned)pdu->transid, (unsigned)pdu->sessid,
+                magic));
+
     pdu = snmp_clone_pdu(pdu);
     if (!pdu)
         return 1;
 
     asi = (struct agent_netsnmp_set_info *) magic;
+
+    DEBUGMSGTL(("agentx/subagent",
+                "set response in mode %d (errstat %d, req_pending %d)\n",
+                asi->mode, pdu->errstat, asi->req_pending));
+
+    asi->req_pending = 0;
     retsess = asi->sess;
+
+    if (!snmp_sess_pointer(retsess) || retsess->sessid != pdu->sessid) {
+        DEBUGMSGTL(("agentx/subagent",
+                    "session id 0x%x gone for set response (transid 0x%x, reqid 0x%x)\n",
+                    (unsigned)pdu->sessid, (unsigned)pdu->transid, (unsigned)pdu->reqid));
+
+        result = 0;
+
+        if (!pdu->errstat) {
+            switch (asi->mode) {
+                case SNMP_MSG_INTERNAL_SET_RESERVE1:
+                case SNMP_MSG_INTERNAL_SET_RESERVE2:
+                    new_mode = SNMP_MSG_INTERNAL_SET_FREE;
+                    break;
+
+                case SNMP_MSG_INTERNAL_SET_ACTION:
+                    new_mode = SNMP_MSG_INTERNAL_SET_UNDO;
+                    break;
+
+                default:
+                    new_mode = 0;
+            }
+
+            if (new_mode) {
+                asi->mode = pdu->command = new_mode;
+                result = snmp_async_send(agentx_callback_sess, pdu,
+                                         handle_subagent_set_response, asi);
+            }
+        }
+
+        if (result == 0) {
+            /* don't need to (or cannot) FREE/UNDO */
+            free_set_vars(retsess, pdu);
+            snmp_free_pdu(pdu);
+        } else {
+            asi->req_pending = 1;
+        }
+        return 1;
+    }
+
     asi->errstat = pdu->errstat;
 
     if (asi->mode == SNMP_MSG_INTERNAL_SET_RESERVE1) {
@@ -669,6 +763,8 @@ handle_subagent_set_response(int op, net
                             handle_subagent_set_response, asi);
             if (result == 0) {
                 snmp_free_pdu( pdu );
+            } else {
+                asi->req_pending = 1;
             }
             DEBUGMSGTL(("agentx/subagent",
                         "  going from RESERVE1 -> RESERVE2\n"));