poll.c:Returned checks for zombie

mysql_backend.c:don't write to session->client if session->client is NULL
mysql_common.c:assert if called with dcb==NULL

server/core/poll.c

@@ -263,11 +263,17 @@ poll_waitevents(void *arg)
 				{
 					atomic_add(&pollStats.n_error, 1);
 					dcb->func.error(dcb);
+                                        if (DCB_ISZOMBIE(dcb)) {
+						continue;
+                                        }
                                 }
 				if (ev & EPOLLHUP)
 				{
                                         atomic_add(&pollStats.n_hup, 1);
 					dcb->func.hangup(dcb);
+                                        if (DCB_ISZOMBIE(dcb)) {
+						continue;
+                                        }
 				}
 				if (ev & EPOLLOUT)
 				{

server/modules/protocol/mysql_backend.c

@@ -360,7 +360,9 @@ static int gw_write_backend_event(DCB *dcb) {
         /**
          * Don't write to backend if backend_dcb is not in poll set anymore.
          */
-        if (dcb->state != DCB_STATE_POLLING) {
+        if (dcb->state != DCB_STATE_POLLING &&
+            dcb->session->client != NULL)
+        {
                 mysql_send_custom_error(
                         dcb->session->client,
                         1,

server/modules/protocol/mysql_common.c

@@ -39,13 +39,33 @@ extern int gw_MySQLWrite_backend(DCB *dcb, GWBUF *queue);
 extern int gw_error_backend_event(DCB *dcb);
 
 
+/** 
+ * @node Creates MySQL protocol structure 
+ *
+ * Parameters:
+ * @param dcb - in, use
+ *          Must be non-NULL.
+ *
+ * @return 
+ *
+ * 
+ * @details (write detailed description here)
+ *
+ */
 MySQLProtocol* mysql_protocol_init(
         DCB* dcb)
 {
         MySQLProtocol* p;
 
-        if (dcb != NULL) {
         CHK_DCB(dcb);
+
+        if (dcb == NULL) {
+            skygw_log_write_flush(
+                    LOGFILE_ERROR,
+                    "%lu [mysql_init_protocol] MySQL protocol init failed : "
+                    "called with DCB == NULL.",
+                    pthread_self());
+            return NULL;
         }
 	p = (MySQLProtocol *) calloc(1, sizeof(MySQLProtocol));
         ss_dassert(p != NULL);