Add more error logging to DCB handling

If an illegal DCB close is done with a backend DCB, it will log the server
where it was connected. This allows us to know whether the DCB was
connected to a master or a slave.

Added more debug assertions to readwritesplit code. The DCBs should never
enter the DCB_STATE_DISCONNECTED.

Removed useless debug log messages. The messages usually just flood the
logs with no use to the developers.

server/core/dcb.c

@@ -1747,6 +1747,38 @@ dcb_grab_writeq(DCB *dcb, bool first_time)
     return local_writeq;
 }
 
+static void log_illegal_dcb(DCB *dcb)
+{
+    const char *connected_to;
+
+    switch (dcb->dcb_role)
+    {
+        case DCB_ROLE_BACKEND_HANDLER:
+            connected_to = dcb->server->unique_name;
+            break;
+
+        case DCB_ROLE_CLIENT_HANDLER:
+            connected_to = dcb->remote;
+            break;
+
+        case DCB_ROLE_INTERNAL:
+            connected_to = "Internal DCB";
+            break;
+
+        case DCB_ROLE_SERVICE_LISTENER:
+            connected_to = dcb->service->name;
+            break;
+
+        default:
+            connected_to = "Illegal DCB role";
+            break;
+    }
+
+    MXS_ERROR("[dcb_close] Error : Removing DCB %p but it is in state %s "
+              "which is not legal for a call to dcb_close. The DCB is connected to: %s",
+              dcb, STRDCBSTATE(dcb->state), connected_to);
+}
+
 /**
  * Removes dcb from poll set, and adds it to zombies list. As a consequence,
  * dcb first moves to DCB_STATE_NOPOLLING, and then to DCB_STATE_ZOMBIE state.
@@ -1766,11 +1798,7 @@ dcb_close(DCB *dcb)
     if (DCB_STATE_UNDEFINED == dcb->state
         || DCB_STATE_DISCONNECTED == dcb->state)
     {
-        MXS_ERROR("%lu [dcb_close] Error : Removing DCB %p but was in state %s "
+        log_illegal_dcb(dcb);
-                  "which is not legal for a call to dcb_close. ",
-                  pthread_self(),
-                  dcb,
-                  STRDCBSTATE(dcb->state));
         raise(SIGABRT);
     }
 

server/modules/protocol/mysql_backend.c

@@ -1032,9 +1032,6 @@ gw_read_and_write(DCB *dcb, MYSQL_session local_session)
         {
             GWBUF* errbuf;
             bool succp;
-#if defined(SS_DEBUG)
-            MXS_ERROR("Backend read error handling #2.");
-#endif
             errbuf = mysql_create_custom_error(1,
                                                0,
                                                "Read from backend failed");
@@ -1554,12 +1551,6 @@ static int gw_backend_hangup(DCB *dcb)
         /* dcb_close(dcb); */
         goto retblock;
     }
-#if defined(SS_DEBUG)
-    if (ses_state != SESSION_STATE_STOPPING)
-    {
-        MXS_ERROR("Backend hangup error handling.");
-    }
-#endif
 
     router->handleError(router_instance,
                         rsession,
@@ -1572,9 +1563,6 @@ static int gw_backend_hangup(DCB *dcb)
     /** There are no required backends available, close session. */
     if (!succp)
     {
-#if defined(SS_DEBUG)
-        MXS_ERROR("Backend hangup -> closing session.");
-#endif
         spinlock_acquire(&session->ses_lock);
         session->state = SESSION_STATE_STOPPING;
         spinlock_release(&session->ses_lock);

server/modules/routing/readwritesplit/readwritesplit.c

@@ -1096,7 +1096,8 @@ static bool get_dcb(DCB **p_dcb, ROUTER_CLIENT_SES *rses, backend_type_t btype,
             {
                 *p_dcb = backend_ref[i].bref_dcb;
                 succp = true;
-                ss_dassert(backend_ref[i].bref_dcb->state != DCB_STATE_ZOMBIE);
+                ss_dassert(backend_ref[i].bref_dcb->state != DCB_STATE_ZOMBIE ||
+                           backend_ref[i].bref_dcb->state != DCB_STATE_DISCONNECTED);
                 break;
             }
         }
@@ -1228,7 +1229,8 @@ static bool get_dcb(DCB **p_dcb, ROUTER_CLIENT_SES *rses, backend_type_t btype,
                 *p_dcb = master_bref->bref_dcb;
                 succp = true;
                 /** if bref is in use DCB should not be closed */
-                ss_dassert(master_bref->bref_dcb->state != DCB_STATE_ZOMBIE);
+                ss_dassert(master_bref->bref_dcb->state != DCB_STATE_ZOMBIE ||
+                           master_bref->bref_dcb->state != DCB_STATE_DISCONNECTED);
             }
             else
             {