MXS-1378 Provide access to the current DCB in 2.1

Conceptually this is a cherry-pick of commit
67efd1daeabbc398b8a8fbc0cd02c2af26e4cb6c (2.2), but too much has
changed to actually be able to cherry-pick that commit.

include/maxscale/dcb.h

@@ -351,6 +351,14 @@ bool dcb_foreach(bool (*func)(DCB *, void *), void *data);
  */
 int dcb_get_port(const DCB *dcb);
 
+/**
+ * @brief Return the DCB currently being handled by the calling thread.
+ *
+ * @return A DCB, or NULL if the calling thread is not currently handling
+ *         a DCB or if the calling thread is not a polling/worker thread.
+ */
+DCB* dcb_get_current();
+
 /**
  * DCB flags values
  */

server/core/poll.c

@@ -49,6 +49,7 @@ extern unsigned long hkheartbeat;
 
 int number_poll_spins;
 int max_poll_sleep;
+static thread_local DCB* current_dcb;
 
 /**
  * @file poll.c  - Abstraction of the epoll functionality
@@ -894,6 +895,7 @@ process_pollq(int thread_id, struct epoll_event *event)
     uint32_t ev = event->events;
     DCB *dcb = event->data.ptr;
     ss_dassert(dcb->thread.id == thread_id || dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER);
+    current_dcb = dcb; // thread local
 
     /** Calculate event queue statistics */
     uint64_t started = hkheartbeat;
@@ -1086,6 +1088,8 @@ process_pollq(int thread_id, struct epoll_event *event)
 
     ts_stats_set_max(queueStats.maxexectime, qtime, thread_id);
 
+    current_dcb = NULL; // thread local
+
     return 1;
 }
 
@@ -1661,3 +1665,8 @@ static void poll_check_message()
         poll_msg[thread_id] &= ~POLL_MSG_CLEAN_PERSISTENT;
     }
 }
+
+DCB* dcb_get_current()
+{
+    return current_dcb;
+}