MXS-1921: Explain why session was closed

When a client connection is closed by MaxScale before the client initiates
a controlled closing of the connection, an error message is sent. This
error message now also explains why the connection was closed to make
problem resolution easier.

include/maxscale/session.h

@@ -135,6 +135,17 @@ typedef struct mxs_upstream
     int32_t (*error)(void *instance, void *session, void *);
 } MXS_UPSTREAM;
 
+/* Specific reasons why a session was closed */
+typedef enum
+{
+    SESSION_CLOSE_NONE = 0,             // No special reason
+    SESSION_CLOSE_TIMEOUT,              // Connection timed out
+    SESSION_CLOSE_HANDLEERROR_FAILED,   // Router returned an error from handleError
+    SESSION_CLOSE_ROUTING_FAILED,       // Router closed DCB
+    SESSION_CLOSE_KILLED,               // Killed by another connection
+    SESSION_CLOSE_TOO_MANY_CONNECTIONS, // Too many connections
+} session_close_t;
+
 /**
  * The session status block
  *
@@ -169,6 +180,7 @@ typedef struct session
     } stmt;  /**< Current statement being executed */
     bool qualifies_for_pooling; /**< Whether this session qualifies for the connection pool */
     SessionStmtQueue*      last_statements;  /*< The N last statements by the client */
+    session_close_t        close_reason; /**< Reason why the session was closed */
     skygw_chk_t     ses_chk_tail;
 } MXS_SESSION;
 
@@ -534,4 +546,15 @@ void session_set_dump_statements(session_dump_statements_t value);
  */
 session_dump_statements_t session_get_dump_statements();
 
+
+/**
+ * Get the reason why a session was closed
+ *
+ * @param session Session to inspect
+ *
+ * @return String representation of the reason why the session was closed. If
+ *         the session was closed normally, an empty string is returned.
+ */
+const char* session_get_close_reason(const MXS_SESSION* session);
+
 MXS_END_DECLS