From 547236b7a4c46d7d30bc7119154b5b14e1779be4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Markus=20M=C3=A4kel=C3=A4?= <markus.makela@mariadb.com>
Date: Wed, 7 Aug 2019 13:23:11 +0300
Subject: [PATCH] MXS-2609: Store history size in Backend

When a connection is created, the size of the history that is about to be
replayed is known. Storing this and decrementing it each time a session
command is completed tells us when the Backend has finished replaying the
session command history. This can then be used to distinguish whether a
session command executed on a master should be retried or whether to
simply discard the connection.
---
 include/maxscale/backend.hh | 13 ++++++++++++-
 server/core/backend.cc      | 11 ++++++++++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/maxscale/backend.hh b/include/maxscale/backend.hh
index a71074878..ab8416317 100644
--- a/include/maxscale/backend.hh
+++ b/include/maxscale/backend.hh
@@ -303,6 +303,16 @@ public:
         return m_state & FATAL_FAILURE;
     }
 
+    /**
+     * Is the backend replaying session command history
+     *
+     * @return If a list of session commands was provided at connect time, the function returns true as long
+     *         as the backend has not completed those session commands.
+     */
+    inline bool is_replaying_history() const
+    {
+        return m_history_size > 0;
+    }
 
     /**
      * @brief Get the object name of this server
@@ -389,7 +399,8 @@ private:
 
     maxbase::StopWatch     m_session_timer;
     maxbase::IntervalTimer m_select_timer;
-    int64_t                m_num_selects = 0;
+    int64_t                m_num_selects {0};
+    int64_t                m_history_size {0};
 };
 
 typedef std::shared_ptr<Backend> SBackend;
diff --git a/server/core/backend.cc b/server/core/backend.cc
index 201ae5c6d..f837f7957 100644
--- a/server/core/backend.cc
+++ b/server/core/backend.cc
@@ -54,6 +54,7 @@ void Backend::close(close_type type)
         m_closed = true;
         m_closed_at = time(NULL);
         m_session_commands.clear();
+        m_history_size = 0;
 
         if (in_use())
         {
@@ -139,6 +140,12 @@ uint64_t Backend::complete_session_command()
 {
     uint64_t rval = m_session_commands.front()->get_position();
     m_session_commands.pop_front();
+
+    if (m_history_size > 0)
+    {
+        --m_history_size;
+    }
+
     return rval;
 }
 
@@ -192,10 +199,12 @@ bool Backend::connect(MXS_SESSION* session, SessionCommandList* sescmd)
         m_state = IN_USE;
         mxb::atomic::add(&m_backend->connections, 1, mxb::atomic::RELAXED);
         rval = true;
+        m_history_size = 0;
 
-        if (sescmd && sescmd->size())
+        if (sescmd && !sescmd->empty())
         {
             append_session_command(*sescmd);
+            m_history_size = sescmd->size();
 
             if (!execute_session_command())
             {