Add ro_reason field

Author: xuniq

doc/reference/reference_lua/box_info.rst

@@ -31,6 +31,8 @@ variables.
 * **ro** is ``true`` if the instance is in "read-only" mode
   (same as :ref:`read_only <cfg_basic-read_only>` in ``box.cfg{}``),
   or if status is 'orphan'.
+* **ro_reason** is ``nil`` if the instance is in writable mode.
+  When the field is ``true``, it reports an error reason.
 * **signature** is the sum of all ``lsn`` values from each :ref:`vector clock <replication-vector>`
   (**vclock**) for all instances in the replica set.
 * **sql().cache.size** is the number of bytes in the SQL prepared statement cache.
@@ -56,7 +58,7 @@ variables.
 * **election** shows the current state of a replica set node regarding leader
   election (see :doc:`here </reference/reference_lua/box_info/election>`).
 
-Below is a list of all ``box.info`` functions.
+Below is a list of all ``box.info`` functions and members.
 
 ..  container:: table
 
@@ -95,6 +97,8 @@ Below is a list of all ``box.info`` functions.
         *  - :doc:`./box_info/synchro`
            - Show the current state of synchronous replication
 
+        *  - :doc:`./box_info/ro_reason`
+           - List of all error reasons for the ``box.info.ro_reason`` field
 
 ..  toctree::
     :hidden:
@@ -107,3 +111,4 @@ Below is a list of all ``box.info`` functions.
     box_info/listen
     box_info/election
     box_info/synchro
+    box_info/ro_reason

doc/reference/reference_lua/box_info/ro_reason.rst

@@ -0,0 +1,22 @@
+.. _box_info_ro-reason:
+
+================================================================================
+box.info.ro_reason
+================================================================================
+
+..  module:: box.info
+
+    Possible error reasons:
+
+    *   ``election`` -- the instance is not a leader.
+        That is, ``box.cfg.election_mode`` is not ``off``.
+        See :ref:`box.info.election <box_info_election>` for details.
+
+    *   ``synchro`` -- the instance is not the owner of the synchronous transaction queue.
+        For details, see :ref:`box.info.synchro <box_info_synchro>`.
+
+    *   ``config`` -- the server instance is in read-only mode.
+        That is, :ref:`box.cfg.read_only <cfg_basic-read_only>` is ``true``.
+
+    *   ``orphan`` -- the instance is in ``orphan`` state.
+        For details, see :ref:`the orphan status page <internals-replication-orphan_status>`.