fabric: switch to maps for view rows

The `#view_row{}` record that is used for capturing messages with
row data is not flexible enough to have it extended easily.  If
one wanted to introduce a fresh field, the change would have to be
propagated through many functions and modules.  Especially, if
support for mixed-version clusters is a concern, this would come
with some degree of duplication.

Leverage Erlang/OTP's built-in maps for mitigating this issue and
offer the view callbacks the `view_row_map` Boolean key in
`#mrargs.extra` to request this communication format.  This way the
old record-based format would be still in use unless requested
otherwise.  This facilitates the smooth interoperability of old
coordinators and new workers.  In parallel to that, the new
coordinator could still receive view rows from old workers.

Author: pgj

src/couch_mrview/src/couch_mrview.erl

@@ -273,7 +273,8 @@ query_all_docs(Db, Args0, Callback, Acc) ->
     all_docs_fold(Db, Args2, Callback, Acc1).
 
 query_view(Db, DDoc, VName) ->
-    query_view(Db, DDoc, VName, #mrargs{}).
+    Args = #mrargs{extra = [{view_row_map, true}]},
+    query_view(Db, DDoc, VName, Args).
 
 query_view(Db, DDoc, VName, Args) when is_list(Args) ->
     query_view(Db, DDoc, VName, to_mrargs(Args), fun default_cb/2, []);
@@ -325,7 +326,7 @@ get_view_info(Db, DDoc, VName) ->
         Db,
         DDoc,
         VName,
-        #mrargs{}
+        #mrargs{extra = [{view_row_map, true}]}
     ),
 
     %% get the total number of rows
@@ -763,7 +764,7 @@ to_mrargs(KeyList) ->
             Index = lookup_index(couch_util:to_existing_atom(Key)),
             setelement(Index, Acc, Value)
         end,
-        #mrargs{},
+        #mrargs{extra = [{view_row_map, true}]},
         KeyList
     ).
 

src/couch_mrview/src/couch_mrview_http.erl

@@ -472,7 +472,7 @@ row_to_json(Id0, Row) ->
 parse_params(#httpd{} = Req, Keys) ->
     parse_params(chttpd:qs(Req), Keys);
 parse_params(Props, Keys) ->
-    Args = #mrargs{},
+    Args = #mrargs{extra = [{view_row_map, true}]},
     parse_params(Props, Keys, Args).
 
 parse_params(Props, Keys, Args) ->
@@ -511,13 +511,16 @@ parse_body_and_query(Req, Keys) ->
         #mrargs{
             keys = Keys,
             group = undefined,
-            group_level = undefined
+            group_level = undefined,
+            extra = [{view_row_map, true}]
         },
         [keep_group_level]
     ).
 
 parse_body_and_query(Req, {Props}, Keys) ->
-    Args = #mrargs{keys = Keys, group = undefined, group_level = undefined},
+    Args = #mrargs{
+        keys = Keys, group = undefined, group_level = undefined, extra = [{view_row_map, true}]
+    },
     BodyArgs0 = parse_params(Props, Keys, Args, [decoded]),
     BodyArgs1 =
         case is_view(Req) of