add support of Server-Sent Events protocol to db changes API.

This patch add support for the new specification of w3c by adding a new
feed type named `eventsource`:

http://www.w3.org/TR/2009/WD-eventsource-20090423/

ex:

    $ curl
'http://127.0.0.1:5984/testdb/_changes?feed=eventsource&heartbeat=true&timeout=10000'
    data:
{"seq":1,"id":"test","changes":[{"rev":"1-967a00dff5e02add41819138abb3284d"}]}
    id: 1

    data:
{"seq":2,"id":"test1","changes":[{"rev":"1-967a00dff5e02add41819138abb3284d"}]}
    id: 2

apps/couch_changes/src/couch_changes.erl

@@ -64,7 +64,8 @@ handle_changes(Args1, Req, Db0) ->
         put(last_changes_heartbeat, now())
     end,
 
-    if Feed == "continuous" orelse Feed == "longpoll" ->
+    case lists:member(Feed, ["continuous", "longpoll", "eventsource"]) of
+    true ->
         fun(CallbackAcc) ->
             {Callback, UserAcc} = get_callback_acc(CallbackAcc),
             Self = self(),
@@ -90,7 +91,7 @@ handle_changes(Args1, Req, Db0) ->
                 get_rest_db_updated(ok) % clean out any remaining update messages
             end
         end;
-    true ->
+    false ->
         fun(CallbackAcc) ->
             {Callback, UserAcc} = get_callback_acc(CallbackAcc),
             UserAcc2 = start_sending_changes(Callback, UserAcc, Feed),
@@ -267,7 +268,9 @@ get_changes_timeout(Args, Callback) ->
             fun(UserAcc) -> {ok, Callback(timeout, ResponseType, UserAcc)} end}
     end.
 
-start_sending_changes(_Callback, UserAcc, "continuous") ->
+start_sending_changes(_Callback, UserAcc, ResponseType)
+        when ResponseType =:= "continuous"
+        orelse ResponseType =:= "eventsource" ->
     UserAcc;
 start_sending_changes(Callback, UserAcc, ResponseType) ->
     Callback(start, ResponseType, UserAcc).
@@ -440,7 +443,9 @@ keep_sending_changes(Args, Acc0, FirstRound) ->
 end_sending_changes(Callback, UserAcc, EndSeq, ResponseType) ->
     Callback({stop, EndSeq}, ResponseType, UserAcc).
 
-changes_enumerator(DocInfo, #changes_acc{resp_type = "continuous"} = Acc) ->
+changes_enumerator(DocInfo, #changes_acc{resp_type = ResponseType} = Acc)
+        when ResponseType =:= "continuous"
+        orelse ResponseType =:= "eventsource" ->
     #changes_acc{
         filter = FilterFun, callback = Callback,
         user_acc = UserAcc, limit = Limit, db = Db,
@@ -462,7 +467,7 @@ changes_enumerator(DocInfo, #changes_acc{resp_type = "continuous"} = Acc) ->
         end;
     _ ->
         ChangesRow = changes_row(Results, DocInfo, Acc),
-        UserAcc2 = Callback({change, ChangesRow, <<>>}, "continuous", UserAcc),
+        UserAcc2 = Callback({change, ChangesRow, <<>>}, ResponseType, UserAcc),
         reset_heartbeat(),
         {Go, Acc#changes_acc{seq = Seq, user_acc = UserAcc2, limit = Limit - 1}}
     end;

apps/couch_changes/src/couch_httpd_changes.erl

@@ -40,14 +40,23 @@ handle_changes_req1(Req, #db{name=DbName}=Db) ->
 
 do_changes_req(Req, Db) ->
     MakeCallback = fun(Resp) ->
-        fun({change, Change, _}, "continuous") ->
+        fun({change, {ChangeProp}=Change, _}, "eventsource") ->
+                Seq = proplists:get_value(<<"seq">>, ChangeProp),
+                couch_httpd:send_chunk(Resp, ["data: ", ?JSON_ENCODE(Change),
+                                              "\n", "id: ", ?JSON_ENCODE(Seq),
+                                              "\n\n"]);
+        ({change, Change, _}, "continuous") ->
             couch_httpd:send_chunk(Resp, [?JSON_ENCODE(Change) | "\n"]);
         ({change, Change, Prepend}, _) ->
             couch_httpd:send_chunk(Resp, [Prepend, ?JSON_ENCODE(Change)]);
+        (start, "eventsource") ->
+            ok;
         (start, "continuous") ->
             ok;
         (start, _) ->
             couch_httpd:send_chunk(Resp, "{\"results\":[\n");
+        ({stop, _EndSeq}, "eventsource") ->
+            couch_httpd:end_json_response(Resp);
         ({stop, EndSeq}, "continuous") ->
             couch_httpd:send_chunk(
                 Resp,
@@ -85,20 +94,30 @@ do_changes_req(Db, Req, #changes_args{feed="normal"}, ChangesFun, MakeCallback)
                     [{"ETag", CurrentEtag}]),
                 ChangesFun(MakeCallback(Resp))
         end);
+do_changes_req(_Db, Req, #changes_args{feed="eventsource"}, ChangesFun,
+               MakeCallback) ->
+    Headers = [
+        {"Content-Type", "text/event-stream"},
+        {"Cache-Control", "no-cache"}
+    ],
+    {ok, Resp} = couch_httpd:start_json_response(Req, 200, Headers),
+    ChangesFun(MakeCallback(Resp));
 do_changes_req(_Db, Req, _ChangesArgs, ChangesFun, MakeCallback) ->
     % "longpoll" or "continuous"
     {ok, Resp} = couch_httpd:start_json_response(Req, 200),
     ChangesFun(MakeCallback(Resp)).
 
 parse_changes_query(Req) ->
     lists:foldl(fun({Key, Value}, Args) ->
-        case {Key, Value} of
+        case {string:to_lower(Key), Value} of
         {"feed", _} ->
             Args#changes_args{feed=Value};
         {"descending", "true"} ->
             Args#changes_args{dir=rev};
         {"since", _} ->
             Args#changes_args{since=list_to_integer(Value)};
+        {"last-event-id", _} ->
+            Args#changes_args{since=list_to_integer(Value)};
         {"limit", _} ->
             Args#changes_args{limit=list_to_integer(Value)};
         {"style", _} ->