Support consumer priority in AMQP

Arguments
* `rabbitmq:stream-offset-spec`,
* `rabbitmq:stream-filter`,
* `rabbitmq:stream-match-unfiltered`
are set in the `filter` field of the `Source`.
This makes sense for these consumer arguments because:
> A filter acts as a function on a message which returns a boolean result
> indicating whether the message can pass through that filter or not.

Consumer priority is not really such a predicate.
Therefore, it makes more sense to set consumer priority in the
`properties` field of the `Attach` frame.

We call the key `rabbitmq:priority` which maps to consumer argument
`x-priority`.

While AMQP 0.9.1 consumers are allowed to set any integer data
type for the priority level, this commit decides to enforce an `int`
value (range -(2^31) to 2^31 - 1 inclusive).
Consumer priority levels outside of this range are not needed in
practice.

deps/amqp10_client/src/amqp10_client_types.erl

@@ -82,8 +82,7 @@ utf8(B) when is_binary(B) -> {utf8, B}.
 uint(N) -> {uint, N}.
 
 make_properties(#{properties := Props})
-  when is_map(Props) andalso
-       map_size(Props) > 0 ->
+  when map_size(Props) > 0 ->
     {map, maps:fold(fun(K, V, L) ->
                             [{{symbol, K}, V} | L]
                     end, [], Props)};

deps/rabbit/src/rabbit_amqp_session.erl

@@ -1086,7 +1086,7 @@ handle_control(#'v1_0.attach'{role = ?AMQP_ROLE_RECEIVER,
                                     mode => Mode,
                                     consumer_tag => handle_to_ctag(HandleInt),
                                     exclusive_consume => false,
-                                    args => source_filters_to_consumer_args(Source),
+                                    args => consumer_arguments(Attach),
                                     ok_msg => undefined,
                                     acting_user => Username},
                            case rabbit_queue_type:consume(Q, Spec, QStates0) of
@@ -2852,19 +2852,36 @@ encode_frames(T, Msg, MaxPayloadSize, Transfers) ->
             lists:reverse([[T, Msg] | Transfers])
     end.
 
-source_filters_to_consumer_args(#'v1_0.source'{filter = {map, KVList}}) ->
-    source_filters_to_consumer_args(
+consumer_arguments(#'v1_0.attach'{
+                      source = #'v1_0.source'{filter = Filter},
+                      properties = Properties}) ->
+    properties_to_consumer_args(Properties) ++
+    filter_to_consumer_args(Filter).
+
+properties_to_consumer_args({map, KVList}) ->
+    Key = {symbol, <<"rabbitmq:priority">>},
+    case proplists:lookup(Key, KVList) of
+        {Key, Val = {int, _Prio}} ->
+            [mc_amqpl:to_091(<<"x-priority">>, Val)];
+        _ ->
+            []
+    end;
+properties_to_consumer_args(_) ->
+    [].
+
+filter_to_consumer_args({map, KVList}) ->
+    filter_to_consumer_args(
       [<<"rabbitmq:stream-offset-spec">>,
        <<"rabbitmq:stream-filter">>,
        <<"rabbitmq:stream-match-unfiltered">>],
       KVList,
       []);
-source_filters_to_consumer_args(_Source) ->
+filter_to_consumer_args(_) ->
     [].
 
-source_filters_to_consumer_args([], _KVList, Acc) ->
+filter_to_consumer_args([], _KVList, Acc) ->
     Acc;
-source_filters_to_consumer_args([<<"rabbitmq:stream-offset-spec">> = H | T], KVList, Acc) ->
+filter_to_consumer_args([<<"rabbitmq:stream-offset-spec">> = H | T], KVList, Acc) ->
     Key = {symbol, H},
     Arg = case keyfind_unpack_described(Key, KVList) of
               {_, {timestamp, Ts}} ->
@@ -2876,8 +2893,8 @@ source_filters_to_consumer_args([<<"rabbitmq:stream-offset-spec">> = H | T], KVL
               _ ->
                   []
           end,
-    source_filters_to_consumer_args(T, KVList, Arg ++ Acc);
-source_filters_to_consumer_args([<<"rabbitmq:stream-filter">> = H | T], KVList, Acc) ->
+    filter_to_consumer_args(T, KVList, Arg ++ Acc);
+filter_to_consumer_args([<<"rabbitmq:stream-filter">> = H | T], KVList, Acc) ->
     Key = {symbol, H},
     Arg = case keyfind_unpack_described(Key, KVList) of
               {_, {list, Filters0}} when is_list(Filters0) ->
@@ -2892,8 +2909,8 @@ source_filters_to_consumer_args([<<"rabbitmq:stream-filter">> = H | T], KVList,
               _ ->
                   []
           end,
-    source_filters_to_consumer_args(T, KVList, Arg ++ Acc);
-source_filters_to_consumer_args([<<"rabbitmq:stream-match-unfiltered">> = H | T], KVList, Acc) ->
+    filter_to_consumer_args(T, KVList, Arg ++ Acc);
+filter_to_consumer_args([<<"rabbitmq:stream-match-unfiltered">> = H | T], KVList, Acc) ->
     Key = {symbol, H},
     Arg = case keyfind_unpack_described(Key, KVList) of
               {_, MU} when is_boolean(MU) ->
@@ -2901,9 +2918,9 @@ source_filters_to_consumer_args([<<"rabbitmq:stream-match-unfiltered">> = H | T]
               _ ->
                   []
           end,
-    source_filters_to_consumer_args(T, KVList, Arg ++ Acc);
-source_filters_to_consumer_args([_ | T], KVList, Acc) ->
-    source_filters_to_consumer_args(T, KVList, Acc).
+    filter_to_consumer_args(T, KVList, Arg ++ Acc);
+filter_to_consumer_args([_ | T], KVList, Acc) ->
+    filter_to_consumer_args(T, KVList, Acc).
 
 keyfind_unpack_described(Key, KvList) ->
     %% filterset values _should_ be described values

deps/rabbit/src/rabbit_fifo.hrl

@@ -110,7 +110,7 @@
          %% command: `{consumer_credit, ReceiverDeliveryCount, Credit}'
          credit_mode :: credit_mode(), % part of snapshot data
          lifetime = once :: once | auto,
-         priority = 0 :: non_neg_integer()}).
+         priority = 0 :: integer()}).
 
 -record(consumer,
         {cfg = #consumer_cfg{},

deps/rabbit/test/amqp_client_SUITE.erl

@@ -81,6 +81,8 @@ groups() ->
        stop_classic_queue,
        stop_quorum_queue,
        stop_stream,
+       consumer_priority_classic_queue,
+       consumer_priority_quorum_queue,
        single_active_consumer_classic_queue,
        single_active_consumer_quorum_queue,
        detach_requeues_one_session_classic_queue,
@@ -1841,6 +1843,95 @@ stop(QType, Config) ->
     #'queue.delete_ok'{} = amqp_channel:call(Ch, #'queue.delete'{queue = QName}),
     ok = rabbit_ct_client_helpers:close_channel(Ch).
 
+consumer_priority_classic_queue(Config) ->
+    consumer_priority(<<"classic">>, Config).
+
+consumer_priority_quorum_queue(Config) ->
+    consumer_priority(<<"quorum">>, Config).
+
+consumer_priority(QType, Config) ->
+    QName = atom_to_binary(?FUNCTION_NAME),
+    {Connection, Session, LinkPair} = init(Config),
+    QProps = #{arguments => #{<<"x-queue-type">> => {utf8, QType}}},
+    {ok, #{type := QType}} = rabbitmq_amqp_client:declare_queue(LinkPair, QName, QProps),
+
+    Address = rabbitmq_amqp_address:queue(QName),
+    {ok, Sender} = amqp10_client:attach_sender_link(Session, <<"sender">>, Address),
+    ok = wait_for_credit(Sender),
+
+    %% We test what our RabbitMQ docs state:
+    %% "Consumers which do not specify a value have priority 0.
+    %% Larger numbers indicate higher priority, and both positive and negative numbers can be used."
+    {ok, ReceiverDefaultPrio} = amqp10_client:attach_receiver_link(
+                                  Session,
+                                  <<"default prio consumer">>,
+                                  Address,
+                                  unsettled),
+    {ok, ReceiverHighPrio} = amqp10_client:attach_receiver_link(
+                               Session,
+                               <<"high prio consumer">>,
+                               Address,
+                               unsettled,
+                               none,
+                               #{},
+                               #{<<"rabbitmq:priority">> => {int, 2_000_000_000}}),
+    {ok, ReceiverLowPrio} = amqp10_client:attach_receiver_link(
+                              Session,
+                              <<"low prio consumer">>,
+                              Address,
+                              unsettled,
+                              none,
+                              #{},
+                              #{<<"rabbitmq:priority">> => {int, -2_000_000_000}}),
+    ok = amqp10_client:flow_link_credit(ReceiverDefaultPrio, 1, never),
+    ok = amqp10_client:flow_link_credit(ReceiverHighPrio, 2, never),
+    ok = amqp10_client:flow_link_credit(ReceiverLowPrio, 1, never),
+
+    NumMsgs = 5,
+    [begin
+         Bin = integer_to_binary(N),
+         ok = amqp10_client:send_msg(Sender, amqp10_msg:new(Bin, Bin))
+     end || N <- lists:seq(1, NumMsgs)],
+    ok = wait_for_accepts(NumMsgs),
+
+    receive {amqp10_msg, Rec1, Msg1} ->
+                ?assertEqual(<<"1">>, amqp10_msg:body_bin(Msg1)),
+                ?assertEqual(ReceiverHighPrio, Rec1),
+                ok = amqp10_client:accept_msg(Rec1, Msg1)
+    after 5000 -> ct:fail({missing_msg, ?LINE})
+    end,
+    receive {amqp10_msg, Rec2, Msg2} ->
+                ?assertEqual(<<"2">>, amqp10_msg:body_bin(Msg2)),
+                ?assertEqual(ReceiverHighPrio, Rec2),
+                ok = amqp10_client:accept_msg(Rec2, Msg2)
+    after 5000 -> ct:fail({missing_msg, ?LINE})
+    end,
+    receive {amqp10_msg, Rec3, Msg3} ->
+                ?assertEqual(<<"3">>, amqp10_msg:body_bin(Msg3)),
+                ?assertEqual(ReceiverDefaultPrio, Rec3),
+                ok = amqp10_client:accept_msg(Rec3, Msg3)
+    after 5000 -> ct:fail({missing_msg, ?LINE})
+    end,
+    receive {amqp10_msg, Rec4, Msg4} ->
+                ?assertEqual(<<"4">>, amqp10_msg:body_bin(Msg4)),
+                ?assertEqual(ReceiverLowPrio, Rec4),
+                ok = amqp10_client:accept_msg(Rec4, Msg4)
+    after 5000 -> ct:fail({missing_msg, ?LINE})
+    end,
+    receive {amqp10_msg, _, _} = Unexpected ->
+                ct:fail({unexpected_msg, Unexpected, ?LINE})
+    after 5 -> ok
+    end,
+
+    ok = amqp10_client:detach_link(Sender),
+    ok = amqp10_client:detach_link(ReceiverDefaultPrio),
+    ok = amqp10_client:detach_link(ReceiverHighPrio),
+    ok = amqp10_client:detach_link(ReceiverLowPrio),
+    {ok, #{message_count := 1}} = rabbitmq_amqp_client:delete_queue(LinkPair, QName),
+    ok = rabbitmq_amqp_client:detach_management_link_pair_sync(LinkPair),
+    ok = end_session_sync(Session),
+    ok = amqp10_client:close_connection(Connection).
+
 single_active_consumer_classic_queue(Config) ->
     single_active_consumer(<<"classic">>, Config).
 
@@ -4899,7 +4990,6 @@ tcp_back_pressure_rabbitmq_internal_flow(QType, Config) ->
     ok = end_session_sync(Session),
     ok = amqp10_client:close_connection(Connection).
 
-
 %% internal
 %%