%% This Source Code Form is subject to the terms of the Mozilla Public
%% License, v. 2.0. If a copy of the MPL was not distributed with this
%% file, You can obtain one at https://mozilla.org/MPL/2.0/.
%%
%% Copyright (c) 2007-2023 VMware, Inc. or its affiliates.  All rights reserved.
%%

-module(rabbit_federation_queue).

-rabbit_boot_step({?MODULE,
                   [{description, "federation queue decorator"},
                    {mfa, {rabbit_queue_decorator, register,
                           [<<"federation">>, ?MODULE]}},
                    {requires, rabbit_registry},
                    {cleanup, {rabbit_queue_decorator, unregister,
                               [<<"federation">>]}},
                    {enables, recovery}]}).

-include_lib("rabbit/include/amqqueue.hrl").
-include_lib("amqp_client/include/amqp_client.hrl").
-include("rabbit_federation.hrl").

-behaviour(rabbit_queue_decorator).

-export([startup/1, shutdown/1, policy_changed/2, active_for/1,
         consumer_state_changed/3]).
-export([policy_changed_local/2]).

%%----------------------------------------------------------------------------

startup(Q) ->
    case active_for(Q) of
        true  -> rabbit_federation_queue_link_sup_sup:start_child(Q);
        false -> ok
    end,
    ok.

shutdown(Q) when ?is_amqqueue(Q) ->
    QName = amqqueue:get_name(Q),
    case active_for(Q) of
        true  -> rabbit_federation_queue_link_sup_sup:stop_child(Q),
                 rabbit_federation_status:remove_exchange_or_queue(QName);
        false -> ok
    end,
    ok.

policy_changed(Q1, Q2) when ?is_amqqueue(Q1) ->
    QName = amqqueue:get_name(Q1),
    case rabbit_amqqueue:lookup(QName) of
        {ok, Q0} when ?is_amqqueue(Q0) ->
            rpc:call(amqqueue:qnode(Q0), rabbit_federation_queue,
                     policy_changed_local, [Q1, Q2]);
        {error, not_found} ->
            ok
    end.

policy_changed_local(Q1, Q2) ->
    shutdown(Q1),
    startup(Q2).

active_for(Q) ->
    Args = amqqueue:get_arguments(Q),
    case rabbit_misc:table_lookup(Args, <<"x-internal-purpose">>) of
        {longstr, _} -> false; %% [0]
        _            -> rabbit_federation_upstream:federate(Q)
    end.
%% [0] Currently the only "internal purpose" is federation, but I
%% suspect if we introduce another one it will also be for something
%% that doesn't want to be federated.

%% We need to reconsider whether we need to run or pause every time
%% the consumer state changes in the queue. But why can the state
%% change?
%%
%% consumer blocked   | We may have no more active consumers, and thus need to
%%                    | pause
%%                    |
%% consumer unblocked | We don't care
%%                    |
%% queue empty        | The queue has become empty therefore we need to run to
%%                    | get more messages
%%                    |
%% basic consume      | We don't care
%%                    |
%% basic cancel       | We may have no more active consumers, and thus need to
%%                    | pause
%%                    |
%% refresh            | We asked for it (we have started a new link after
%%                    | failover and need something to prod us into action
%%                    | (or not)).
%%
%% In the cases where we don't care it's not prohibitively expensive
%% for us to be here anyway, so never mind.
%%
%% Note that there is no "queue became non-empty" state change - that's
%% because of the queue invariant. If the queue transitions from empty to
%% non-empty then it must have no active consumers - in which case it stays
%% the same from our POV.

consumer_state_changed(Q, MaxActivePriority, IsEmpty) ->
    QName = amqqueue:get_name(Q),
    _ = case IsEmpty andalso active_unfederated(MaxActivePriority) of
        true  -> rabbit_federation_queue_link:run(QName);
        false -> rabbit_federation_queue_link:pause(QName)
    end,
    ok.

active_unfederated(empty)         -> false;
active_unfederated(P) when P >= 0 -> true;
active_unfederated(_P)            -> false.
