%% 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) 2010-2020 VMware, Inc. or its affiliates.  All rights reserved.
%%

-module(rabbit_mirror_queue_sync).

-include_lib("rabbit_common/include/rabbit.hrl").

-export([master_prepare/4, master_go/8, slave/7, conserve_resources/3]).

-define(SYNC_PROGRESS_INTERVAL, 1000000).

%% There are three processes around, the master, the syncer and the
%% slave(s). The syncer is an intermediary, linked to the master in
%% order to make sure we do not mess with the master's credit flow or
%% set of monitors.
%%
%% Interactions
%% ------------
%%
%% '*' indicates repeating messages. All are standard Erlang messages
%% except sync_start which is sent over GM to flush out any other
%% messages that we might have sent that way already. (credit) is the
%% usual credit_flow bump message every so often.
%%
%%               Master             Syncer                 Slave(s)
%% sync_mirrors -> ||                                         ||
%%                 || -- (spawns) --> ||                      ||
%%                 || --------- sync_start (over GM) -------> ||
%%                 ||                 || <--- sync_ready ---- ||
%%                 ||                 ||         (or)         ||
%%                 ||                 || <--- sync_deny ----- ||
%%                 || <--- ready ---- ||                      ||
%%                 || <--- next* ---- ||                      ||  }
%%                 || ---- msg* ----> ||                      ||  } loop
%%                 ||                 || ---- sync_msgs* ---> ||  }
%%                 ||                 || <--- (credit)* ----- ||  }
%%                 || <--- next  ---- ||                      ||
%%                 || ---- done ----> ||                      ||
%%                 ||                 || -- sync_complete --> ||
%%                 ||               (Dies)                    ||

-type log_fun() :: fun ((string(), [any()]) -> 'ok').
-type bq() :: atom().
-type bqs() :: any().
-type ack() :: any().
-type slave_sync_state() :: {[{rabbit_types:msg_id(), ack()}], timer:tref(),
                             bqs()}.

%% ---------------------------------------------------------------------------
%% Master

-spec master_prepare(reference(), rabbit_amqqueue:name(),
                               log_fun(), [pid()]) -> pid().

master_prepare(Ref, QName, Log, SPids) ->
    MPid = self(),
    spawn_link(fun () ->
                       ?store_proc_name(QName),
                       syncer(Ref, Log, MPid, SPids)
               end).

-spec master_go(pid(), reference(), log_fun(),
                      rabbit_mirror_queue_master:stats_fun(),
                      rabbit_mirror_queue_master:stats_fun(),
                      non_neg_integer(),
                      bq(), bqs()) ->
                          {'already_synced', bqs()} | {'ok', bqs()} |
                          {'cancelled', bqs()} |
                          {'shutdown', any(), bqs()} |
                          {'sync_died', any(), bqs()}.

master_go(Syncer, Ref, Log, HandleInfo, EmitStats, SyncBatchSize, BQ, BQS) ->
    Args = {Syncer, Ref, Log, HandleInfo, EmitStats, rabbit_misc:get_parent()},
    receive
        {'EXIT', Syncer, normal} -> {already_synced, BQS};
        {'EXIT', Syncer, Reason} -> {sync_died, Reason, BQS};
        {ready, Syncer}          -> EmitStats({syncing, 0}),
                                    master_batch_go0(Args, SyncBatchSize,
                                                     BQ, BQS)
    end.

master_batch_go0(Args, BatchSize, BQ, BQS) ->
    FoldFun =
        fun (Msg, MsgProps, Unacked, Acc) ->
                Acc1 = append_to_acc(Msg, MsgProps, Unacked, Acc),
                case maybe_master_batch_send(Acc1, BatchSize) of
                    true  -> master_batch_send(Args, Acc1);
                    false -> {cont, Acc1}
                end
        end,
    FoldAcc = {[], 0, {0, BQ:depth(BQS)}, erlang:monotonic_time()},
    bq_fold(FoldFun, FoldAcc, Args, BQ, BQS).

master_batch_send({Syncer, Ref, Log, HandleInfo, EmitStats, Parent},
                  {Batch, I, {Curr, Len}, Last}) ->
    T = maybe_emit_stats(Last, I, EmitStats, Log),
    HandleInfo({syncing, I}),
    handle_set_maximum_since_use(),
    SyncMsg = {msgs, Ref, lists:reverse(Batch)},
    NewAcc = {[], I + length(Batch), {Curr, Len}, T},
    master_send_receive(SyncMsg, NewAcc, Syncer, Ref, Parent).

%% Either send messages when we reach the last one in the queue or
%% whenever we have accumulated BatchSize messages.
maybe_master_batch_send({_, _, {Len, Len}, _}, _BatchSize) ->
    true;
maybe_master_batch_send({_, _, {Curr, _Len}, _}, BatchSize)
  when Curr rem BatchSize =:= 0 ->
    true;
maybe_master_batch_send(_Acc, _BatchSize) ->
    false.

bq_fold(FoldFun, FoldAcc, Args, BQ, BQS) ->
    case BQ:fold(FoldFun, FoldAcc, BQS) of
        {{shutdown,  Reason}, BQS1} -> {shutdown,  Reason, BQS1};
        {{sync_died, Reason}, BQS1} -> {sync_died, Reason, BQS1};
        {_,                   BQS1} -> master_done(Args, BQS1)
    end.

append_to_acc(Msg, MsgProps, Unacked, {Batch, I, {Curr, Len}, T}) ->
    {[{Msg, MsgProps, Unacked} | Batch], I, {Curr + 1, Len}, T}.

master_send_receive(SyncMsg, NewAcc, Syncer, Ref, Parent) ->
    receive
        {'$gen_call', From,
         cancel_sync_mirrors}    -> stop_syncer(Syncer, {cancel, Ref}),
                                    gen_server2:reply(From, ok),
                                    {stop, cancelled};
        {next, Ref}              -> Syncer ! SyncMsg,
                                    {cont, NewAcc};
        {'EXIT', Parent, Reason} -> {stop, {shutdown,  Reason}};
        {'EXIT', Syncer, Reason} -> {stop, {sync_died, Reason}}
    end.

master_done({Syncer, Ref, _Log, _HandleInfo, _EmitStats, Parent}, BQS) ->
    receive
        {'$gen_call', From,
         cancel_sync_mirrors}    ->
            stop_syncer(Syncer, {cancel, Ref}),
            gen_server2:reply(From, ok),
            {cancelled, BQS};
        {cancelled, Ref} ->
            {cancelled, BQS};
        {next, Ref}              ->
            stop_syncer(Syncer, {done, Ref}),
            {ok, BQS};
        {'EXIT', Parent, Reason} ->
            {shutdown,  Reason, BQS};
        {'EXIT', Syncer, Reason} ->
            {sync_died, Reason, BQS}
    end.

stop_syncer(Syncer, Msg) ->
    unlink(Syncer),
    Syncer ! Msg,
    receive {'EXIT', Syncer, _} -> ok
    after 0 -> ok
    end.

maybe_emit_stats(Last, I, EmitStats, Log) ->
    Interval = erlang:convert_time_unit(
                 erlang:monotonic_time() - Last, native, micro_seconds),
    case Interval > ?SYNC_PROGRESS_INTERVAL of
        true  -> EmitStats({syncing, I}),
                 Log("~p messages", [I]),
                 erlang:monotonic_time();
        false -> Last
    end.

handle_set_maximum_since_use() ->
    receive
        {'$gen_cast', {set_maximum_since_use, Age}} ->
            ok = file_handle_cache:set_maximum_since_use(Age)
    after 0 ->
            ok
    end.

%% Master
%% ---------------------------------------------------------------------------
%% Syncer

syncer(Ref, Log, MPid, SPids) ->
    [erlang:monitor(process, SPid) || SPid <- SPids],
    %% We wait for a reply from the mirrors so that we know they are in
    %% a receive block and will thus receive messages we send to them
    %% *without* those messages ending up in their gen_server2 pqueue.
    case await_slaves(Ref, SPids) of
        []     -> Log("all mirrors already synced", []);
        SPids1 -> MPid ! {ready, self()},
                  Log("mirrors ~p to sync", [[node(SPid) || SPid <- SPids1]]),
                  syncer_check_resources(Ref, MPid, SPids1)
    end.

await_slaves(Ref, SPids) ->
    [SPid || SPid <- SPids,
             rabbit_mnesia:on_running_node(SPid) andalso %% [0]
                 receive
                     {sync_ready, Ref, SPid}       -> true;
                     {sync_deny,  Ref, SPid}       -> false;
                     {'DOWN', _, process, SPid, _} -> false
                 end].
%% [0] This check is in case there's been a partition which has then
%% healed in between the master retrieving the mirror pids from Mnesia
%% and sending 'sync_start' over GM. If so there might be mirrors on the
%% other side of the partition which we can monitor (since they have
%% rejoined the distributed system with us) but which did not get the
%% 'sync_start' and so will not reply. We need to act as though they are
%% down.

syncer_check_resources(Ref, MPid, SPids) ->
    rabbit_alarm:register(self(), {?MODULE, conserve_resources, []}),
    %% Before we ask the master node to send the first batch of messages
    %% over here, we check if one node is already short on memory. If
    %% that's the case, we wait for the alarm to be cleared before
    %% starting the syncer loop.
    AlarmedNodes = lists:any(
      fun
          ({{resource_limit, memory, _}, _}) -> true;
          ({_, _})                           -> false
      end, rabbit_alarm:get_alarms()),
    if
        not AlarmedNodes ->
            MPid ! {next, Ref},
            syncer_loop(Ref, MPid, SPids);
        true ->
            case wait_for_resources(Ref, SPids) of
                cancel -> MPid ! {cancelled, Ref};
                SPids1 -> MPid ! {next, Ref},
                          syncer_loop(Ref, MPid, SPids1)
            end
    end.

syncer_loop(Ref, MPid, SPids) ->
    receive
        {conserve_resources, memory, true} ->
            case wait_for_resources(Ref, SPids) of
                cancel -> MPid ! {cancelled, Ref};
                SPids1 -> syncer_loop(Ref, MPid, SPids1)
            end;
        {conserve_resources, _, _} ->
            %% Ignore other alerts.
            syncer_loop(Ref, MPid, SPids);
        {msgs, Ref, Msgs} ->
            SPids1 = wait_for_credit(SPids),
            case SPids1 of
                [] ->
                    % Die silently because there are no mirrors left.
                    ok;
                _  ->
                    broadcast(SPids1, {sync_msgs, Ref, Msgs}),
                    MPid ! {next, Ref},
                    syncer_loop(Ref, MPid, SPids1)
            end;
        {cancel, Ref} ->
            %% We don't tell the mirrors we will die - so when we do
            %% they interpret that as a failure, which is what we
            %% want.
            ok;
        {done, Ref} ->
            [SPid ! {sync_complete, Ref} || SPid <- SPids]
    end.

broadcast(SPids, Msg) ->
    [begin
         credit_flow:send(SPid),
         SPid ! Msg
     end || SPid <- SPids].

conserve_resources(Pid, Source, {_, Conserve, _}) ->
    Pid ! {conserve_resources, Source, Conserve},
    ok.

wait_for_credit(SPids) ->
    case credit_flow:blocked() of
        true  -> receive
                     {bump_credit, Msg} ->
                         credit_flow:handle_bump_msg(Msg),
                         wait_for_credit(SPids);
                     {'DOWN', _, process, SPid, _} ->
                         credit_flow:peer_down(SPid),
                         wait_for_credit(lists:delete(SPid, SPids))
                 end;
        false -> SPids
    end.

wait_for_resources(Ref, SPids) ->
    receive
        {conserve_resources, memory, false} ->
            SPids;
        {conserve_resources, _, _} ->
            %% Ignore other alerts.
            wait_for_resources(Ref, SPids);
        {cancel, Ref} ->
            %% We don't tell the mirrors we will die - so when we do
            %% they interpret that as a failure, which is what we
            %% want.
            cancel;
        {'DOWN', _, process, SPid, _} ->
            credit_flow:peer_down(SPid),
            SPids1 = wait_for_credit(lists:delete(SPid, SPids)),
            wait_for_resources(Ref, SPids1)
    end.

%% Syncer
%% ---------------------------------------------------------------------------
%% Slave

-spec slave(non_neg_integer(), reference(), timer:tref(), pid(),
                  bq(), bqs(), fun((bq(), bqs()) -> {timer:tref(), bqs()})) ->
                      'denied' |
                      {'ok' | 'failed', slave_sync_state()} |
                      {'stop', any(), slave_sync_state()}.

slave(0, Ref, _TRef, Syncer, _BQ, _BQS, _UpdateRamDuration) ->
    Syncer ! {sync_deny, Ref, self()},
    denied;

slave(_DD, Ref, TRef, Syncer, BQ, BQS, UpdateRamDuration) ->
    MRef = erlang:monitor(process, Syncer),
    Syncer ! {sync_ready, Ref, self()},
    {_MsgCount, BQS1} = BQ:purge(BQ:purge_acks(BQS)),
    slave_sync_loop({Ref, MRef, Syncer, BQ, UpdateRamDuration,
                     rabbit_misc:get_parent()}, {[], TRef, BQS1}).

slave_sync_loop(Args = {Ref, MRef, Syncer, BQ, UpdateRamDuration, Parent},
                State = {MA, TRef, BQS}) ->
    receive
        {'DOWN', MRef, process, Syncer, _Reason} ->
            %% If the master dies half way we are not in the usual
            %% half-synced state (with messages nearer the tail of the
            %% queue); instead we have ones nearer the head. If we then
            %% sync with a newly promoted master, or even just receive
            %% messages from it, we have a hole in the middle. So the
            %% only thing to do here is purge.
            {_MsgCount, BQS1} = BQ:purge(BQ:purge_acks(BQS)),
            credit_flow:peer_down(Syncer),
            {failed, {[], TRef, BQS1}};
        {bump_credit, Msg} ->
            credit_flow:handle_bump_msg(Msg),
            slave_sync_loop(Args, State);
        {sync_complete, Ref} ->
            erlang:demonitor(MRef, [flush]),
            credit_flow:peer_down(Syncer),
            {ok, State};
        {'$gen_cast', {set_maximum_since_use, Age}} ->
            ok = file_handle_cache:set_maximum_since_use(Age),
            slave_sync_loop(Args, State);
        {'$gen_cast', {set_ram_duration_target, Duration}} ->
            BQS1 = BQ:set_ram_duration_target(Duration, BQS),
            slave_sync_loop(Args, {MA, TRef, BQS1});
        {'$gen_cast', {run_backing_queue, Mod, Fun}} ->
            BQS1 = BQ:invoke(Mod, Fun, BQS),
            slave_sync_loop(Args, {MA, TRef, BQS1});
        update_ram_duration ->
            {TRef1, BQS1} = UpdateRamDuration(BQ, BQS),
            slave_sync_loop(Args, {MA, TRef1, BQS1});
        {sync_msgs, Ref, Batch} ->
            credit_flow:ack(Syncer),
            {MA1, BQS1} = process_batch(Batch, MA, BQ, BQS),
            slave_sync_loop(Args, {MA1, TRef, BQS1});
        {'EXIT', Parent, Reason} ->
            {stop, Reason, State};
        %% If the master throws an exception
        {'$gen_cast', {gm, {delete_and_terminate, Reason}}} ->
            BQ:delete_and_terminate(Reason, BQS),
            {stop, Reason, {[], TRef, undefined}}
    end.

%% We are partitioning messages by the Unacked element in the tuple.
%% when unacked = true, then it's a publish_delivered message,
%% otherwise it's a publish message.
%%
%% Note that we can't first partition the batch and then publish each
%% part, since that would result in re-ordering messages, which we
%% don't want to do.
process_batch([], MA, _BQ, BQS) ->
    {MA, BQS};
process_batch(Batch, MA, BQ, BQS) ->
    {_Msg, _MsgProps, Unacked} = hd(Batch),
    process_batch(Batch, Unacked, [], MA, BQ, BQS).

process_batch([{Msg, Props, true = Unacked} | Rest], true = Unacked,
              Acc, MA, BQ, BQS) ->
    %% publish_delivered messages don't need the IsDelivered flag,
    %% therefore we just add {Msg, Props} to the accumulator.
    process_batch(Rest, Unacked, [{Msg, props(Props)} | Acc],
                  MA, BQ, BQS);
process_batch([{Msg, Props, false = Unacked} | Rest], false = Unacked,
              Acc, MA, BQ, BQS) ->
    %% publish messages needs the IsDelivered flag which is set to true
    %% here.
    process_batch(Rest, Unacked, [{Msg, props(Props), true} | Acc],
                  MA, BQ, BQS);
process_batch(Batch, Unacked, Acc, MA, BQ, BQS) ->
    {MA1, BQS1} = publish_batch(Unacked, lists:reverse(Acc), MA, BQ, BQS),
    process_batch(Batch, MA1, BQ, BQS1).

%% Unacked msgs are published via batch_publish.
publish_batch(false, Batch, MA, BQ, BQS) ->
    batch_publish(Batch, MA, BQ, BQS);
%% Acked msgs are published via batch_publish_delivered.
publish_batch(true, Batch, MA, BQ, BQS) ->
    batch_publish_delivered(Batch, MA, BQ, BQS).


batch_publish(Batch, MA, BQ, BQS) ->
    BQS1 = BQ:batch_publish(Batch, none, noflow, BQS),
    {MA, BQS1}.

batch_publish_delivered(Batch, MA, BQ, BQS) ->
    {AckTags, BQS1} = BQ:batch_publish_delivered(Batch, none, noflow, BQS),
    MA1 = BQ:zip_msgs_and_acks(Batch, AckTags, MA, BQS1),
    {MA1, BQS1}.

props(Props) ->
    Props#message_properties{needs_confirming = false}.
