Make empty CQ init faster in case of clean shutdown (backport #13856)

deps/rabbit/src/rabbit_classic_queue_index_v2.erl

@@ -20,7 +20,10 @@
 %% queue implementation itself.
 -export([pre_publish/7, flush_pre_publish_cache/2,
          sync/1, needs_sync/1, flush/1,
-         bounds/1, next_segment_boundary/1]).
+         bounds/2, next_segment_boundary/1]).
+
+%% Only used by tests
+-export([bounds/1]).
 
 %% Used to upgrade/downgrade from/to the v1 index.
 -export([init_for_conversion/3]).
@@ -480,7 +483,7 @@ recover_index_v1_common(State0 = #qi{ queue_name = Name, dir = DirBin },
     %% When resuming after a crash we need to double check the messages that are both
     %% in the v1 and v2 index (effectively the messages below the upper bound of the
     %% v1 index that are about to be written to it).
-    {_, V2HiSeqId, _} = bounds(State0),
+    {_, V2HiSeqId, _} = bounds(State0, undefined),
     SkipFun = fun
         (SeqId, FunState0) when SeqId < V2HiSeqId ->
             case read(SeqId, SeqId + 1, FunState0) of
@@ -1188,14 +1191,22 @@ flush_pre_publish_cache(TargetRamCount, State) ->
 %% the test suite to pass. This can probably be made more accurate
 %% in the future.
 
+%% `bounds/1` is only used by tests
 -spec bounds(State) ->
                        {non_neg_integer(), non_neg_integer(), State}
                        when State::state().
+bounds(State) ->
+    bounds(State, undefined).
 
-bounds(State = #qi{ segments = Segments }) ->
+-spec bounds(State, non_neg_integer() | undefined) ->
+                       {non_neg_integer(), non_neg_integer(), State}
+                       when State::state().
+bounds(State = #qi{ segments = Segments }, NextSeqIdHint) ->
     ?DEBUG("~0p", [State]),
     %% We must special case when we are empty to make tests happy.
     if
+        Segments =:= #{} andalso is_integer(NextSeqIdHint) ->
+            {NextSeqIdHint, NextSeqIdHint, State};
         Segments =:= #{} ->
             {0, 0, State};
         true ->

deps/rabbit/src/rabbit_variable_queue.erl

@@ -1172,7 +1172,13 @@ expand_delta(_SeqId, #delta { count       = Count,
 
 init(IsDurable, IndexState, StoreState, DeltaCount, DeltaBytes, Terms,
      PersistentClient, TransientClient, VHost) ->
-    {LowSeqId, HiSeqId, IndexState1} = rabbit_classic_queue_index_v2:bounds(IndexState),
+    NextSeqIdHint =
+        case Terms of
+            non_clean_shutdown -> undefined;
+            _ -> proplists:get_value(next_seq_id, Terms)
+        end,
+
+    {LowSeqId, HiSeqId, IndexState1} = rabbit_classic_queue_index_v2:bounds(IndexState, NextSeqIdHint),
 
     {NextSeqId, NextDeliverSeqId, DeltaCount1, DeltaBytes1} =
         case Terms of

deps/rabbit/test/backing_queue_SUITE.erl

@@ -29,6 +29,7 @@
     variable_queue_dropfetchwhile,
     variable_queue_dropwhile_restart,
     variable_queue_dropwhile_sync_restart,
+    variable_queue_restart_large_seq_id,
     variable_queue_ack_limiting,
     variable_queue_purge,
     variable_queue_requeue,
@@ -1421,6 +1422,45 @@ variable_queue_dropwhile_sync_restart2(VQ0, QName) ->
 
     VQ5.
 
+variable_queue_restart_large_seq_id(Config) ->
+    passed = rabbit_ct_broker_helpers:rpc(Config, 0,
+      ?MODULE, variable_queue_restart_large_seq_id1, [Config]).
+
+variable_queue_restart_large_seq_id1(Config) ->
+    with_fresh_variable_queue(
+      fun variable_queue_restart_large_seq_id2/2,
+      ?config(variable_queue_type, Config)).
+
+variable_queue_restart_large_seq_id2(VQ0, QName) ->
+    Count = 1,
+
+    %% publish and consume a message
+    VQ1 = publish_fetch_and_ack(Count, 0, VQ0),
+    %% should be empty now
+    true = rabbit_variable_queue:is_empty(VQ1),
+
+    _VQ2 = rabbit_variable_queue:terminate(shutdown, VQ1),
+    Terms = variable_queue_read_terms(QName),
+    Count = proplists:get_value(next_seq_id, Terms),
+
+    %% set a very high next_seq_id as if 100M messages have been
+    %% published and consumed
+    Terms2 = lists:keyreplace(next_seq_id, 1, Terms, {next_seq_id, 100_000_000}),
+
+    {TInit, VQ3} =
+        timer:tc(
+          fun() -> variable_queue_init(test_amqqueue(QName, true), Terms2) end,
+          millisecond),
+    %% even with a very high next_seq_id start of an empty queue
+    %% should be quick (few milliseconds, but let's give it 100ms, to
+    %% avoid flaking on slow servers)
+    {true, _} = {TInit < 100, TInit},
+
+    %% should be empty now
+    true = rabbit_variable_queue:is_empty(VQ3),
+
+    VQ3.
+
 variable_queue_ack_limiting(Config) ->
     passed = rabbit_ct_broker_helpers:rpc(Config, 0,
       ?MODULE, variable_queue_ack_limiting1, [Config]).