Fix delete_replica bug

It caused a lot of flakiness on the rabbit_stream_queue_SUITE, both on `delete_replica`
and `delete_last_replica` test cases.

deps/rabbit/src/rabbit_stream_coordinator.erl

@@ -1018,16 +1018,14 @@ update_stream0(#{system_time := _Ts},
                             _ ->
                                 false
                         end,
-
     case maps:get(Node, Members0) of
         #member{role = {replica, Epoch},
                 current = {stopping, Idx},
                 state = _} = Member0
           when IsLeaderInCurrent ->
             %% A leader has already been selected so skip straight to ready state
-            Member = Member0#member{state = {ready, Epoch},
+            Member = update_target(Member0#member{state = {ready, Epoch},
-                                    target = Target,
+                                                  current = undefined}, Target),
-                                    current = undefined},
             Members1 = Members0#{Node => Member},
             Stream0#stream{members = Members1};
         #member{role = {_, Epoch},
@@ -1037,9 +1035,8 @@ update_stream0(#{system_time := _Ts},
             %% epoch
             Member = case StoppedEpoch of
                          Epoch ->
-                             Member0#member{state = {stopped, StoppedEpoch, Tail},
+                             update_target(Member0#member{state = {stopped, StoppedEpoch, Tail},
-                                            target = Target,
+                                                          current = undefined}, Target);
-                                            current = undefined};
                          _ ->
                              %% if stopped epoch is from another epoch
                              %% leave target as is to retry stop in current term
@@ -1518,3 +1515,8 @@ set_running_to_stopped(Members) ->
                      M
              end, Members).
 
+update_target(#member{target = deleted} = Member, _) ->
+    %% A deleted member can never transition to another state
+    Member;
+update_target(Member, Target) ->
+    Member#member{target = Target}.

deps/rabbit/test/rabbit_stream_coordinator_SUITE.erl

@@ -30,6 +30,7 @@ all_tests() ->
      delete_stream,
      delete_replica_leader,
      delete_replica,
+     delete_two_replicas,
      delete_replica_2,
      leader_start_failed
     ].
@@ -907,6 +908,79 @@ delete_replica(_) ->
     {S4, []} = evaluate_stream(meta(?LINE), S4, []),
     ok.
 
+delete_two_replicas(_) ->
+    %% There was a race condition on the rabbit_stream_queue_SUITE testcases delete_replica
+    %% and delete_last_replica. A replica can sometimes restart after deletion as it transitions
+    %% again to running state. This test reproduces it. See `rabbit_stream_coordinator.erl`
+    %% line 1039, the processing of `member_stopped` command. The new function `update_target`
+    %% ensures this transition never happens.
+    %% This test reproduces the trace that leads to that error.
+    E = 1,
+    StreamId = atom_to_list(?FUNCTION_NAME),
+    LeaderPid = fake_pid(n1),
+    [Replica1, Replica2] = [fake_pid(n2), fake_pid(n3)],
+    N1 = node(LeaderPid),
+    N2 = node(Replica1),
+    %% this is to be added
+    N3 = node(Replica2),
+
+    S0 = started_stream(StreamId, LeaderPid, [Replica1, Replica2]),
+    From = {self(), make_ref()},
+    Idx1 = ?LINE,
+    Meta1 = (meta(Idx1))#{from => From},
+    S1 = update_stream(Meta1, {delete_replica, StreamId, #{node => N3}}, S0),
+    ?assertMatch(#stream{target = running,
+                         nodes = [N1, N2],
+                         members = #{N1 := #member{target = stopped,
+                                                   current = undefined,
+                                                   state = {running, _, _}},
+                                     N2 := #member{target = stopped,
+                                                   current = undefined,
+                                                   state = {running, _, _}},
+                                     N3 := #member{target = deleted,
+                                                   current = undefined,
+                                                   state = {running, _, _}}
+                                    }},
+                 S1),
+    {S2, Actions1} = evaluate_stream(Meta1, S1, []),
+    ?assertMatch([{aux, {delete_member, StreamId, #{node := N3}, _}},
+                  {aux, {stop, StreamId, #{node := N1, epoch := E}, _}},
+                  {aux, {stop, StreamId, #{node := N2, epoch := E}, _}}],
+                 lists:sort(Actions1)),
+
+    Idx2 = ?LINE,
+    Meta2 = (meta(Idx2))#{from => From},
+    S3 = update_stream(Meta2, {delete_replica, StreamId, #{node => N2}}, S2),
+    ?assertMatch(#stream{target = running,
+                         nodes = [N1],
+                         members = #{N1 := #member{target = stopped,
+                                                   current = {stopping, _},
+                                                   state = {running, _, _}},
+                                     N2 := #member{target = deleted,
+                                                   current = {stopping, _},
+                                                   state = {running, _, _}},
+                                     N3 := #member{target = deleted,
+                                                   current = {deleting, _},
+                                                   state = {running, _, _}}
+                                    }},
+                 S3),
+    {S4, []} = evaluate_stream(Meta2, S3, []),
+
+
+    Idx3 = ?LINE,
+    S5 = update_stream(meta(Idx3),
+                       {member_stopped, StreamId, #{node => N2,
+                                                    index => Idx1,
+                                                    epoch => E,
+                                                    tail => {E, 101}}},
+                       S4),
+    %% A deleted member can never transition to another target.
+    ?assertMatch(#stream{members = #{N2 := #member{target = deleted,
+                                                   current = undefined,
+                                                   state = {stopped, _, _}}}},
+                 S5),
+    ok.
+
 delete_replica_2(_) ->
     %% replica is deleted before it has been fully started
     E = 1,

deps/rabbit/test/rabbit_stream_queue_SUITE.erl

@@ -186,7 +186,6 @@ merge_app_env(Config) ->
                                       {rabbit, [{core_metrics_gc_interval, 100}]}).
 
 end_per_testcase(Testcase, Config) ->
-    Q = ?config(queue_name, Config),
     Config1 = rabbit_ct_helpers:run_steps(
                 Config,
                 rabbit_ct_client_helpers:teardown_steps()),