From 37dd46271a805a90b49d993d49ade8b70c47f1ef Mon Sep 17 00:00:00 2001
From: Simon MacMullen <simon@rabbitmq.com>
Date: Tue, 12 Feb 2013 18:04:31 +0000
Subject: Tweak nodes policy to allow master removal and thus queue migration

---
 src/rabbit_mirror_queue_misc.erl | 49 ++++++++++++++++++++++++----------------
 1 file changed, 30 insertions(+), 19 deletions(-)

(limited to 'src/rabbit_mirror_queue_misc.erl')

diff --git a/src/rabbit_mirror_queue_misc.erl b/src/rabbit_mirror_queue_misc.erl
index 05036d35..cc2d7c77 100644
--- a/src/rabbit_mirror_queue_misc.erl
+++ b/src/rabbit_mirror_queue_misc.erl
@@ -235,13 +235,13 @@ suggested_queue_nodes(Q) ->
 %% rabbit_mnesia:cluster_nodes(running) out of a loop or
 %% transaction or both.
 suggested_queue_nodes(Q, PossibleNodes) ->
-    {MNode0, SNodes} = actual_queue_nodes(Q),
+    {MNode0, SNodes, SSNodes} = actual_queue_nodes(Q),
     MNode = case MNode0 of
                 none -> node();
                 _    -> MNode0
             end,
     suggested_queue_nodes(policy(<<"ha-mode">>, Q), policy(<<"ha-params">>, Q),
-                          {MNode, SNodes}, PossibleNodes).
+                          {MNode, SNodes, SSNodes}, PossibleNodes).
 
 policy(Policy, Q) ->
     case rabbit_policy:get(Policy, Q) of
@@ -249,15 +249,20 @@ policy(Policy, Q) ->
         _       -> none
     end.
 
-suggested_queue_nodes(<<"all">>, _Params, {MNode, _SNodes}, Possible) ->
-    {MNode, Possible -- [MNode]};
-suggested_queue_nodes(<<"nodes">>, Nodes0, {MNode, _SNodes}, Possible) ->
+suggested_queue_nodes(<<"all">>, _Params, {MNode, _SNodes, _SSNodes}, Poss) ->
+    {MNode, Poss -- [MNode]};
+suggested_queue_nodes(<<"nodes">>, Nodes0, {MNode, _SNodes, SSNodes}, Poss) ->
     Nodes1 = [list_to_atom(binary_to_list(Node)) || Node <- Nodes0],
-    %% If the current master is currently not in the nodes specified,
-    %% act like it is for the purposes below - otherwise we will not
-    %% return it in the results...
-    Nodes = lists:usort([MNode | Nodes1]),
-    Unavailable = Nodes -- Possible,
+    %% If the current master is not in the nodes specified, then what we want
+    %% to do depends on whether there are any synchronised slaves. If there
+    %% are then we can just kill the current master - the admin has asked for
+    %% a migration and we should give it to them. If there are not however
+    %% then we must keep the master around so as not to lose messages.
+    Nodes = case SSNodes of
+                [] -> lists:usort([MNode | Nodes1]);
+                _  -> Nodes1
+            end,
+    Unavailable = Nodes -- Poss,
     Available = Nodes -- Unavailable,
     case Available of
         [] -> %% We have never heard of anything? Not much we can do but
@@ -265,21 +270,24 @@ suggested_queue_nodes(<<"nodes">>, Nodes0, {MNode, _SNodes}, Possible) ->
               {MNode, []};
         _  -> case lists:member(MNode, Available) of
                   true  -> {MNode, Available -- [MNode]};
-                  false -> promote_slave(Available)
+                  false -> %% Make the sure new master is synced! In order to
+                           %% get here SSNodes must not be empty.
+                           [NewMNode | _] = SSNodes,
+                           {NewMNode, Available -- [NewMNode]}
               end
     end;
 %% When we need to add nodes, we randomise our candidate list as a
 %% crude form of load-balancing. TODO it would also be nice to
-%% randomise the list of ones to remove when we have too many - but
-%% that would fail to take account of synchronisation...
-suggested_queue_nodes(<<"exactly">>, Count, {MNode, SNodes}, Possible) ->
+%% randomise the list of ones to remove when we have too many - we
+%% would have to take account of synchronisation though.
+suggested_queue_nodes(<<"exactly">>, Count, {MNode, SNodes, _SSNodes}, Poss) ->
     SCount = Count - 1,
     {MNode, case SCount > length(SNodes) of
-                true  -> Cand = shuffle((Possible -- [MNode]) -- SNodes),
+                true  -> Cand = shuffle((Poss -- [MNode]) -- SNodes),
                          SNodes ++ lists:sublist(Cand, SCount - length(SNodes));
                 false -> lists:sublist(SNodes, SCount)
             end};
-suggested_queue_nodes(_, _, {MNode, _}, _) ->
+suggested_queue_nodes(_, _, {MNode, _, _}, _) ->
     {MNode, []}.
 
 shuffle(L) ->
@@ -288,11 +296,14 @@ shuffle(L) ->
     {_, L1} = lists:unzip(lists:keysort(1, [{random:uniform(), N} || N <- L])),
     L1.
 
-actual_queue_nodes(#amqqueue{pid = MPid, slave_pids = SPids}) ->
+actual_queue_nodes(#amqqueue{pid             = MPid,
+                             slave_pids      = SPids,
+                             sync_slave_pids = SSPids}) ->
+    Nodes = fun (L) -> [node(Pid) || Pid <- L] end,
     {case MPid of
          none -> none;
          _    -> node(MPid)
-     end, [node(Pid) || Pid <- SPids]}.
+     end, Nodes(SPids), Nodes(SSPids)}.
 
 is_mirrored(Q) ->
     case policy(<<"ha-mode">>, Q) of
@@ -313,7 +324,7 @@ update_mirrors(OldQ = #amqqueue{pid = QPid},
 
 update_mirrors0(OldQ = #amqqueue{name = QName},
                 NewQ = #amqqueue{name = QName}) ->
-    All = fun ({A,B}) -> [A|B] end,
+    All = fun (Tuple) -> [element(1, Tuple) | element(2, Tuple)] end,
     OldNodes = All(actual_queue_nodes(OldQ)),
     NewNodes = All(suggested_queue_nodes(NewQ)),
     add_mirrors(QName, NewNodes -- OldNodes),
-- 
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