Priority queue: Fix a crash when querying 'head_message_timestamp'

Before the fix, the code would consider it an integer which could be
summed. It crashed if the head message had no timestamp at all (the
underlying queue returned '').

Now, we have a special case, exactly like 'backing_queue_status'. The
code first finds a priority with queued messages, then it queries the
underlying queue for the 'head_message_timestamp'.

Fixes #245.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/rabbit_priority_queue.erl b/src/rabbit_priority_queue.erl
index 206d674abc..a839badfc4 100644
--- a/src/rabbit_priority_queue.erl
+++ b/src/rabbit_priority_queue.erl
@@ -376,6 +376,8 @@ info(backing_queue_status, #state{bq = BQ, bqss = BQSs}) ->
     fold0(fun (P, BQSN, Acc) ->
                   combine_status(P, BQ:info(backing_queue_status, BQSN), Acc)
           end, nothing, BQSs);
+info(head_message_timestamp, #state{bq = BQ, bqss = BQSs}) ->
+    find_head_message_timestamp(BQ, BQSs, '');
 info(Item, #state{bq = BQ, bqss = BQSs}) ->
     fold0(fun (_P, BQSN, Acc) ->
                   Acc + BQ:info(Item, BQSN)
@@ -582,3 +584,17 @@ cse(_, infinity)            -> infinity;
 cse(A, B) when is_number(A) -> A + B;
 cse({delta, _, _, _}, _)    -> {delta, todo, todo, todo};
 cse(A, B)                   -> exit({A, B}).
+
+%% When asked about 'head_message_timestamp' fro this priority queue, we
+%% walk all the backing queues, starting by the highest priority. Once a
+%% backing queue having messages (ready or unacknowledged) is found, its
+%% 'head_message_timestamp' is returned even if it is null.
+
+find_head_message_timestamp(BQ, [{_, BQSN} | Rest], Timestamp) ->
+    MsgCount = BQ:len(BQSN) + BQ:info(messages_unacknowledged_ram, BQSN),
+    if
+        MsgCount =/= 0 -> BQ:info(head_message_timestamp, BQSN);
+        true           -> find_head_message_timestamp(BQ, Rest, Timestamp)
+    end;
+find_head_message_timestamp(_, [], Timestamp) ->
+    Timestamp.