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enb_proxy: fix stale SCTP events misprocessed during MME pool selection

When closing a connection to one MME and opening a new one to the next,
a SHUTDOWN_COMP event from the old (now-closed) socket can still be
pending in the process mailbox.  The sctp_assoc_change handlers in the
'connecting' and 'wait_s1setup_rsp' states were matching '_Socket',
ignoring the socket identity.  A stale SHUTDOWN_COMP would therefore
fall into the '_ -> repeat_state_and_data' branch, triggering a spurious
re-entry of 'connecting', which immediately closed the newly established
connection to the next MME and skipped to yet another pool entry.

Fix this by matching the Socket against the current S#state.sock in both
handlers.  Events arriving on a previously closed socket no longer match
these clauses and are silently dropped by the catch-all handle_event/4.

This was found thanks to the MME pooling TCs in ttcn3-s1gw-test.

Change-Id: I4211dd343607f045cf4dd33fa568ed580c79dd9f
Related: SYS#7052
---
M src/enb_proxy.erl
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/enb_proxy.erl b/src/enb_proxy.erl
index 5ab79d1..3ce205f 100644
--- a/src/enb_proxy.erl
+++ b/src/enb_proxy.erl
@@ -235,9 +235,11 @@
     keep_state_and_data;
 
 %% Handle an #sctp_assoc_change event (connection state)
-connecting(info, {sctp, _Socket, MmeAddr, MmePort,
+%% Match on Socket to avoid processing stale events from a previously closed socket
+connecting(info, {sctp, Socket, MmeAddr, MmePort,
                   {[], #sctp_assoc_change{state = ConnState,
-                                          assoc_id = Aid}}}, S) ->
+                                          assoc_id = Aid}}},
+           #state{sock = Socket} = S) ->
     MmeName = hd(S#state.tried_mmes),
     case ConnState of
         comm_up ->
@@ -315,9 +317,11 @@
 
 %% Handle an #sctp_assoc_change event (MME connection state)
 %% We may loose connection while waiting for the S1 SETUP RESPONSE
-wait_s1setup_rsp(info, {sctp, _Socket, MmeAddr, MmePort,
+%% Match on Socket to avoid processing stale events from a previously closed socket
+wait_s1setup_rsp(info, {sctp, Socket, MmeAddr, MmePort,
                  {[], #sctp_assoc_change{state = ConnState,
-                                         assoc_id = Aid}}}, S) ->
+                                         assoc_id = Aid}}},
+                #state{sock = Socket} = S) ->
     case ConnState of
         comm_up ->
             ?LOG_NOTICE("MME connection (id=~p, ~p:~p) is already established?!?",

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