Defer MBus DestinationSession registration during content node init

Creating a `DestinationSession` that is immediately registered as available
for business means we may theoretically start receiving messages over the
session even before the call returns to the caller. Either way there would
be no memory barrier that ensures that `_messageBusSession` would be fully
visible to the MessageBus threads (since it's written after return).

To avoid this sneaky scenario, defer registration (and thus introduce a
barrier) until _after_ we've initialized our internal member variables.

This addresses a TSan warning.

1 files changed, 9 insertions, 0 deletions

diff --git a/storage/src/vespa/storage/storageserver/communicationmanager.cpp b/storage/src/vespa/storage/storageserver/communicationmanager.cpp
index 621f8a2ecc0..4458bf8a92d 100644
--- a/storage/src/vespa/storage/storageserver/communicationmanager.cpp
+++ b/storage/src/vespa/storage/storageserver/communicationmanager.cpp
@@ -396,6 +396,7 @@ void CommunicationManager::configure(std::unique_ptr<CommunicationManagerConfig>
         mbus::DestinationSessionParams dstParams;
         dstParams.setName("default");
         dstParams.setBroadcastName(true);
+        dstParams.defer_registration(true); // Deferred session registration; see rationale below
         dstParams.setMessageHandler(*this);
         _messageBusSession = _mbus->getMessageBus().createDestinationSession(dstParams);
 
@@ -403,6 +404,14 @@ void CommunicationManager::configure(std::unique_ptr<CommunicationManagerConfig>
         srcParams.setThrottlePolicy(mbus::IThrottlePolicy::SP());
         srcParams.setReplyHandler(*this);
         _sourceSession = _mbus->getMessageBus().createSourceSession(srcParams);
+
+        // Creating a DestinationSession that is immediately registered as available for business
+        // means we may theoretically start receiving messages over the session even before the call returns
+        // to the caller. Either way there would be no memory barrier that ensures that _messageBusSession
+        // would be fully visible to the MessageBus threads (since it's written after return).
+        // To avoid this sneaky scenario, defer registration (and thus introduce a barrier) until
+        // _after_ we've initialized our internal member variables.
+        _messageBusSession->register_session_deferred();
     }
 }