mqb: fix ClientSession::initiateShutdown crash (bloomberg#349)

Signed-off-by: dorjesinpo <129227380+dorjesinpo@users.noreply.github.com>

src/groups/mqb/mqba/mqba_clientsession.cpp

@@ -2816,16 +2816,41 @@ void ClientSession::initiateShutdown(const ShutdownCb&         callback,
 
     BALL_LOG_INFO << description() << ": initiateShutdown";
 
-    dispatcher()->execute(
-        bdlf::BindUtil::bind(
-            bdlf::MemFnUtil::memFn(&ClientSession::initiateShutdownDispatched,
-                                   d_self.acquire()),
-            callback,
-            timeout),
-        this,
-        mqbi::DispatcherEventType::e_DISPATCHER);
-    // Use 'mqbi::DispatcherEventType::e_DISPATCHER' to avoid (re)enabling
-    // 'd_flushList'
+    // The 'd_self.acquire()' return 'shared_ptr<ClientSession>' but that does
+    // not relate to the 'shared_ptr<mqbnet::Session>' acquired by
+    // 'mqbnet::TransportManagerIterator'.  The latter is bound to the
+    // 'initiateShutdown'.  The former can be null after 'd_self.invalidate()'
+    // call. ('invalidate()' waits for all _acquired_ 'shared_ptr' references
+    // to drop).
+    //
+    // We have a choice, either 1) bind the latter to 'initiateShutdown' to
+    // make sure 'd_self.acquire()' returns not null, or 2) invoke the
+    // 'callback' earlier if fail to 'acquire()' because of 'invalidate()', or
+    // 3) bind _acquired_ 'shared_ptr' to 'initiateShutdown'.
+    //
+    // Choosing 2), assuming that calling the (completion) callback from a
+    // thread other than the *CLIENT* dispatcher thread is ok.  The
+    // 'mwcu::OperationChainLink' expects the completion callback from multiple
+    // sessions anyway.
+
+    bsl::shared_ptr<ClientSession> ptr = d_self.acquire();
+
+    if (!ptr) {
+        callback();
+    }
+    else {
+        dispatcher()->execute(
+            bdlf::BindUtil::bind(
+                bdlf::MemFnUtil::memFn(
+                    &ClientSession::initiateShutdownDispatched,
+                    d_self.acquire()),
+                callback,
+                timeout),
+            this,
+            mqbi::DispatcherEventType::e_DISPATCHER);
+        // Use 'mqbi::DispatcherEventType::e_DISPATCHER' to avoid (re)enabling
+        // 'd_flushList'
+    }
 }
 
 void ClientSession::invalidate()