[UnboundedBlockingMpscQueue] Fix message loss

Avoid messages getting lost in the queue.

emper/UnboundedBlockingMpscQueue.hpp

@@ -63,9 +63,14 @@ class UnboundedBlockingMpscQueue : public Blockable<LogSubsystem::U_B_MPSC_Q> {
 		if (!tPopped) {
 			Context* context = Context::getCurrentContext();
 			block([this, context, postRetrieve] {
+				tryToGetElement(postRetrieve);
+
+				// N.B. It is important that we set blockedContext *after* we
+				// called tryToGetElement(), as otherwise, multiple threads
+				// may call tryToGetElement() which results in messages put in
+				// the queue getting lost.
 				blockedContext = context;
 
-				tryToGetElement(postRetrieve);
 				if (tPopped) {
 					tryToWakeupBlockedContext();
 				}