fix(txns): omit writeConcern when in a transaction

Author: mbroadst

lib/utils.js

@@ -455,6 +455,15 @@ function applyWriteConcern(target, sources, options) {
   const db = sources.db;
   const coll = sources.collection;
 
+  if (options.session && options.session.inTransaction()) {
+    // writeConcern is not allowed within a multi-statement transaction
+    if (target.writeConcern) {
+      delete target.writeConcern;
+    }
+
+    return target;
+  }
+
   // NOTE: there is probably a much better place for this
   if (db && db.s.options.retryWrites) {
     target.retryWrites = true;