fix(BulkOp): run unordered bulk ops in serial

UnorderedBulkWrites that use either retryableWrites or transactions
and require more than one batch have a chance to fail if the batches
arrive at the server out of order. We will now conform to other
drivers by executing batches serially.

Fixes NODE-1934

Author: daprahamian

lib/bulk/unordered.js

@@ -139,19 +139,25 @@ class UnorderedBulkOperation extends BulkOperationBase {
     options = ret.options;
     callback = ret.callback;
 
-    return executeOperation(this.s.topology, executeBatches, [this, options, callback]);
+    return executeOperation(this.s.topology, executeCommands, [this, options, callback]);
   }
 }
 
 /**
- * Execute the command
+ * Execute next write command in a chain
  *
- * @param {UnorderedBulkOperation} bulkOperation
- * @param {object} batch
+ * @param {OrderedBulkOperation} bulkOperation
  * @param {object} options
  * @param {function} callback
  */
-function executeBatch(bulkOperation, batch, options, callback) {
+function executeCommands(bulkOperation, options, callback) {
+  if (bulkOperation.s.batches.length === 0) {
+    return handleCallback(callback, null, new BulkWriteResult(bulkOperation.s.bulkResult));
+  }
+
+  // Ordered execution of the command
+  const batch = bulkOperation.s.batches.shift();
+
   function resultHandler(err, result) {
     // Error is a driver related error not a bulk op error, terminate
     if (((err && err.driver) || (err && err.message)) && !(err instanceof MongoWriteConcernError)) {
@@ -161,54 +167,23 @@ function executeBatch(bulkOperation, batch, options, callback) {
     // If we have and error
     if (err) err.ok = 0;
     if (err instanceof MongoWriteConcernError) {
-      return handleMongoWriteConcernError(batch, bulkOperation.s.bulkResult, false, err, callback);
+      return handleMongoWriteConcernError(batch, bulkOperation.s.bulkResult, true, err, callback);
     }
-    handleCallback(
-      callback,
-      null,
-      mergeBatchResults(false, batch, bulkOperation.s.bulkResult, err, result)
-    );
-  }
 
-  bulkOperation.finalOptionsHandler({ options, batch, resultHandler }, callback);
-}
+    // Merge the results together
+    const writeResult = new BulkWriteResult(bulkOperation.s.bulkResult);
+    const mergeResult = mergeBatchResults(true, batch, bulkOperation.s.bulkResult, err, result);
+    if (mergeResult != null) {
+      return handleCallback(callback, null, writeResult);
+    }
 
-/**
- * Execute all the commands
- *
- * @param {UnorderedBulkOperation} bulkOperation
- * @param {object} options
- * @param {function} callback
- */
-function executeBatches(bulkOperation, options, callback) {
-  let numberOfCommandsToExecute = bulkOperation.s.batches.length;
-  let hasErrored = false;
-  // Execute over all the batches
-  for (let i = 0; i < bulkOperation.s.batches.length; i++) {
-    executeBatch(bulkOperation, bulkOperation.s.batches[i], options, function(err) {
-      if (hasErrored) {
-        return;
-      }
-
-      if (err) {
-        hasErrored = true;
-        return handleCallback(callback, err);
-      }
-      // Count down the number of commands left to execute
-      numberOfCommandsToExecute = numberOfCommandsToExecute - 1;
-
-      // Execute
-      if (numberOfCommandsToExecute === 0) {
-        // Driver level error
-        if (err) return handleCallback(callback, err);
-
-        const writeResult = new BulkWriteResult(bulkOperation.s.bulkResult);
-        if (bulkOperation.handleWriteError(callback, writeResult)) return;
-
-        return handleCallback(callback, null, writeResult);
-      }
-    });
+    if (bulkOperation.handleWriteError(callback, writeResult)) return;
+
+    // Execute the next command in line
+    executeCommands(bulkOperation, options, callback);
   }
+
+  bulkOperation.finalOptionsHandler({ options, batch, resultHandler }, callback);
 }
 
 /**