fix: always include writeErrors on a BulkWriteError instance

We special case situations where there is only one error returned
from a bulk operation, but that can be quite confusing for users of
the bulk API. Promoting the single error to the top-level error
message is reasonable, but in all cases we should use a consistent
shape, and return `writeErrors` as a field on the object

NODE-2625

lib/bulk/common.js

@@ -1200,19 +1200,10 @@ class BulkOperationBase {
    * @ignore
    * @param {function} callback
    * @param {BulkWriteResult} writeResult
-   * @param {class} self either OrderedBulkOperation or UnorderdBulkOperation
+   * @param {class} self either OrderedBulkOperation or UnorderedBulkOperation
    */
   handleWriteError(callback, writeResult) {
     if (this.s.bulkResult.writeErrors.length > 0) {
-      if (this.s.bulkResult.writeErrors.length === 1) {
-        handleCallback(
-          callback,
-          new BulkWriteError(toError(this.s.bulkResult.writeErrors[0]), writeResult),
-          null
-        );
-        return true;
-      }
-
       const msg = this.s.bulkResult.writeErrors[0].errmsg
         ? this.s.bulkResult.writeErrors[0].errmsg
         : 'write operation failed';
@@ -1230,7 +1221,9 @@ class BulkOperationBase {
         null
       );
       return true;
-    } else if (writeResult.getWriteConcernError()) {
+    }
+
+    if (writeResult.getWriteConcernError()) {
       handleCallback(
         callback,
         new BulkWriteError(toError(writeResult.getWriteConcernError()), writeResult),

lib/core/error.js

@@ -19,6 +19,10 @@ class MongoError extends Error {
       } else {
         super(message.message || message.errmsg || message.$err || 'n/a');
         for (var name in message) {
+          if (name === 'errmsg') {
+            continue;
+          }
+
           this[name] = message[name];
         }
       }
@@ -29,6 +33,13 @@ class MongoError extends Error {
     this.name = 'MongoError';
   }
 
+  /**
+   * Legacy name for server error responses
+   */
+  get errmsg() {
+    return this.message;
+  }
+
   /**
    * Creates a new MongoError object
    *

test/functional/bulk.test.js

@@ -4,6 +4,7 @@ const test = require('./shared').assert,
   expect = require('chai').expect;
 
 const MongoError = require('../../index').MongoError;
+const ignoreNsNotFound = require('./shared').ignoreNsNotFound;
 
 describe('Bulk', function() {
   before(function() {
@@ -1616,16 +1617,41 @@ describe('Bulk', function() {
       .then(() => client.close());
   });
 
+  it('should promote a single error to the top-level message, and preserve writeErrors', function() {
+    const client = this.configuration.newClient();
+    return client.connect().then(() => {
+      this.defer(() => client.close());
+
+      const coll = client.db().collection('single_bulk_write_error');
+      return coll
+        .drop()
+        .catch(ignoreNsNotFound)
+        .then(() => coll.insert(Array.from({ length: 4 }, (_, i) => ({ _id: i, a: i }))))
+        .then(() =>
+          coll.bulkWrite([{ insertOne: { _id: 5, a: 0 } }, { insertOne: { _id: 5, a: 0 } }])
+        )
+        .then(
+          () => {
+            throw new Error('expected a bulk error');
+          },
+          err => {
+            expect(err)
+              .property('message')
+              .to.match(/E11000/);
+            expect(err)
+              .to.have.property('writeErrors')
+              .with.length(1);
+          }
+        );
+    });
+  });
+
   it('should preserve order of operation index in unordered bulkWrite', function() {
     const client = this.configuration.newClient();
     return client.connect().then(() => {
       this.defer(() => client.close());
 
       const coll = client.db().collection('bulk_write_ordering_test');
-      function ignoreNsNotFound(err) {
-        if (!err.message.match(/ns not found/)) throw err;
-      }
-
       return coll
         .drop()
         .catch(ignoreNsNotFound)
@@ -1667,10 +1693,6 @@ describe('Bulk', function() {
       this.defer(() => client.close());
 
       const coll = client.db().collection('unordered_preserve_order');
-      function ignoreNsNotFound(err) {
-        if (!err.message.match(/ns not found/)) throw err;
-      }
-
       return coll
         .drop()
         .catch(ignoreNsNotFound)
@@ -1704,10 +1726,6 @@ describe('Bulk', function() {
       this.defer(() => client.close());
 
       const coll = client.db().collection('bulk_op_ordering_test');
-      function ignoreNsNotFound(err) {
-        if (!err.message.match(/ns not found/)) throw err;
-      }
-
       return coll
         .drop()
         .catch(ignoreNsNotFound)