fix: Server crashes on startup with cryptic error when a Mongo _SCHEMA field has a non-string type

spec/MongoSchemaCollectionAdapter.spec.js

@@ -2,6 +2,7 @@
 
 const MongoSchemaCollection = require('../lib/Adapters/Storage/Mongo/MongoSchemaCollection')
   .default;
+const Parse = require('parse/node');
 
 describe('MongoSchemaCollection', () => {
   it('can transform legacy _client_permissions keys to parse format', done => {
@@ -96,4 +97,28 @@ describe('MongoSchemaCollection', () => {
     });
     done();
   });
+
+  it('throws a clear error for a null field type instead of crashing (#9847)', () => {
+    expect(() =>
+      MongoSchemaCollection._TESTmongoSchemaToParseSchema({
+        _id: 'SomeClass',
+        goodField: 'string',
+        badField: null,
+      })
+    ).toThrowMatching(
+      e =>
+        e.code === Parse.Error.INCORRECT_TYPE &&
+        e.message.includes('SomeClass') &&
+        e.message.includes('badField')
+    );
+  });
+
+  it('throws a clear error for a non-string object field type (#9847)', () => {
+    expect(() =>
+      MongoSchemaCollection._TESTmongoSchemaToParseSchema({
+        _id: 'SomeClass',
+        badField: { foo: 1 },
+      })
+    ).toThrowMatching(e => e.code === Parse.Error.INCORRECT_TYPE);
+  });
 });

src/Adapters/Storage/Mongo/MongoSchemaCollection.js

@@ -1,7 +1,14 @@
 import MongoCollection from './MongoCollection';
 import Parse from 'parse/node';
 
-function mongoFieldToParseSchemaField(type) {
+function mongoFieldToParseSchemaField(type, fieldName, className) {
+  if (typeof type !== 'string') {
+    throw new Parse.Error(
+      Parse.Error.INCORRECT_TYPE,
+      `Invalid schema for class '${className}': field '${fieldName}' has an invalid type ` +
+        `(${type === null ? 'null' : typeof type}). Expected a string type descriptor.`
+    );
+  }
   if (type[0] === '*') {
     return {
       type: 'Pointer',
@@ -43,7 +50,7 @@ const nonFieldSchemaKeys = ['_id', '_metadata', '_client_permissions'];
 function mongoSchemaFieldsToParseSchemaFields(schema) {
   var fieldNames = Object.keys(schema).filter(key => nonFieldSchemaKeys.indexOf(key) === -1);
   var response = fieldNames.reduce((obj, fieldName) => {
-    obj[fieldName] = mongoFieldToParseSchemaField(schema[fieldName]);
+    obj[fieldName] = mongoFieldToParseSchemaField(schema[fieldName], fieldName, schema._id);
     if (
       schema._metadata &&
       schema._metadata.fields_options &&