Adds ability to login with email when provided as username (#4420)

src/Routers/UsersRouter.js

@@ -84,13 +84,26 @@ export class UsersRouter extends ClassesRouter {
 
     let user;
     let isValidPassword = false;
-    const query = Object.assign({}, username ? { username } : {}, email ? { email } : {});
+    let query;
+    if (email && username) {
+      query = { email, username };
+    } else if (email) {
+      query = { email };
+    } else {
+      query = { $or: [{ username } , { email: username }] };
+    }
     return req.config.database.find('_User', query)
       .then((results) => {
         if (!results.length) {
           throw new Parse.Error(Parse.Error.OBJECT_NOT_FOUND, 'Invalid username/password.');
         }
-        user = results[0];
+
+        if (results.length > 1) { // corner case where user1 has username == user2 email
+          req.config.loggerController.warn('There is a user which email is the same as another user\'s username, logging in based on username');
+          user = results.filter((user) =>  user.username === username)[0];
+        } else {
+          user = results[0];
+        }
 
         if (req.config.verifyUserEmails && req.config.preventLoginWithUnverifiedEmail && !user.emailVerified) {
           throw new Parse.Error(Parse.Error.EMAIL_NOT_FOUND, 'User email is not verified.');