Validate password length

We can't do this using the ValidatesBeforeSave trait because the
password has been hashed by then. Instead, we must validate the
original password as it comes in.

src/Core/Users/User.php

@@ -32,6 +32,7 @@ use Flarum\Core\Support\Locked;
 use Flarum\Core\Support\VisibleScope;
 use Flarum\Core\Support\EventGenerator;
 use Flarum\Core\Support\ValidatesBeforeSave;
+use Flarum\Core\Exceptions\ValidationException;
 
 /**
  * @todo document database columns with @property
@@ -149,6 +150,8 @@ class User extends Model
     {
         $user = new static;
 
+        $this->assertValidPassword($password);
+
         $user->username  = $username;
         $user->email     = $email;
         $user->password  = $password;
@@ -225,6 +228,8 @@ class User extends Model
      */
     public function changePassword($password)
     {
+        $this->assertValidPassword($password);
+
         $this->password = $password;
 
         $this->raise(new UserPasswordWasChanged($this));
@@ -232,6 +237,20 @@ class User extends Model
         return $this;
     }
 
+    /**
+     * Validate password input.
+     *
+     * @param string $password
+     * @return void
+     * @throws \Flarum\Core\Exceptions\ValidationException
+     */
+    protected function assertValidPassword($password)
+    {
+        if (strlen($password) < 8) {
+            throw new ValidationException(['password' => 'Password must be at least 8 characters']);
+        }
+    }
+
     /**
      * Set the password attribute, storing it as a hash.
      *