antispam: Make a setting for default Realm.max_invites.

This makes this value much easier for a server admin to change than it
was when embedded directly in the code.  (Note this entire mechanism
already only applies on a server open for anyone to create a realm.)

Doing this also means getting the default out of the database.
Instead, we make the column nullable, and when it's NULL in the
database, treat that as whatever the current default is.  This better
matches anyway the likely model where there are a few realms with
specially-set values, and everything else should be treated uniformly.

The migration contains a `RenameField` step, which sounds scary
operationally -- but it really does mean just the *field*, in
the model within the Python code.  The underlying column's name
doesn't change.

zerver/migrations/0148_max_invites_forget_default.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2018-02-10 02:59
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0147_realm_disallow_disposable_email_addresses'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='realm',
+            name='max_invites',
+            field=models.IntegerField(null=True, db_column='max_invites'),
+        ),
+        migrations.RenameField(
+            model_name='realm',
+            old_name='max_invites',
+            new_name='_max_invites',
+        ),
+    ]

zerver/models.py

@@ -185,8 +185,7 @@ class Realm(models.Model):
     authentication_methods = BitField(flags=AUTHENTICATION_FLAGS,
                                       default=2**31 - 1)  # type: BitHandler
     waiting_period_threshold = models.PositiveIntegerField(default=0)  # type: int
-    DEFAULT_MAX_INVITES = 100
-    max_invites = models.IntegerField(default=DEFAULT_MAX_INVITES)  # type: int
+    _max_invites = models.IntegerField(null=True, db_column='max_invites')  # type: int
     message_visibility_limit = models.IntegerField(null=True)  # type: int
     # See upload_quota_bytes; don't interpret upload_quota_gb directly.
     upload_quota_gb = models.IntegerField(null=True)  # type: Optional[int]
@@ -285,6 +284,16 @@ class Realm(models.Model):
             return self.signup_notifications_stream
         return None
 
+    @property
+    def max_invites(self) -> int:
+        if self._max_invites is None:
+            return settings.INVITES_DEFAULT_REALM_DAILY_MAX
+        return self._max_invites
+
+    @max_invites.setter
+    def max_invites(self, value: int) -> None:
+        self._max_invites = value
+
     def upload_quota_bytes(self) -> Optional[int]:
         if self.upload_quota_gb is None:
             return None

zerver/tests/test_signup.py

@@ -647,6 +647,15 @@ earl-test@zulip.com""", ["Denmark"]))
         self.check_sent_emails(["bob-test@zulip.com", "carol-test@zulip.com",
                                 "dave-test@zulip.com", "earl-test@zulip.com"])
 
+    def test_max_invites_model(self) -> None:
+        realm = get_realm("zulip")
+        self.assertEqual(realm.max_invites, settings.INVITES_DEFAULT_REALM_DAILY_MAX)
+        realm.max_invites = 3
+        realm.save()
+        self.assertEqual(get_realm("zulip").max_invites, 3)
+        realm.max_invites = settings.INVITES_DEFAULT_REALM_DAILY_MAX
+        realm.save()
+
     def test_invite_too_many_users(self) -> None:
         # Only a light test of this pathway; e.g. doesn't test that
         # the limit gets reset after 24 hours

zproject/settings.py

@@ -306,6 +306,9 @@ DEFAULT_SETTINGS.update({
     #
     # A non-admin user who's joined an open realm this recently can't invite at all.
     'INVITES_MIN_USER_AGE_DAYS': 3,
+    # Default for a realm's `max_invites`; which applies per day,
+    # and only applies if OPEN_REALM_CREATION is true.
+    'INVITES_DEFAULT_REALM_DAILY_MAX': 100,
 
     # Controls for which links are published in portico footers/headers/etc.
     'EMAIL_DELIVERER_DISABLED': False,