fix(accounts): the admin's Email OTP sent twice at once makes one device

get_user_totp_device looked for the user's TOTP device and made one if
there was none; two requests at once could both make one, and both
devices' codes then logged in. It now does so under a lock on the user's
row.

accounts/tests/test_utils.py

@@ -82,6 +82,14 @@ class AccountsUtilsTestCase(TestCase):
         device = TOTPDevice.objects.create(user=self.user, confirmed=True)
         self.assertEqual(get_existing_totp_device(self.user).id, device.id)
 
+    def test_the_device_is_made_under_a_lock_on_the_user(self):
+        from django.db.models.query import QuerySet
+
+        with patch.object(QuerySet, "select_for_update", autospec=True, side_effect=QuerySet.select_for_update) as lock:
+            device = get_user_totp_device(self.user)
+        lock.assert_called_once()
+        self.assertEqual(get_user_totp_device(self.user), device)
+
     @override_settings(OTP_ENABLED=True)
     def test_the_mail_links_to_the_qr_code_and_holds_no_secret(self):
         device = get_user_totp_device(self.user)

accounts/utils.py

@@ -4,8 +4,10 @@ import struct
 from decimal import Decimal
 
 import segno
+from django.contrib.auth import get_user_model
 from django.core.mail import send_mail
 from django.core.signing import TimestampSigner
+from django.db import transaction
 from django.urls import reverse
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext as _
@@ -24,13 +26,15 @@ def get_existing_totp_device(user):
 
 
 def get_user_totp_device(user):
-    devices = devices_for_user(user)
-    for device in devices:
-        if isinstance(device, TOTPDevice):
-            return device
-
-    device = user.totpdevice_set.create()
-    return device
+    """The user's TOTP device, made if they have none. The user's row is
+    locked meanwhile: two requests at once (a double click) make one device,
+    not two that both log in."""
+    with transaction.atomic():
+        get_user_model().objects.select_for_update().get(pk=user.pk)
+        for device in devices_for_user(user):
+            if isinstance(device, TOTPDevice):
+                return device
+        return user.totpdevice_set.create()
 
 
 def validate_ssh_key(key):