performance: Add get_users_by_delivery_email().

The main purpose of this new function is to allow
us to validate emails in bulk, which we don't do
yet (still setting the stage for that).

This is still a speedup, though, since in our
caller we grab only three fields now.

And other than that, we're essentially doing
the same query for the single-email case, just
outside the loop.

zerver/lib/email_validation.py

@@ -9,14 +9,13 @@ from zerver.lib.name_restrictions import is_disposable_domain
 from zerver.models import (
     email_to_username,
     email_to_domain,
-    get_user_by_delivery_email,
+    get_users_by_delivery_email,
     is_cross_realm_bot_email,
     DisposableEmailError,
     DomainNotAllowedForRealmError,
     EmailContainsPlusError,
     Realm,
     RealmDomain,
-    UserProfile,
 )
 
 def validate_disposable(email: str) -> None:
@@ -126,8 +125,25 @@ def get_existing_user_errors(
     already exist.  There's a bit of fiddly logic related
     to cross-realm bots and mirror dummies too.
     '''
+
     errors = {}  # type: Dict[str, Tuple[str, Optional[str], bool]]
 
+    users = get_users_by_delivery_email(emails, target_realm).only(
+        'email',
+        'is_active',
+        'is_mirror_dummy',
+    )
+
+    '''
+    A note on casing: We will preserve the casing used by
+    the user for email in most of this code.  The only
+    exception is when we do existence checks against
+    the `user_dict` dictionary.  (We don't allow two
+    users in the same realm to have the same effective
+    delivery email.)
+    '''
+    user_dict = {user.email.lower(): user for user in users}
+
     def process_email(email: str) -> None:
         if is_cross_realm_bot_email(email):
             msg = email_reserved_for_system_bots_error(email)
@@ -136,9 +152,9 @@ def get_existing_user_errors(
             errors[email] = (msg, code, deactivated)
             return
 
-        try:
-            existing_user_profile = get_user_by_delivery_email(email, target_realm)
-        except UserProfile.DoesNotExist:
+        existing_user_profile = user_dict.get(email.lower())
+
+        if existing_user_profile is None:
             # HAPPY PATH!  Most people invite users that don't exist yet.
             return
 

zerver/models.py

@@ -3,7 +3,7 @@ from typing import Any, DefaultDict, Dict, List, Set, Tuple, TypeVar, \
 
 from django.db import models
 from django.db.models.query import QuerySet
-from django.db.models import Manager, Sum, CASCADE
+from django.db.models import Manager, Q, Sum, CASCADE
 from django.conf import settings
 from django.contrib.auth.models import AbstractBaseUser, UserManager, \
     PermissionsMixin
@@ -2144,6 +2144,28 @@ def get_user_by_delivery_email(email: str, realm: Realm) -> UserProfile:
     return UserProfile.objects.select_related().get(
         delivery_email__iexact=email.strip(), realm=realm)
 
+def get_users_by_delivery_email(emails: Set[str], realm: Realm) -> QuerySet:
+    """This is similar to get_users_by_delivery_email, and
+    it has the same security caveats.  It gets multiple
+    users and returns a QuerySet, since most callers
+    will only need two or three fields.
+
+    If you are using this to get large UserProfile objects, you are
+    probably making a mistake, but if you must,
+    then use `select_related`.
+    """
+
+    '''
+    Django doesn't support delivery_email__iexact__in, so
+    we simply OR all the filters that we'd do for the
+    one-email case.
+    '''
+    email_filter = Q()
+    for email in emails:
+        email_filter |= Q(delivery_email__iexact=email.strip())
+
+    return UserProfile.objects.filter(realm=realm).filter(email_filter)
+
 @cache_with_key(user_profile_cache_key, timeout=3600*24*7)
 def get_user(email: str, realm: Realm) -> UserProfile:
     """Fetches the user by its visible-to-other users username (in the