performance: Optimize get_realm_email_validator.

We now query RealmDomain objects up front.  This
change is minor in most circumstances--it sometimes
saves a round trip to the database; other times,
it actually brings back slightly more data
(optimistically).

The big win will come in a subsequent commit,
where we avoid running these queries in a loop
for every callback.

Note that I'm not sure if we intentionally
omitted checks for emails with "+" in them
for some circumstances, but I just preserved
the behavior.

zerver/models.py

@@ -575,28 +575,69 @@ class EmailContainsPlusError(Exception):
 # So for invite-only realms, this is the test for whether a user can be invited,
 # not whether the user can sign up currently.)
 def email_allowed_for_realm(email: str, realm: Realm) -> None:
+    '''
+    Avoid calling this in a loop!
+    Instead, call get_realm_email_validator()
+    outside of the loop.
+    '''
     get_realm_email_validator(realm)(email)
 
+def validate_disposable(email: str) -> None:
+    if is_disposable_domain(email_to_domain(email)):
+        raise DisposableEmailError
+
 def get_realm_email_validator(realm: Realm) -> Callable[[str], None]:
+    if not realm.emails_restricted_to_domains:
+        # Should we also do '+' check for non-resticted realms?
+        if realm.disallow_disposable_email_addresses:
+            return validate_disposable
+
+        # allow any email through
+        return lambda email: None
+
+    '''
+    RESTRICTIVE REALMS:
+
+    Some realms only allow emails within a set
+    of domains that are configured in RealmDomain.
+
+    We get the set of domains up front so that
+    folks can validate multiple emails without
+    multiple round trips to the database.
+    '''
+
+    query = RealmDomain.objects.filter(realm=realm)
+    rows = list(query.values('allow_subdomains', 'domain'))
+
+    allowed_domains = {
+        r['domain'] for r in rows
+    }
+
+    allowed_subdomains = {
+        r['domain'] for r in rows
+        if r['allow_subdomains']
+    }
+
     def validate(email: str) -> None:
-        if not realm.emails_restricted_to_domains:
-            if realm.disallow_disposable_email_addresses and \
-                    is_disposable_domain(email_to_domain(email)):
-                raise DisposableEmailError
-            return
-        elif '+' in email_to_username(email):
+        '''
+        We don't have to do a "disposable" check for restricted
+        domains, since the realm is already giving us
+        a small whitelist.
+        '''
+
+        if '+' in email_to_username(email):
             raise EmailContainsPlusError
 
         domain = email_to_domain(email)
-        query = RealmDomain.objects.filter(realm=realm)
-        if query.filter(domain=domain).exists():
+
+        if domain in allowed_domains:
             return
-        else:
-            query = query.filter(allow_subdomains=True)
-            while len(domain) > 0:
-                subdomain, sep, domain = domain.partition('.')
-                if query.filter(domain=domain).exists():
-                    return
+
+        while len(domain) > 0:
+            subdomain, sep, domain = domain.partition('.')
+            if domain in allowed_subdomains:
+                return
+
         raise DomainNotAllowedForRealmError
 
     return validate