email digests: Introduce bulk methods for digest.

Note that we are not changing anything semantically
or algorithmically yet.  The only overhead here
for the single-user case is boxing and unboxing
data into single-item dicts and lists.

The interfaces for callers in the view and the
queue processor remain the same for now.

zerver/lib/digest.py

@@ -218,10 +218,13 @@ def gather_new_streams(user_profile: UserProfile,
 def enough_traffic(hot_conversations: str, new_streams: int) -> bool:
     return bool(hot_conversations or new_streams)
 
-def get_digest_context(user: UserProfile, cutoff: float) -> Dict[str, Any]:
+def bulk_get_digest_context(users: List[UserProfile], cutoff: float) -> Dict[int, Dict[str, Any]]:
     # Convert from epoch seconds to a datetime object.
     cutoff_date = datetime.datetime.fromtimestamp(int(cutoff), tz=datetime.timezone.utc)
 
+    result: Dict[int, Dict[str, Any]] = {}
+
+    for user in users:
         context = common_context(user)
 
         # Start building email template data.
@@ -251,11 +254,19 @@ def get_digest_context(user: UserProfile, cutoff: float) -> Dict[str, Any]:
         context["new_streams"] = new_streams
         context["new_streams_count"] = new_streams_count
 
-    return context
+        result[user.id] = context
 
-def handle_digest_email(user_id: int, cutoff: float) -> None:
-    user = get_user_profile_by_id(user_id)
-    context = get_digest_context(user, cutoff)
+    return result
+
+def get_digest_context(user: UserProfile, cutoff: float) -> Dict[str, Any]:
+    return bulk_get_digest_context([user], cutoff)[user.id]
+
+def bulk_handle_digest_email(user_ids: List[int], cutoff: float) -> None:
+    users = [get_user_profile_by_id(user_id) for user_id in user_ids]
+    context_map = bulk_get_digest_context(users, cutoff)
+
+    for user in users:
+        context = context_map[user.id]
 
         # We don't want to send emails containing almost no information.
         if enough_traffic(context["hot_conversations"], context["new_streams_count"]):
@@ -270,6 +281,8 @@ def handle_digest_email(user_id: int, cutoff: float) -> None:
                 context=context,
             )
 
+def handle_digest_email(user_id: int, cutoff: float) -> None:
+    bulk_handle_digest_email([user_id], cutoff)
 
 def exclude_subscription_modified_streams(user_profile: UserProfile,
                                           stream_ids: List[int],

zerver/tests/test_digest.py

@@ -9,6 +9,7 @@ from django.utils.timezone import now as timezone_now
 from confirmation.models import one_click_unsubscribe_link
 from zerver.lib.actions import do_create_user
 from zerver.lib.digest import (
+    bulk_handle_digest_email,
     enqueue_emails,
     exclude_subscription_modified_streams,
     gather_new_streams,
@@ -168,17 +169,14 @@ class TestDigestEmailMessages(ZulipTestCase):
         one_click_unsubscribe_link(digest_users[0], 'digest')
 
         with mock.patch('zerver.lib.digest.send_future_email') as mock_send_future_email:
-            keep_cache_warm = False
+            digest_user_ids = [user.id for user in digest_users]
 
-            for digest_user in digest_users:
-                with queries_captured(keep_cache_warm=keep_cache_warm) as queries:
+            with queries_captured() as queries:
                 with cache_tries_captured() as cache_tries:
-                        handle_digest_email(digest_user.id, cutoff)
+                    bulk_handle_digest_email(digest_user_ids, cutoff)
 
-                keep_cache_warm = True
-
-                self.assert_length(queries, 10)
-                self.assert_length(cache_tries, 1)
+            self.assert_length(queries, 40)
+            self.assert_length(cache_tries, 4)
 
         self.assertEqual(mock_send_future_email.call_count, len(digest_users))