zilencer: Avoid repeated COUNT(*) queries.

Because Django does not support returning the inserted row-ids with a
`bulk_create(..., ignore_conflicts=True)`, we previously counted the
total rows before and after insertion.  This is rather inefficient,
and can lead to database contention when many servers are reporting
statistics at once.

Switch to reaching into the private `_insert` method, which does
support what we need.  While relying a private method is poor form, it
is mildly preferable to attempting to re-implement all of the
complexities of it.

zilencer/views.py

@@ -11,6 +11,7 @@ from django.core.exceptions import ValidationError
 from django.core.validators import URLValidator, validate_email
 from django.db import IntegrityError, transaction
 from django.db.models import Model
+from django.db.models.constants import OnConflict
 from django.http import HttpRequest, HttpResponse
 from django.utils.crypto import constant_time_compare
 from django.utils.timezone import now as timezone_now
@@ -796,20 +797,28 @@ def batch_create_table_data(
     row_objects: list[ModelT],
 ) -> None:
     # We ignore previously-existing data, in case it was truncated and
-    # re-created on the remote server.  `ignore_conflicts=True`
-    # cannot return the ids, or count thereof, of the new inserts,
-    # (see https://code.djangoproject.com/ticket/0138) so we rely on
-    # having a lock to accurately count them before and after.  This
-    # query is also well-indexed.
-    before_count = model._default_manager.filter(server=server).count()
-    model._default_manager.bulk_create(row_objects, batch_size=1000, ignore_conflicts=True)
-    after_count = model._default_manager.filter(server=server).count()
-    inserted_count = after_count - before_count
-    if inserted_count < len(row_objects):
+    # re-created on the remote server.  Because the existing
+    # `bulk_create(..., ignore_conflicts=True)` cannot yet return the
+    # ids, or count thereof, of the new inserts, (see
+    # https://code.djangoproject.com/ticket/30138), we reach in and
+    # call _insert with `returning_fields` in batches ourselves.
+    inserted_count = 0
+    expected_count = len(row_objects)
+    fields = [f for f in model._meta.fields if f.concrete and not f.generated and f.name != "id"]
+    while row_objects:
+        to_insert, row_objects = row_objects[:1000], row_objects[1000:]
+        result = model._default_manager._insert(  # type:ignore[attr-defined]  # This is a private method
+            to_insert,
+            fields=fields,
+            returning_fields=[model._meta.get_field("id")],
+            on_conflict=OnConflict.IGNORE,
+        )
+        inserted_count += len(result)
+    if inserted_count < expected_count:
         logging.warning(
             "Dropped %d duplicated rows while saving %d rows of %s for server %s/%s",
-            len(row_objects) - inserted_count,
-            len(row_objects),
+            expected_count - inserted_count,
+            expected_count,
             model._meta.db_table,
             server.hostname,
             server.uuid,