retention: Limit number of ids passed to db in delete messages query.

If do_delete_messages (and friends) are called for a massive number of
messages, the giant list of message ids is passed to Postgres even
though chunk_size makes all but the first chunk_size of message ids
useless.

zerver/lib/retention.py

@@ -124,7 +124,7 @@ def run_archiving(
     query: SQL,
     type: int,
     realm: Realm | None = None,
-    chunk_size: int = MESSAGE_BATCH_SIZE,
+    chunk_size: int | None = MESSAGE_BATCH_SIZE,
     **kwargs: Composable,
 ) -> int:
     # This function is carefully designed to achieve our
@@ -135,6 +135,9 @@ def run_archiving(
     # (such as UserMessage, Reaction, and Attachment) inside the same transaction.atomic() block.
     assert type in (ArchiveTransaction.MANUAL, ArchiveTransaction.RETENTION_POLICY_BASED)
 
+    if chunk_size is not None:
+        kwargs["chunk_size"] = Literal(chunk_size)
+
     message_count = 0
     while True:
         start_time = time.time()
@@ -144,7 +147,6 @@ def run_archiving(
                 Message,
                 query,
                 src_db_table=None,
-                chunk_size=Literal(chunk_size),
                 returning_id=True,
                 archive_transaction_id=Literal(archive_transaction.id),
                 **kwargs,
@@ -169,8 +171,9 @@ def run_archiving(
             )
 
         # We run the loop, until the query returns fewer results than chunk_size,
-        # which means we are done:
-        if len(new_chunk) < chunk_size:
+        # which means we are done; or if we're not chunking, we're done
+        # after one iteration.
+        if chunk_size is None or len(new_chunk) < chunk_size:
             break
 
     return message_count
@@ -458,35 +461,48 @@ def get_realms_and_streams_for_archiving() -> list[tuple[Realm, list[Stream]]]:
 def move_messages_to_archive(
     message_ids: list[int], realm: Realm | None = None, chunk_size: int = MESSAGE_BATCH_SIZE
 ) -> None:
-    # Uses index: zerver_message_pkey
-    query = SQL(
-        """
-    INSERT INTO zerver_archivedmessage ({dst_fields}, archive_transaction_id)
-        SELECT {src_fields}, {archive_transaction_id}
-        FROM zerver_message
-        WHERE zerver_message.id IN {message_ids}
-        LIMIT {chunk_size}
-    ON CONFLICT (id) DO UPDATE SET archive_transaction_id = {archive_transaction_id}
-    RETURNING id
     """
-    )
-    count = run_archiving(
-        query,
-        type=ArchiveTransaction.MANUAL,
-        message_ids=Literal(tuple(message_ids)),
-        realm=realm,
-        chunk_size=chunk_size,
-    )
+    Callers using this to archive a large amount of messages should ideally make sure the message_ids are
+    ordered, as that'll allow better performance here by keeping the batches that'll be sent to the database
+    ordered rather than randomly scattered.
+    """
+    count = 0
+    # In order to avoid sending a massive list of message ids to the database,
+    # we'll handle chunking the list of ids directly here.
+    message_ids_head = message_ids
+    while message_ids_head:
+        message_ids_chunk = message_ids_head[0:chunk_size]
+        message_ids_head = message_ids_head[chunk_size:]
+
+        # Uses index: zerver_message_pkey
+        query = SQL(
+            """
+        INSERT INTO zerver_archivedmessage ({dst_fields}, archive_transaction_id)
+            SELECT {src_fields}, {archive_transaction_id}
+            FROM zerver_message
+            WHERE zerver_message.id IN {message_ids}
+        ON CONFLICT (id) DO UPDATE SET archive_transaction_id = {archive_transaction_id}
+        RETURNING id
+        """
+        )
+
+        count += run_archiving(
+            query,
+            type=ArchiveTransaction.MANUAL,
+            message_ids=Literal(tuple(message_ids_chunk)),
+            realm=realm,
+            chunk_size=None,
+        )
+        # Clean up attachments:
+        archived_attachments = ArchivedAttachment.objects.filter(
+            messages__id__in=message_ids_chunk
+        ).distinct()
+        Attachment.objects.filter(
+            messages__isnull=True, scheduled_messages__isnull=True, id__in=archived_attachments
+        ).delete()
 
     if count == 0:
         raise Message.DoesNotExist
-    # Clean up attachments:
-    archived_attachments = ArchivedAttachment.objects.filter(
-        messages__id__in=message_ids
-    ).distinct()
-    Attachment.objects.filter(
-        messages__isnull=True, scheduled_messages__isnull=True, id__in=archived_attachments
-    ).delete()
 
 
 def restore_messages_from_archive(archive_transaction_id: int) -> list[int]: