feat: add notification deduplication trigger (#14172)

coderd/database/migrations/000245_notifications_dedupe.down.sql

@@ -0,0 +1,4 @@
+DROP TRIGGER IF EXISTS update_notification_message_dedupe_hash ON notification_messages;
+DROP FUNCTION IF EXISTS compute_notification_message_dedupe_hash();
+ALTER TABLE IF EXISTS notification_messages
+    DROP COLUMN IF EXISTS dedupe_hash;

coderd/database/migrations/000245_notifications_dedupe.up.sql

@@ -0,0 +1,33 @@
+-- Add a column to store the hash.
+ALTER TABLE IF EXISTS notification_messages
+    ADD COLUMN IF NOT EXISTS dedupe_hash TEXT NULL;
+
+COMMENT ON COLUMN notification_messages.dedupe_hash IS 'Auto-generated by insert/update trigger, used to prevent duplicate notifications from being enqueued on the same day';
+
+-- Ensure that multiple notifications with identical hashes cannot be inserted into the table.
+CREATE UNIQUE INDEX ON notification_messages (dedupe_hash);
+
+-- Computes a hash from all unique messages fields and the current day; this will help prevent duplicate messages from being sent within the same day.
+-- It is possible that a message could be sent at 23:59:59 and again at 00:00:00, but this should be good enough for now.
+-- This could have been a unique index, but we cannot immutably create an index on a timestamp with a timezone.
+CREATE OR REPLACE FUNCTION compute_notification_message_dedupe_hash() RETURNS TRIGGER AS
+$$
+BEGIN
+    NEW.dedupe_hash := MD5(CONCAT_WS(':',
+                                     NEW.notification_template_id,
+                                     NEW.user_id,
+                                     NEW.method,
+                                     NEW.payload::text,
+                                     ARRAY_TO_STRING(NEW.targets, ','),
+                                     DATE_TRUNC('day', NEW.created_at AT TIME ZONE 'UTC')::text
+                           ));
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+COMMENT ON FUNCTION compute_notification_message_dedupe_hash IS 'Computes a unique hash which will be used to prevent duplicate messages from being enqueued on the same day';
+CREATE TRIGGER update_notification_message_dedupe_hash
+    BEFORE INSERT OR UPDATE
+    ON notification_messages
+    FOR EACH ROW
+EXECUTE FUNCTION compute_notification_message_dedupe_hash();