Previously robots.txt was fetched lazily on the first request per
domain, causing early concurrent requests to each stall waiting for
the same network fetch. The cache is now warmed before the crawl loop
starts, making all subsequent robots.txt lookups a local read.
- RobotsTxtManager gains a prefetch(urls, sid) method that fetches all domains concurrently via a task group
- CrawlerEngine._prefetch_robots_txt() is called after on_start():
uses allowed_domains if configured, otherwise falls back to unique
domains extracted from start_urls
- Mid-crawl domain discovery (not covered by prefetch) still fetches
lazily; two concurrent callbacks on the same new domain can each
trigger a fetch — accepted tradeoff, documented in _get_domain_delay
Files: scrapling/spiders/robotstxt.py, scrapling/spiders/engine.py, tests/spiders/test_engine.py
Karim shoair