Restore and deprecate the spider arg of Downloader._get_slot(). (#7178)

scrapy/core/downloader/__init__.py

@@ -151,7 +151,10 @@ class Downloader:
     def needs_backout(self) -> bool:
         return len(self.active) >= self.total_concurrency
 
-    def _get_slot(self, request: Request) -> tuple[str, Slot]:
+    @_warn_spider_arg
+    def _get_slot(
+        self, request: Request, spider: Spider | None = None
+    ) -> tuple[str, Slot]:
         key = self.get_slot_key(request)
         if key not in self.slots:
             assert self.crawler.spider

tests/test_downloaderslotssettings.py

@@ -7,6 +7,7 @@ from twisted.internet.defer import inlineCallbacks
 from scrapy import Request
 from scrapy.core.downloader import Downloader, Slot
 from scrapy.crawler import CrawlerRunner
+from scrapy.exceptions import ScrapyDeprecationWarning
 from scrapy.utils.defer import deferred_f_from_coro_f, maybe_deferred_to_future
 from scrapy.utils.spider import DefaultSpider
 from scrapy.utils.test import get_crawler
@@ -108,6 +109,24 @@ def test_params():
         )
 
 
+def test_get_slot_deprecated_spider_arg():
+    crawler = get_crawler(DefaultSpider)
+    crawler.spider = crawler._create_spider()
+    downloader = Downloader(crawler)
+    downloader._slot_gc_loop.stop()  # Prevent an unclean reactor.
+    request = Request("https://example.com")
+
+    with pytest.warns(
+        ScrapyDeprecationWarning,
+        match=r"Passing a 'spider' argument to Downloader\._get_slot\(\) is deprecated",
+    ):
+        key1, slot1 = downloader._get_slot(request, spider=crawler.spider)
+    key2, slot2 = downloader._get_slot(request)
+
+    assert key1 == key2
+    assert slot1 == slot2
+
+
 @pytest.mark.parametrize(
     "priority_queue_class",
     [