If TWISTED_REACTOR is None, reuse any pre-installed reactor

docs/topics/settings.rst

@@ -1638,9 +1638,10 @@ which raises :exc:`Exception`, becomes::
 
 
 The default value of the :setting:`TWISTED_REACTOR` setting is ``None``, which
-means that Scrapy will install the default reactor defined by Twisted for the
-current platform. This is to maintain backward compatibility and avoid possible
-problems caused by using a non-default reactor.
+means that Scrapy will use the existing reactor if one is already installed, or
+install the default reactor defined by Twisted for the current platform. This
+is to maintain backward compatibility and avoid possible problems caused by
+using a non-default reactor.
 
 For additional information, see :doc:`core/howto/choosing-reactor`.
 

scrapy/crawler.py

@@ -78,8 +78,7 @@ def __init__(self, spidercls, settings=None, init_reactor: bool = False):
             if reactor_class:
                 install_reactor(reactor_class, self.settings["ASYNCIO_EVENT_LOOP"])
             else:
-                from twisted.internet import default
-                default.install()
+                from twisted.internet import reactor
             log_reactor_info()
         if reactor_class:
             verify_installed_reactor(reactor_class)

tests/CrawlerProcess/reactor_default.py

@@ -0,0 +1,17 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+from twisted.internet import reactor
+
+
+class NoRequestsSpider(scrapy.Spider):
+    name = 'no_request'
+
+    def start_requests(self):
+        return []
+
+
+process = CrawlerProcess(settings={})
+
+process.crawl(NoRequestsSpider)
+process.start()
+

tests/CrawlerProcess/reactor_default_twisted_reactor_select.py

@@ -0,0 +1,20 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+from twisted.internet import reactor
+
+
+class NoRequestsSpider(scrapy.Spider):
+    name = 'no_request'
+
+    def start_requests(self):
+        return []
+
+
+process = CrawlerProcess(settings={
+    "TWISTED_REACTOR": "twisted.internet.selectreactor.SelectReactor",
+})
+
+process.crawl(NoRequestsSpider)
+process.start()
+
+

tests/CrawlerProcess/reactor_select.py

@@ -0,0 +1,19 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+from twisted.internet import selectreactor
+selectreactor.install()
+
+
+class NoRequestsSpider(scrapy.Spider):
+    name = 'no_request'
+
+    def start_requests(self):
+        return []
+
+
+process = CrawlerProcess(settings={})
+
+process.crawl(NoRequestsSpider)
+process.start()
+
+

tests/CrawlerProcess/reactor_select_twisted_reactor_select.py

@@ -0,0 +1,22 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+from twisted.internet import selectreactor
+selectreactor.install()
+
+
+class NoRequestsSpider(scrapy.Spider):
+    name = 'no_request'
+
+    def start_requests(self):
+        return []
+
+
+process = CrawlerProcess(settings={
+    "TWISTED_REACTOR": "twisted.internet.selectreactor.SelectReactor",
+})
+
+process.crawl(NoRequestsSpider)
+process.start()
+
+
+

tests/test_crawler.py

@@ -308,6 +308,33 @@ def test_multi(self):
         self.assertNotIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
         self.assertNotIn("ReactorAlreadyInstalledError", log)
 
+    def test_reactor_default(self):
+        log = self.run_script('reactor_default.py')
+        self.assertIn('Spider closed (finished)', log)
+        self.assertNotIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
+        self.assertNotIn("ReactorAlreadyInstalledError", log)
+
+    def test_reactor_default_twisted_reactor_select(self):
+        log = self.run_script('reactor_default_twisted_reactor_select.py')
+        self.assertNotIn('Spider closed (finished)', log)
+        self.assertIn(
+            (
+                "does not match the requested one "
+                "(twisted.internet.selectreactor.SelectReactor)"
+            ),
+            log,
+        )
+
+    def test_reactor_select(self):
+        log = self.run_script('reactor_select.py')
+        self.assertIn('Spider closed (finished)', log)
+        self.assertNotIn("ReactorAlreadyInstalledError", log)
+
+    def test_reactor_select_twisted_reactor_select(self):
+        log = self.run_script('reactor_select_twisted_reactor_select.py')
+        self.assertIn('Spider closed (finished)', log)
+        self.assertNotIn("ReactorAlreadyInstalledError", log)
+
     def test_asyncio_enabled_no_reactor(self):
         log = self.run_script('asyncio_enabled_no_reactor.py')
         self.assertIn('Spider closed (finished)', log)
@@ -340,33 +367,33 @@ def test_caching_hostname_resolver_finite_execution(self):
             self.assertNotIn("TimeoutError", log)
             self.assertNotIn("twisted.internet.error.DNSLookupError", log)
 
-    def test_reactor_select(self):
+    def test_twisted_reactor_select(self):
         log = self.run_script("twisted_reactor_select.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.selectreactor.SelectReactor", log)
 
     @mark.skipif(platform.system() == 'Windows', reason="PollReactor is not supported on Windows")
-    def test_reactor_poll(self):
+    def test_twisted_reactor_poll(self):
         log = self.run_script("twisted_reactor_poll.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.pollreactor.PollReactor", log)
 
-    def test_reactor_asyncio(self):
+    def test_twisted_reactor_asyncio(self):
         log = self.run_script("twisted_reactor_asyncio.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
-    def test_reactor_asyncio_custom_settings(self):
+    def test_twisted_reactor_asyncio_custom_settings(self):
         log = self.run_script("twisted_reactor_custom_settings.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
-    def test_reactor_asyncio_custom_settings_same(self):
+    def test_twisted_reactor_asyncio_custom_settings_same(self):
         log = self.run_script("twisted_reactor_custom_settings_same.py")
         self.assertIn("Spider closed (finished)", log)
         self.assertIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
-    def test_reactor_asyncio_custom_settings_conflict(self):
+    def test_twisted_reactor_asyncio_custom_settings_conflict(self):
         log = self.run_script("twisted_reactor_custom_settings_conflict.py")
         self.assertIn("Using reactor: twisted.internet.selectreactor.SelectReactor", log)
         self.assertIn("(twisted.internet.selectreactor.SelectReactor) does not match the requested one", log)