Don't install the reactor in is_asyncio_reactor_installed(). (#6732)

docs/topics/asyncio.rst

@@ -116,6 +116,8 @@ example:
                     f"of Scrapy for more information."
                 )
 
+.. autofunction:: scrapy.utils.reactor.is_asyncio_reactor_installed
+
 
 .. _asyncio-windows:
 

scrapy/utils/reactor.py

@@ -175,7 +175,20 @@ def verify_installed_asyncio_event_loop(loop_path: str) -> None:
     )
 
 
+def is_reactor_installed() -> bool:
+    return "twisted.internet.reactor" in sys.modules
+
+
 def is_asyncio_reactor_installed() -> bool:
+    """Check whether the installed reactor is :class:`~twisted.internet.asyncioreactor.AsyncioSelectorReactor`.
+
+    Raise a :exc:`RuntimeError` if no reactor is installed.
+    """
+    if not is_reactor_installed():
+        raise RuntimeError(
+            "is_asyncio_reactor_installed() called without an installed reactor."
+        )
+
     from twisted.internet import reactor
 
     return isinstance(reactor, asyncioreactor.AsyncioSelectorReactor)

tests/CrawlerProcess/asyncio_enabled_no_reactor.py

@@ -1,5 +1,12 @@
 import scrapy
 from scrapy.crawler import CrawlerProcess
+from scrapy.utils.reactor import is_asyncio_reactor_installed
+
+
+class ReactorCheckExtension:
+    def __init__(self):
+        if not is_asyncio_reactor_installed():
+            raise RuntimeError("ReactorCheckExtension requires the asyncio reactor.")
 
 
 class NoRequestsSpider(scrapy.Spider):
@@ -12,6 +19,7 @@ class NoRequestsSpider(scrapy.Spider):
 process = CrawlerProcess(
     settings={
         "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+        "EXTENSIONS": {ReactorCheckExtension: 0},
     }
 )
 process.crawl(NoRequestsSpider)

tests/CrawlerProcess/asyncio_enabled_reactor.py

@@ -1,9 +1,39 @@
 import scrapy
 from scrapy.crawler import CrawlerProcess
-from scrapy.utils.reactor import install_reactor
+from scrapy.utils.reactor import (
+    install_reactor,
+    is_asyncio_reactor_installed,
+    is_reactor_installed,
+)
+
+if is_reactor_installed():
+    raise RuntimeError(
+        "Reactor already installed before is_asyncio_reactor_installed()."
+    )
+
+try:
+    is_asyncio_reactor_installed()
+except RuntimeError:
+    pass
+else:
+    raise RuntimeError("is_asyncio_reactor_installed() did not raise RuntimeError.")
+
+if is_reactor_installed():
+    raise RuntimeError(
+        "Reactor already installed after is_asyncio_reactor_installed()."
+    )
 
 install_reactor("twisted.internet.asyncioreactor.AsyncioSelectorReactor")
 
+if not is_asyncio_reactor_installed():
+    raise RuntimeError("Wrong reactor installed after install_reactor().")
+
+
+class ReactorCheckExtension:
+    def __init__(self):
+        if not is_asyncio_reactor_installed():
+            raise RuntimeError("ReactorCheckExtension requires the asyncio reactor.")
+
 
 class NoRequestsSpider(scrapy.Spider):
     name = "no_request"
@@ -15,6 +45,7 @@ class NoRequestsSpider(scrapy.Spider):
 process = CrawlerProcess(
     settings={
         "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
+        "EXTENSIONS": {ReactorCheckExtension: 0},
     }
 )
 process.crawl(NoRequestsSpider)

tests/test_crawler.py

@@ -747,6 +747,7 @@ class TestCrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
             "Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor"
             in log
         )
+        assert "RuntimeError" not in log
 
     def test_asyncio_enabled_reactor(self):
         log = self.run_script("asyncio_enabled_reactor.py")
@@ -755,6 +756,7 @@ class TestCrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
             "Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor"
             in log
         )
+        assert "RuntimeError" not in log
 
     @pytest.mark.skipif(
         parse_version(w3lib_version) >= parse_version("2.0.0"),