Add the ASYNCIO_SUPPORT setting, reshuffle other logic accordingly.

docs/topics/settings.rst

@@ -160,6 +160,31 @@ to any particular component. In that case the module of that component will be
 shown, typically an extension, middleware or pipeline. It also means that the
 component must be enabled in order for the setting to have any effect.
 
+.. setting:: ASYNCIO_SUPPORT
+
+ASYNCIO_SUPPORT
+---------------
+
+Default: ``False``
+
+Whether to support ``async def`` methods and callbacks which use code that
+requires an asyncio loop.
+
+If an ``async def`` coroutine doesn't require the asyncio loop, it will work
+even if this is set to ``False``. Coroutines that require the asyncio loop may
+silently fail to run or raise errors unless this is set to ``True``.
+
+When this option is set to ``True``, Scrapy will require
+:class:`~twisted.internet.asyncioreactor.AsyncioSelectorReactor`. It will
+install this reactor if no reactor is installed yet, such as when using the
+``scrapy`` script or :class:`~scrapy.crawler.CrawlerProcess`. If you are using
+:class:`~scrapy.crawler.CrawlerRunner`, you need to install the correct reactor
+manually.
+
+The default value for this option is currently ``False`` to maintain backward
+compatibility and avoid possible problems caused by using a different Twisted
+reactor.
+
 .. setting:: AWS_ACCESS_KEY_ID
 
 AWS_ACCESS_KEY_ID

scrapy/cmdline.py

@@ -9,7 +9,6 @@ import pkg_resources
 import scrapy
 from scrapy.commands import ScrapyCommand
 from scrapy.exceptions import UsageError
-from scrapy.utils.asyncio import install_asyncio_reactor
 from scrapy.utils.misc import walk_modules
 from scrapy.utils.project import inside_project, get_project_settings
 from scrapy.utils.python import garbage_collect
@@ -121,10 +120,6 @@ def execute(argv=None, settings=None):
             settings['EDITOR'] = editor
     check_deprecated_settings(settings)
 
-    # needs to be before _get_commands_dict() as that imports the command modules
-    # which may import twisted.internet.reactor
-    install_asyncio_reactor()
-
     inproject = inside_project()
     cmds = _get_commands_dict(settings, inproject)
     cmdname = _pop_command_name(argv)
@@ -146,7 +141,8 @@ def execute(argv=None, settings=None):
     opts, args = parser.parse_args(args=argv[1:])
     _run_print_help(parser, cmd.process_options, args, opts)
 
-    # needs to be after install_asyncio_reactor() as it imports twisted.internet.reactor
+    # needs to be after cmd.process_options() as it imports twisted.internet.reactor
+    # while commands may want to install the asyncio reactor
     from scrapy.crawler import CrawlerProcess
     cmd.crawler_process = CrawlerProcess(settings)
     _run_print_help(parser, _run_command, cmd, args, opts)

scrapy/commands/crawl.py

@@ -1,5 +1,6 @@
 import os
 from scrapy.commands import ScrapyCommand
+from scrapy.utils.asyncio import install_asyncio_reactor
 from scrapy.utils.conf import arglist_to_dict
 from scrapy.utils.python import without_none_values
 from scrapy.exceptions import UsageError
@@ -26,6 +27,8 @@ class Command(ScrapyCommand):
 
     def process_options(self, args, opts):
         ScrapyCommand.process_options(self, args, opts)
+        if self.settings.getbool('ASYNCIO_SUPPORT'):
+            install_asyncio_reactor()
         try:
             opts.spargs = arglist_to_dict(opts.spargs)
         except ValueError:

scrapy/commands/runspider.py

@@ -2,6 +2,7 @@ import sys
 import os
 from importlib import import_module
 
+from scrapy.utils.asyncio import install_asyncio_reactor
 from scrapy.utils.spider import iter_spider_classes
 from scrapy.commands import ScrapyCommand
 from scrapy.exceptions import UsageError
@@ -50,6 +51,8 @@ class Command(ScrapyCommand):
 
     def process_options(self, args, opts):
         ScrapyCommand.process_options(self, args, opts)
+        if self.settings.getbool('ASYNCIO_SUPPORT'):
+            install_asyncio_reactor()
         try:
             opts.spargs = arglist_to_dict(opts.spargs)
         except ValueError:

scrapy/settings/default_settings.py

@@ -19,6 +19,8 @@ from os.path import join, abspath, dirname
 
 AJAXCRAWL_ENABLED = False
 
+ASYNCIO_SUPPORT = False
+
 AUTOTHROTTLE_ENABLED = False
 AUTOTHROTTLE_DEBUG = False
 AUTOTHROTTLE_MAX_DELAY = 60.0

scrapy/utils/asyncio.py

@@ -14,7 +14,7 @@ def install_asyncio_reactor():
             pass
 
 
-def is_asyncio_supported():
+def is_asyncio_reactor_installed():
     try:
         import twisted.internet.reactor
         from twisted.internet import asyncioreactor

scrapy/utils/defer.py

@@ -9,8 +9,7 @@ from twisted.internet import defer, reactor, task
 from twisted.python import failure
 
 from scrapy.exceptions import IgnoreRequest
-from scrapy.utils.asyncio import is_asyncio_supported
-
+from scrapy.utils.asyncio import is_asyncio_reactor_installed
 
 
 def defer_fail(_failure):
@@ -127,12 +126,18 @@ def isfuture(o):
     return isinstance(o, asyncio.futures.Future)
 
 
-def deferred_from_coro(o):
+def deferred_from_coro(o, asyncio_enabled=False):
     """Converts a coroutine into a Deferred, or returns the object as is if it isn't a coroutine"""
     if isinstance(o, defer.Deferred):
         return o
     if isfuture(o) or inspect.isawaitable(o):
-        if not is_asyncio_supported():
-            raise TypeError('Using coroutines requires installing AsyncioSelectorReactor')
-        return defer.Deferred.fromFuture(asyncio.ensure_future(o))
+        if not asyncio_enabled:
+            # wrapping the coroutine directly into a Deferred, this doesn't work correctly with coroutines
+            # that use asyncio, e.g. "await asyncio.sleep(1)"
+            return defer.ensureDeferred(o)
+        else:
+            # wrapping the coroutine into a Future and then into a Deferred, this requires AsyncioSelectorReactor
+            if not is_asyncio_reactor_installed():
+                raise TypeError('Using coroutines requires installing AsyncioSelectorReactor')
+            return defer.Deferred.fromFuture(asyncio.ensure_future(o))
     return o

scrapy/utils/log.py

@@ -11,7 +11,7 @@ from twisted.python import log as twisted_log
 import scrapy
 from scrapy.settings import Settings
 from scrapy.exceptions import ScrapyDeprecationWarning
-from scrapy.utils.asyncio import is_asyncio_supported
+from scrapy.utils.asyncio import is_asyncio_reactor_installed
 from scrapy.utils.versions import scrapy_components_versions
 
 
@@ -149,8 +149,13 @@ def log_scrapy_info(settings):
                 {'versions': ", ".join("%s %s" % (name, version)
                     for name, version in scrapy_components_versions()
                     if name != "Scrapy")})
-    if is_asyncio_supported():
-        logger.debug("Asyncio support enabled")
+    if settings.getbool('ASYNCIO_SUPPORT'):
+        if is_asyncio_reactor_installed():
+            logger.debug("Asyncio support enabled")
+        else:
+            logger.error("ASYNCIO_SUPPORT is on but the Twisted asyncio "
+                         "reactor is not installed, this is not supported "
+                         "and asyncio coroutines will not work.")
 
 
 class StreamLogger(object):

tests/test_commands.py

@@ -9,7 +9,6 @@ from tempfile import mkdtemp
 from contextlib import contextmanager
 from threading import Timer
 
-from pytest import mark
 from twisted.trial import unittest
 
 import scrapy
@@ -179,7 +178,6 @@ class MiscCommandsTest(CommandTest):
         self.assertEqual(0, self.call('list'))
 
 
-@mark.usefixtures('reactor_pytest')
 class RunSpiderCommandTest(CommandTest):
 
     debug_log_spider = """
@@ -297,10 +295,13 @@ class BadSpider(scrapy.Spider):
         self.assertIn("start_requests", log)
         self.assertIn("badspider.py", log)
 
-    def test_asyncio_supported(self):
-        if self.reactor_pytest == 'asyncio':
-            log = self.get_log(self.debug_log_spider)
-            self.assertIn("DEBUG: Asyncio support enabled", log)
+    def test_asyncio_support_true(self):
+        log = self.get_log(self.debug_log_spider, args=['-s', 'ASYNCIO_SUPPORT=True'])
+        self.assertIn("DEBUG: Asyncio support enabled", log)
+
+    def test_asyncio_support_false(self):
+        log = self.get_log(self.debug_log_spider, args=['-s', 'ASYNCIO_SUPPORT=False'])
+        self.assertNotIn("DEBUG: Asyncio support enabled", log)
 
 
 class BenchCommandTest(CommandTest):

tests/test_crawler.py

@@ -10,7 +10,7 @@ import scrapy
 from scrapy.crawler import Crawler, CrawlerRunner, CrawlerProcess
 from scrapy.settings import Settings, default_settings
 from scrapy.spiderloader import SpiderLoader
-from scrapy.utils.asyncio import is_asyncio_supported
+from scrapy.utils.asyncio import is_asyncio_reactor_installed
 from scrapy.utils.log import configure_logging, get_scrapy_root_handler
 from scrapy.utils.spider import DefaultSpider
 from scrapy.utils.misc import load_object
@@ -209,7 +209,7 @@ class AsyncioSpider(scrapy.Spider):
     name = 'asyncio'
 
     def start_requests(self):
-        self.logger.info('Asyncio support: %s', is_asyncio_supported())
+        self.logger.info('Asyncio support: %s', is_asyncio_reactor_installed())
         return []
 
 
@@ -258,7 +258,25 @@ class CrawlerRunnerHasSpider(unittest.TestCase):
         self.assertEqual(runner.bootstrap_failed, True)
 
     @defer.inlineCallbacks
-    def test_asyncio_supported(self):
+    def test_crawler_process_asyncio_supported_true(self):
+        with LogCapture(level=logging.DEBUG) as log:
+            runner = CrawlerProcess(settings={'ASYNCIO_SUPPORT': True})
+            yield runner.crawl(NoRequestsSpider)
+            if self.reactor_pytest == 'asyncio':
+                self.assertIn("Asyncio support enabled", str(log))
+            else:
+                self.assertNotIn("Asyncio support enabled", str(log))
+                self.assertIn("ASYNCIO_SUPPORT is on but the Twisted asyncio reactor is not installed", str(log))
+
+    @defer.inlineCallbacks
+    def test_crawler_process_asyncio_supported_false(self):
+        runner = CrawlerProcess(settings={'ASYNCIO_SUPPORT': False})
+        with LogCapture(level=logging.DEBUG) as log:
+            yield runner.crawl(NoRequestsSpider)
+            self.assertNotIn("Asyncio support enabled", str(log))
+
+    @defer.inlineCallbacks
+    def test_crawler_runner_asyncio_supported(self):
         runner = CrawlerRunner()
         with LogCapture() as log:
             yield runner.crawl(AsyncioSpider)

tests/test_utils_asyncio.py

@@ -2,15 +2,15 @@ from unittest import TestCase
 
 from pytest import mark
 
-from scrapy.utils.asyncio import is_asyncio_supported, install_asyncio_reactor
+from scrapy.utils.asyncio import is_asyncio_reactor_installed, install_asyncio_reactor
 
 
 @mark.usefixtures('reactor_pytest')
 class AsyncioTest(TestCase):
 
-    def test_is_asyncio_supported(self):
+    def test_is_asyncio_reactor_installed(self):
         # the result should depend only on the pytest --reactor argument
-        self.assertEquals(is_asyncio_supported(), self.reactor_pytest == 'asyncio')
+        self.assertEquals(is_asyncio_reactor_installed(), self.reactor_pytest == 'asyncio')
 
     def test_install_asyncio_reactor(self):
         # this should do nothing