TWISTED_REACTOR setting not honored from Spider.custom_settings

Description

The value of the TWISTED_REACTOR setting is not taken into account if the setting is specified in a spider’s custom_settings attribute. It works well if the setting is specified in a project’s settings file, as a parameter when creating a CrawlerProcess object (as the tests show) or as a CLI argument with the -s option.

Steps to Reproduce

1. Create a file with the following contents:

# asyncio_spider.py
import asyncio

from scrapy import Spider


class AsyncIOSpider(Spider):
    name = "asyncio"
    start_urls = ["https://example.org"]
    custom_settings = {
        "TWISTED_REACTOR": "twisted.internet.asyncioreactor.AsyncioSelectorReactor",
    }

    async def parse(self, response):
        await asyncio.sleep(1)
        yield {"foo": "bar"}

1. Execute the spider: scrapy runspider asyncio_spider.py

Expected behavior: The spider should run fine, without exceptions

Actual behavior: The following exception is raised:

2020-04-12 00:04:23 [scrapy.core.scraper] ERROR: Spider error processing <GET https://example.org> (referer: None)
Traceback (most recent call last):
  File "/.../scrapy/venv-scrapy/lib/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/.../scrapy/scrapy/utils/py36.py", line 8, in collect_asyncgen
    async for x in result:
  File "/.../scrapy/test-spiders/asyncio_spider.py", line 14, in parse
    await asyncio.sleep(1)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/tasks.py", line 595, in sleep
    return await future
RuntimeError: await wasn't used with future

This is because the asyncio-based reactor is not actually installed, as the third log line of the job shows:

2020-04-12 00:04:23 [scrapy.utils.log] DEBUG: Using reactor: twisted.internet.selectreactor.SelectReactor

Full logs:

2020-04-12 00:04:23 [scrapy.utils.log] INFO: Scrapy 2.0.1 started (bot: scrapybot)
2020-04-12 00:04:23 [scrapy.utils.log] INFO: Versions: lxml 4.5.0.0, libxml2 2.9.10, cssselect 1.1.0, parsel 1.5.2, w3lib 1.21.0, Twisted 20.3.0, Python 3.7.4 (v3.7.4:e09359112e, Jul  8 2019, 14:54:52) - [Clang 6.0 (clang-600.0.57)], pyOpenSSL 19.1.0 (OpenSSL 1.1.1d  10 Sep 2019), cryptography 2.8, Platform Darwin-18.7.0-x86_64-i386-64bit
2020-04-12 00:04:23 [scrapy.utils.log] DEBUG: Using reactor: twisted.internet.selectreactor.SelectReactor
2020-04-12 00:04:23 [scrapy.crawler] INFO: Overridden settings:
{'EDITOR': 'nano',
 'SPIDER_LOADER_WARN_ONLY': True,
 'TWISTED_REACTOR': 'twisted.internet.asyncioreactor.AsyncioSelectorReactor'}
2020-04-12 00:04:23 [scrapy.extensions.telnet] INFO: Telnet Password: a2bd966307b319f0
2020-04-12 00:04:23 [scrapy.middleware] INFO: Enabled extensions:
['scrapy.extensions.corestats.CoreStats',
 'scrapy.extensions.telnet.TelnetConsole',
 'scrapy.extensions.memusage.MemoryUsage',
 'scrapy.extensions.logstats.LogStats']
2020-04-12 00:04:23 [scrapy.middleware] INFO: Enabled downloader middlewares:
['scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware',
 'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware',
 'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware',
 'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware',
 'scrapy.downloadermiddlewares.retry.RetryMiddleware',
 'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware',
 'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware',
 'scrapy.downloadermiddlewares.redirect.RedirectMiddleware',
 'scrapy.downloadermiddlewares.cookies.CookiesMiddleware',
 'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware',
 'scrapy.downloadermiddlewares.stats.DownloaderStats']
2020-04-12 00:04:23 [scrapy.middleware] INFO: Enabled spider middlewares:
['scrapy.spidermiddlewares.httperror.HttpErrorMiddleware',
 'scrapy.spidermiddlewares.offsite.OffsiteMiddleware',
 'scrapy.spidermiddlewares.referer.RefererMiddleware',
 'scrapy.spidermiddlewares.urllength.UrlLengthMiddleware',
 'scrapy.spidermiddlewares.depth.DepthMiddleware']
2020-04-12 00:04:23 [scrapy.middleware] INFO: Enabled item pipelines:
[]
2020-04-12 00:04:23 [scrapy.core.engine] INFO: Spider opened
2020-04-12 00:04:23 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)
2020-04-12 00:04:23 [scrapy.extensions.telnet] INFO: Telnet console listening on 127.0.0.1:6023
2020-04-12 00:04:23 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://example.org> (referer: None)
2020-04-12 00:04:23 [asyncio] DEBUG: Using selector: KqueueSelector
2020-04-12 00:04:23 [scrapy.core.scraper] ERROR: Spider error processing <GET https://example.org> (referer: None)
Traceback (most recent call last):
  File "/.../scrapy/venv-scrapy/lib/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/.../scrapy/scrapy/utils/py36.py", line 8, in collect_asyncgen
    async for x in result:
  File "/.../scrapy/test-spiders/asyncio_spider.py", line 14, in parse
    await asyncio.sleep(1)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/tasks.py", line 595, in sleep
    return await future
RuntimeError: await wasn't used with future
2020-04-12 00:04:23 [scrapy.core.engine] INFO: Closing spider (finished)
2020-04-12 00:04:23 [scrapy.statscollectors] INFO: Dumping Scrapy stats:
{'downloader/request_bytes': 211,
 'downloader/request_count': 1,
 'downloader/request_method_count/GET': 1,
 'downloader/response_bytes': 1001,
 'downloader/response_count': 1,
 'downloader/response_status_count/200': 1,
 'elapsed_time_seconds': 0.778025,
 'finish_reason': 'finished',
 'finish_time': datetime.datetime(2020, 4, 12, 3, 4, 23, 925629),
 'log_count/DEBUG': 2,
 'log_count/ERROR': 1,
 'log_count/INFO': 10,
 'memusage/max': 54767616,
 'memusage/startup': 54767616,
 'response_received_count': 1,
 'scheduler/dequeued': 1,
 'scheduler/dequeued/memory': 1,
 'scheduler/enqueued': 1,
 'scheduler/enqueued/memory': 1,
 'spider_exceptions/RuntimeError': 1,
 'start_time': datetime.datetime(2020, 4, 12, 3, 4, 23, 147604)}
2020-04-12 00:04:23 [scrapy.core.engine] INFO: Spider closed (finished)

Reproduces how often: 100% of the times.

Versions

Scrapy       : 2.0.1
lxml         : 4.5.0.0
libxml2      : 2.9.10
cssselect    : 1.1.0
parsel       : 1.5.2
w3lib        : 1.21.0
Twisted      : 20.3.0
Python       : 3.7.4 (v3.7.4:e09359112e, Jul  8 2019, 14:54:52) - [Clang 6.0 (clang-600.0.57)]
pyOpenSSL    : 19.1.0 (OpenSSL 1.1.1d  10 Sep 2019)
cryptography : 2.8
Platform     : Darwin-18.7.0-x86_64-i386-64bit

Scrapy       : 2.0.1
lxml         : 4.5.0.0
libxml2      : 2.9.10
cssselect    : 1.1.0
parsel       : 1.5.2
w3lib        : 1.21.0
Twisted      : 19.10.0
Python       : 3.6.9 (default, Nov  7 2019, 10:44:02) - [GCC 8.3.0]
pyOpenSSL    : 19.1.0 (OpenSSL 1.1.1d  10 Sep 2019)
cryptography : 2.8
Platform     : Linux-4.15.0-96-generic-x86_64-with-LinuxMint-19.1-tessa

Additional context

From a quick look, it seems like this happens because the custom reactor is installed in CrawlerRunner.__init__, but the spider class is only available in CrawlerRunner.crawl or CrawlerRunner.create_crawler.

[wRAR]

I think we should only support matching settings and take custom_settings from the first spider, while showing an error if the following spiders have different custom_settings.

[GeorgeA92]

Running multiple spiders in the same process

...
process = CrawlerProcess()
process.crawl(MySpider1)
process.crawl(MySpider2)
process.start()

As far as I know: Running multiple spiders in the same process also means that multiple spiders will use single instantse of reactor(1 reactor - per process).

As result: In case if CrawlerProcess will have spiders with different reactor settings - it is not clear, what is expected outcome of this?

process = CrawlerProcess()
process.crawl(MySpider1)
process.crawl(MySpider2_with_TWISTED_REACTOR_in_custom_settings)
process.start()