CrawlerProcess: initialize the reactor only once

[Gallaecio]

Fixes #5435

Pending:

• Test coverage
• Check if CrawlerRunner is working as expected (should it initialize the reactor in any scenario?)

[codecov]

Codecov Report

Merging #5436 (edbf397) into 2.6 (23537a0) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head edbf397 differs from pull request most recent head 3bf6bae. Consider uploading reports for the commit 3bf6bae to get more accurate results

@@            Coverage Diff             @@
##              2.6    #5436      +/-   ##
==========================================
- Coverage   88.77%   88.70%   -0.08%     
==========================================
  Files         163      163              
  Lines       10666    10667       +1     
  Branches     1818     1788      -30     
==========================================
- Hits         9469     9462       -7     
- Misses        922      929       +7     
- Partials      275      276       +1     

Impacted Files	Coverage Δ
scrapy/crawler.py	88.82% <100.00%> (+0.18%)	⬆️
scrapy/core/downloader/handlers/__init__.py	83.63% <0.00%> (-9.10%)	⬇️
scrapy/downloadermiddlewares/cookies.py	95.78% <0.00%> (-2.11%)	⬇️
scrapy/shell.py	67.96% <0.00%> (-0.79%)	⬇️
scrapy/http/response/__init__.py	97.43% <0.00%> (-0.04%)	⬇️
scrapy/http/request/__init__.py	97.77% <0.00%> (-0.03%)	⬇️
scrapy/utils/python.py	87.64% <0.00%> (ø)
scrapy/commands/check.py	71.01% <0.00%> (ø)

[Gallaecio]

CrawlerRunner is fine, and unlike CrawlerProcess, existing tests would have failed had it initialized the reactor.

[odimko]

@wRAR when is it going to be released?

[Gallaecio]

2 weeks ago according to plan :)

We have had some busy weeks. My current hopes are to release before the end of the month.

[odimko]

Awesome @Gallaecio 💯
thanks for letting me know. Looking forward to 👍🏻