diff --git a/.bumpversion.cfg b/.bumpversion.cfg
index 1d9b9c02f..2e2f7949a 100644
--- a/.bumpversion.cfg
+++ b/.bumpversion.cfg
@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 2.6.1
+current_version = 2.6.2
 commit = True
 tag = True
 tag_name = {new_version}
diff --git a/docs/news.rst b/docs/news.rst
index 2d0ab485e..d27c105a5 100644
--- a/docs/news.rst
+++ b/docs/news.rst
@@ -3,6 +3,78 @@
 Release notes
 =============
 
+.. _release-2.6.2:
+
+Scrapy 2.6.2 (2022-07-25)
+-------------------------
+
+**Security bug fix:**
+
+-   When :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware`
+    processes a request with :reqmeta:`proxy` metadata, and that
+    :reqmeta:`proxy` metadata includes proxy credentials,
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` sets
+    the ``Proxy-Authentication`` header, but only if that header is not already
+    set.
+
+    There are third-party proxy-rotation downloader middlewares that set
+    different :reqmeta:`proxy` metadata every time they process a request.
+
+    Because of request retries and redirects, the same request can be processed
+    by downloader middlewares more than once, including both
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` and
+    any third-party proxy-rotation downloader middleware.
+
+    These third-party proxy-rotation downloader middlewares could change the
+    :reqmeta:`proxy` metadata of a request to a new value, but fail to remove
+    the ``Proxy-Authentication`` header from the previous value of the
+    :reqmeta:`proxy` metadata, causing the credentials of one proxy to be sent
+    to a different proxy.
+
+    To prevent the unintended leaking of proxy credentials, the behavior of
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` is now
+    as follows when processing a request:
+
+    -   If the request being processed defines :reqmeta:`proxy` metadata that
+        includes credentials, the ``Proxy-Authorization`` header is always
+        updated to feature those credentials.
+
+    -   If the request being processed defines :reqmeta:`proxy` metadata
+        without credentials, the ``Proxy-Authorization`` header is removed
+        *unless* it was originally defined for the same proxy URL.
+
+        To remove proxy credentials while keeping the same proxy URL, remove
+        the ``Proxy-Authorization`` header.
+
+    -   If the request has no :reqmeta:`proxy` metadata, or that metadata is a
+        falsy value (e.g. ``None``), the ``Proxy-Authorization`` header is
+        removed.
+
+        It is no longer possible to set a proxy URL through the
+        :reqmeta:`proxy` metadata but set the credentials through the
+        ``Proxy-Authorization`` header. Set proxy credentials through the
+        :reqmeta:`proxy` metadata instead.
+
+Also fixes the following regressions introduced in 2.6.0:
+
+-   :class:`~scrapy.crawler.CrawlerProcess` supports again crawling multiple
+    spiders (:issue:`5435`, :issue:`5436`)
+
+-   Installing a Twisted reactor before Scrapy does (e.g. importing
+    :mod:`twisted.internet.reactor` somewhere at the module level) no longer
+    prevents Scrapy from starting, as long as a different reactor is not
+    specified in :setting:`TWISTED_REACTOR` (:issue:`5525`, :issue:`5528`)
+
+-   Fixed an exception that was being logged after the spider finished under
+    certain conditions (:issue:`5437`, :issue:`5440`)
+
+-   The ``--output``/``-o`` command-line parameter supports again a value
+    starting with a hyphen (:issue:`5444`, :issue:`5445`)
+
+-   The ``scrapy parse -h`` command no longer throws an error (:issue:`5481`,
+    :issue:`5482`)
+
+
 .. _release-2.6.1:
 
 Scrapy 2.6.1 (2022-03-01)
@@ -113,6 +185,9 @@ Backward-incompatible changes
     meet expectations, :exc:`TypeError` is now raised at startup time. Before,
     other exceptions would be raised at run time. (:issue:`3559`)
 
+-   The ``_encoding`` field of serialized :class:`~scrapy.http.Request` objects
+    is now named ``encoding``, in line with all other fields (:issue:`5130`)
+
 
 Deprecation removals
 ~~~~~~~~~~~~~~~~~~~~
@@ -1897,6 +1972,59 @@ affect subclasses:
 (:issue:`3884`)
 
 
+.. _release-1.8.3:
+
+Scrapy 1.8.3 (2022-07-25)
+-------------------------
+
+**Security bug fix:**
+
+-   When :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware`
+    processes a request with :reqmeta:`proxy` metadata, and that
+    :reqmeta:`proxy` metadata includes proxy credentials,
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` sets
+    the ``Proxy-Authentication`` header, but only if that header is not already
+    set.
+
+    There are third-party proxy-rotation downloader middlewares that set
+    different :reqmeta:`proxy` metadata every time they process a request.
+
+    Because of request retries and redirects, the same request can be processed
+    by downloader middlewares more than once, including both
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` and
+    any third-party proxy-rotation downloader middleware.
+
+    These third-party proxy-rotation downloader middlewares could change the
+    :reqmeta:`proxy` metadata of a request to a new value, but fail to remove
+    the ``Proxy-Authentication`` header from the previous value of the
+    :reqmeta:`proxy` metadata, causing the credentials of one proxy to be sent
+    to a different proxy.
+
+    To prevent the unintended leaking of proxy credentials, the behavior of
+    :class:`~scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware` is now
+    as follows when processing a request:
+
+    -   If the request being processed defines :reqmeta:`proxy` metadata that
+        includes credentials, the ``Proxy-Authorization`` header is always
+        updated to feature those credentials.
+
+    -   If the request being processed defines :reqmeta:`proxy` metadata
+        without credentials, the ``Proxy-Authorization`` header is removed
+        *unless* it was originally defined for the same proxy URL.
+
+        To remove proxy credentials while keeping the same proxy URL, remove
+        the ``Proxy-Authorization`` header.
+
+    -   If the request has no :reqmeta:`proxy` metadata, or that metadata is a
+        falsy value (e.g. ``None``), the ``Proxy-Authorization`` header is
+        removed.
+
+        It is no longer possible to set a proxy URL through the
+        :reqmeta:`proxy` metadata but set the credentials through the
+        ``Proxy-Authorization`` header. Set proxy credentials through the
+        :reqmeta:`proxy` metadata instead.
+
+
 .. _release-1.8.2:
 
 Scrapy 1.8.2 (2022-03-01)
diff --git a/docs/requirements.txt b/docs/requirements.txt
index a0930ba1e..9f9aef711 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,4 +1,4 @@
-Sphinx>=3.0
-sphinx-hoverxref>=0.2b1
-sphinx-notfound-page>=0.4
-sphinx-rtd-theme>=0.5.2
\ No newline at end of file
+sphinx==5.0.2
+sphinx-hoverxref==1.1.1
+sphinx-notfound-page==0.8
+sphinx-rtd-theme==1.0.0
diff --git a/docs/topics/settings.rst b/docs/topics/settings.rst
index 2046c6446..6722ce9ed 100644
--- a/docs/topics/settings.rst
+++ b/docs/topics/settings.rst
@@ -1634,9 +1634,10 @@ 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`.
 
diff --git a/pylintrc b/pylintrc
index 0d29dc709..18819feba 100644
--- a/pylintrc
+++ b/pylintrc
@@ -9,11 +9,9 @@ disable=abstract-method,
         arguments-renamed,
         attribute-defined-outside-init,
         bad-classmethod-argument,
-        bad-continuation,
         bad-indentation,
         bad-mcs-classmethod-argument,
         bad-super-call,
-        bad-whitespace,
         bare-except,
         blacklisted-name,
         broad-except,
@@ -53,7 +51,6 @@ disable=abstract-method,
         logging-not-lazy,
         lost-exception,
         method-hidden,
-        misplaced-comparison-constant,
         missing-docstring,
         missing-final-newline,
         multiple-imports,
@@ -61,12 +58,10 @@ disable=abstract-method,
         no-else-continue,
         no-else-raise,
         no-else-return,
-        no-init,
         no-member,
         no-method-argument,
         no-name-in-module,
         no-self-argument,
-        no-self-use,
         no-value-for-parameter,
         not-an-iterable,
         not-callable,
@@ -103,6 +98,7 @@ disable=abstract-method,
         ungrouped-imports,
         unidiomatic-typecheck,
         unnecessary-comprehension,
+        unnecessary-dunder-call,
         unnecessary-lambda,
         unnecessary-pass,
         unreachable,
diff --git a/scrapy/VERSION b/scrapy/VERSION
index 6a6a3d8e3..097a15a2a 100644
--- a/scrapy/VERSION
+++ b/scrapy/VERSION
@@ -1 +1 @@
-2.6.1
+2.6.2
diff --git a/scrapy/cmdline.py b/scrapy/cmdline.py
index 491c4beab..5ee1f0f44 100644
--- a/scrapy/cmdline.py
+++ b/scrapy/cmdline.py
@@ -14,6 +14,15 @@ from scrapy.utils.project import inside_project, get_project_settings
 from scrapy.utils.python import garbage_collect
 
 
+class ScrapyArgumentParser(argparse.ArgumentParser):
+    def _parse_optional(self, arg_string):
+        # if starts with -: it means that is a parameter not a argument
+        if arg_string[:2] == '-:':
+            return None
+
+        return super()._parse_optional(arg_string)
+
+
 def _iter_command_classes(module_name):
     # TODO: add `name` attribute to commands and and merge this function with
     # scrapy.utils.spider.iter_spider_classes
@@ -131,10 +140,10 @@ def execute(argv=None, settings=None):
         sys.exit(2)
 
     cmd = cmds[cmdname]
-    parser = argparse.ArgumentParser(formatter_class=ScrapyHelpFormatter,
-                                     usage=f"scrapy {cmdname} {cmd.syntax()}",
-                                     conflict_handler='resolve',
-                                     description=cmd.long_desc())
+    parser = ScrapyArgumentParser(formatter_class=ScrapyHelpFormatter,
+                                  usage=f"scrapy {cmdname} {cmd.syntax()}",
+                                  conflict_handler='resolve',
+                                  description=cmd.long_desc())
     settings.setdict(cmd.default_settings, priority='command')
     cmd.settings = settings
     cmd.add_options(parser)
diff --git a/scrapy/commands/parse.py b/scrapy/commands/parse.py
index 99fc8f955..9b4fb0ed6 100644
--- a/scrapy/commands/parse.py
+++ b/scrapy/commands/parse.py
@@ -51,7 +51,7 @@ class Command(BaseRunSpiderCommand):
         parser.add_argument("--cbkwargs", dest="cbkwargs",
                             help="inject extra callback kwargs into the Request, it must be a valid raw json string")
         parser.add_argument("-d", "--depth", dest="depth", type=int, default=1,
-                            help="maximum depth for parsing requests [default: %default]")
+                            help="maximum depth for parsing requests [default: %(default)s]")
         parser.add_argument("-v", "--verbose", dest="verbose", action="store_true",
                             help="print each depth level one by one")
 
diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index f9de7ee23..6602f661d 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -136,7 +136,9 @@ class ExecutionEngine:
         self.paused = False
 
     def _next_request(self) -> None:
-        assert self.slot is not None  # typing
+        if self.slot is None:
+            return
+
         assert self.spider is not None  # typing
 
         if self.paused:
@@ -184,7 +186,8 @@ class ExecutionEngine:
         d.addErrback(lambda f: logger.info('Error while removing request from slot',
                                            exc_info=failure_to_exc_info(f),
                                            extra={'spider': self.spider}))
-        d.addBoth(lambda _: self.slot.nextcall.schedule())
+        slot = self.slot
+        d.addBoth(lambda _: slot.nextcall.schedule())
         d.addErrback(lambda f: logger.info('Error while scheduling new request',
                                            exc_info=failure_to_exc_info(f),
                                            extra={'spider': self.spider}))
diff --git a/scrapy/crawler.py b/scrapy/crawler.py
index fdca7b335..e768bca12 100644
--- a/scrapy/crawler.py
+++ b/scrapy/crawler.py
@@ -85,8 +85,7 @@ class Crawler:
             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  # noqa: F401
             log_reactor_info()
         if reactor_class:
             verify_installed_reactor(reactor_class)
@@ -297,6 +296,7 @@ class CrawlerProcess(CrawlerRunner):
         super().__init__(settings)
         configure_logging(self.settings, install_root_handler)
         log_scrapy_info(self.settings)
+        self._initialized_reactor = False
 
     def _signal_shutdown(self, signum, _):
         from twisted.internet import reactor
@@ -317,7 +317,9 @@ class CrawlerProcess(CrawlerRunner):
     def _create_crawler(self, spidercls):
         if isinstance(spidercls, str):
             spidercls = self.spider_loader.load(spidercls)
-        return Crawler(spidercls, self.settings, init_reactor=True)
+        init_reactor = not self._initialized_reactor
+        self._initialized_reactor = True
+        return Crawler(spidercls, self.settings, init_reactor=init_reactor)
 
     def start(self, stop_after_crawl=True, install_signal_handlers=True):
         """
diff --git a/scrapy/downloadermiddlewares/httpproxy.py b/scrapy/downloadermiddlewares/httpproxy.py
index d2665b655..1deda42bd 100644
--- a/scrapy/downloadermiddlewares/httpproxy.py
+++ b/scrapy/downloadermiddlewares/httpproxy.py
@@ -45,31 +45,37 @@ class HttpProxyMiddleware:
         return creds, proxy_url
 
     def process_request(self, request, spider):
-        # ignore if proxy is already set
+        creds, proxy_url = None, None
         if 'proxy' in request.meta:
-            if request.meta['proxy'] is None:
-                return
-            # extract credentials if present
-            creds, proxy_url = self._get_proxy(request.meta['proxy'], '')
+            if request.meta['proxy'] is not None:
+                creds, proxy_url = self._get_proxy(request.meta['proxy'], '')
+        elif self.proxies:
+            parsed = urlparse_cached(request)
+            scheme = parsed.scheme
+            if (
+                (
+                    # 'no_proxy' is only supported by http schemes
+                    scheme not in ('http', 'https')
+                    or not proxy_bypass(parsed.hostname)
+                )
+                and scheme in self.proxies
+            ):
+                creds, proxy_url = self.proxies[scheme]
+
+        self._set_proxy_and_creds(request, proxy_url, creds)
+
+    def _set_proxy_and_creds(self, request, proxy_url, creds):
+        if proxy_url:
             request.meta['proxy'] = proxy_url
-            if creds and not request.headers.get('Proxy-Authorization'):
-                request.headers['Proxy-Authorization'] = b'Basic ' + creds
-            return
-        elif not self.proxies:
-            return
-
-        parsed = urlparse_cached(request)
-        scheme = parsed.scheme
-
-        # 'no_proxy' is only supported by http schemes
-        if scheme in ('http', 'https') and proxy_bypass(parsed.hostname):
-            return
-
-        if scheme in self.proxies:
-            self._set_proxy(request, scheme)
-
-    def _set_proxy(self, request, scheme):
-        creds, proxy = self.proxies[scheme]
-        request.meta['proxy'] = proxy
+        elif request.meta.get('proxy') is not None:
+            request.meta['proxy'] = None
         if creds:
-            request.headers['Proxy-Authorization'] = b'Basic ' + creds
+            request.headers[b'Proxy-Authorization'] = b'Basic ' + creds
+            request.meta['_auth_proxy'] = proxy_url
+        elif '_auth_proxy' in request.meta:
+            if proxy_url != request.meta['_auth_proxy']:
+                if b'Proxy-Authorization' in request.headers:
+                    del request.headers[b'Proxy-Authorization']
+                del request.meta['_auth_proxy']
+        elif b'Proxy-Authorization' in request.headers:
+            del request.headers[b'Proxy-Authorization']
diff --git a/scrapy/utils/reactor.py b/scrapy/utils/reactor.py
index 96395543c..bc543b230 100644
--- a/scrapy/utils/reactor.py
+++ b/scrapy/utils/reactor.py
@@ -83,7 +83,7 @@ def verify_installed_reactor(reactor_path):
     path."""
     from twisted.internet import reactor
     reactor_class = load_object(reactor_path)
-    if not isinstance(reactor, reactor_class):
+    if not reactor.__class__ == reactor_class:
         msg = ("The installed reactor "
                f"({reactor.__module__}.{reactor.__class__.__name__}) does not "
                f"match the requested one ({reactor_path})")
diff --git a/tests/CrawlerProcess/multi.py b/tests/CrawlerProcess/multi.py
new file mode 100644
index 000000000..aaa1af5c5
--- /dev/null
+++ b/tests/CrawlerProcess/multi.py
@@ -0,0 +1,16 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+
+
+class NoRequestsSpider(scrapy.Spider):
+    name = 'no_request'
+
+    def start_requests(self):
+        return []
+
+
+process = CrawlerProcess(settings={})
+
+process.crawl(NoRequestsSpider)
+process.crawl(NoRequestsSpider)
+process.start()
diff --git a/tests/CrawlerProcess/reactor_default.py b/tests/CrawlerProcess/reactor_default.py
new file mode 100644
index 000000000..5a21a3717
--- /dev/null
+++ b/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()
+
diff --git a/tests/CrawlerProcess/reactor_default_twisted_reactor_select.py b/tests/CrawlerProcess/reactor_default_twisted_reactor_select.py
new file mode 100644
index 000000000..c476722ef
--- /dev/null
+++ b/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()
+
+
diff --git a/tests/CrawlerProcess/reactor_select.py b/tests/CrawlerProcess/reactor_select.py
new file mode 100644
index 000000000..eac6e2f89
--- /dev/null
+++ b/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()
+
+
diff --git a/tests/CrawlerProcess/reactor_select_subclass_twisted_reactor_select.py b/tests/CrawlerProcess/reactor_select_subclass_twisted_reactor_select.py
new file mode 100644
index 000000000..47f480605
--- /dev/null
+++ b/tests/CrawlerProcess/reactor_select_subclass_twisted_reactor_select.py
@@ -0,0 +1,31 @@
+import scrapy
+from scrapy.crawler import CrawlerProcess
+from twisted.internet.main import installReactor
+from twisted.internet.selectreactor import SelectReactor
+
+
+class SelectReactorSubclass(SelectReactor):
+    pass
+
+
+reactor = SelectReactorSubclass()
+installReactor(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()
+
+
+
+
diff --git a/tests/CrawlerProcess/reactor_select_twisted_reactor_select.py b/tests/CrawlerProcess/reactor_select_twisted_reactor_select.py
new file mode 100644
index 000000000..e0d2dab26
--- /dev/null
+++ b/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()
+
+
+
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 7cd19b29a..76d5f3935 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -770,6 +770,21 @@ class MySpider(scrapy.Spider):
         log = self.get_log(spider_code, args=args)
         self.assertIn("error: Please use only one of -o/--output and -O/--overwrite-output", log)
 
+    def test_output_stdout(self):
+        spider_code = """
+import scrapy
+
+class MySpider(scrapy.Spider):
+    name = 'myspider'
+
+    def start_requests(self):
+        self.logger.debug('FEEDS: {}'.format(self.settings.getdict('FEEDS')))
+        return []
+"""
+        args = ['-o', '-:json']
+        log = self.get_log(spider_code, args=args)
+        self.assertIn("[myspider] DEBUG: FEEDS: {'stdout:': {'format': 'json'}}", log)
+
 
 @skipIf(platform.system() != 'Windows', "Windows required for .pyw files")
 class WindowsRunSpiderCommandTest(RunSpiderCommandTest):
@@ -915,3 +930,17 @@ class MySpider(scrapy.Spider):
         args = ['-o', 'example1.json', '-O', 'example2.json']
         log = self.get_log(spider_code, args=args)
         self.assertIn("error: Please use only one of -o/--output and -O/--overwrite-output", log)
+
+
+class HelpMessageTest(CommandTest):
+
+    def setUp(self):
+        super().setUp()
+        self.commands = ["parse", "startproject", "view", "crawl", "edit",
+                         "list", "fetch", "settings", "shell", "runspider",
+                         "version", "genspider", "check", "bench"]
+
+    def test_help_messages(self):
+        for command in self.commands:
+            _, out, _ = self.proc(command, "-h")
+            self.assertIn("Usage", out)
diff --git a/tests/test_crawler.py b/tests/test_crawler.py
index d67abed7c..4e9ef7740 100644
--- a/tests/test_crawler.py
+++ b/tests/test_crawler.py
@@ -311,6 +311,63 @@ class CrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
         self.assertIn('Spider closed (finished)', log)
         self.assertNotIn("Using reactor: twisted.internet.asyncioreactor.AsyncioSelectorReactor", log)
 
+    def test_multi(self):
+        log = self.run_script('multi.py')
+        self.assertIn('Spider closed (finished)', log)
+        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')
+        if platform.system() == 'Windows':
+            # The goal of this test function is to test that, when a reactor is
+            # installed (the default one here) and a different reactor is
+            # configured (select here), an error raises.
+            #
+            # In Windows the default reactor is the select reactor, so that
+            # error does not raise.
+            #
+            # If that ever becomes the case on more platforms (i.e. if Linux
+            # also starts using the select reactor by default in a future
+            # version of Twisted), then we will need to rethink this test.
+            self.assertIn('Spider closed (finished)', log)
+        else:
+            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_reactor_select_subclass_twisted_reactor_select(self):
+        log = self.run_script('reactor_select_subclass_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_asyncio_enabled_no_reactor(self):
         log = self.run_script('asyncio_enabled_no_reactor.py')
         self.assertIn('Spider closed (finished)', log)
@@ -343,33 +400,33 @@ class CrawlerProcessSubprocess(ScriptRunnerMixin, unittest.TestCase):
             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)
diff --git a/tests/test_downloadermiddleware_httpproxy.py b/tests/test_downloadermiddleware_httpproxy.py
index 4ac85c1ec..70eb94d77 100644
--- a/tests/test_downloadermiddleware_httpproxy.py
+++ b/tests/test_downloadermiddleware_httpproxy.py
@@ -65,12 +65,12 @@ class TestHttpProxyMiddleware(TestCase):
         mw = HttpProxyMiddleware()
         req = Request('http://scrapytest.org')
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic dXNlcjpwYXNz')
         # proxy from request.meta
         req = Request('http://scrapytest.org', meta={'proxy': 'https://username:password@proxy:3128'})
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic dXNlcm5hbWU6cGFzc3dvcmQ=')
 
     def test_proxy_auth_empty_passwd(self):
@@ -78,12 +78,12 @@ class TestHttpProxyMiddleware(TestCase):
         mw = HttpProxyMiddleware()
         req = Request('http://scrapytest.org')
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic dXNlcjo=')
         # proxy from request.meta
         req = Request('http://scrapytest.org', meta={'proxy': 'https://username:@proxy:3128'})
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic dXNlcm5hbWU6')
 
     def test_proxy_auth_encoding(self):
@@ -92,26 +92,26 @@ class TestHttpProxyMiddleware(TestCase):
         mw = HttpProxyMiddleware(auth_encoding='utf-8')
         req = Request('http://scrapytest.org')
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic bcOhbjpwYXNz')
 
         # proxy from request.meta
         req = Request('http://scrapytest.org', meta={'proxy': 'https://\u00FCser:pass@proxy:3128'})
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic w7xzZXI6cGFzcw==')
 
         # default latin-1 encoding
         mw = HttpProxyMiddleware(auth_encoding='latin-1')
         req = Request('http://scrapytest.org')
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic beFuOnBhc3M=')
 
         # proxy from request.meta, latin-1 encoding
         req = Request('http://scrapytest.org', meta={'proxy': 'https://\u00FCser:pass@proxy:3128'})
         assert mw.process_request(req, spider) is None
-        self.assertEqual(req.meta, {'proxy': 'https://proxy:3128'})
+        self.assertEqual(req.meta['proxy'], 'https://proxy:3128')
         self.assertEqual(req.headers.get('Proxy-Authorization'), b'Basic /HNlcjpwYXNz')
 
     def test_proxy_already_seted(self):
@@ -152,3 +152,300 @@ class TestHttpProxyMiddleware(TestCase):
         # '/var/run/docker.sock' may be used by the user for
         # no_proxy value but is not parseable and should be skipped
         assert 'no' not in mw.proxies
+
+    def test_add_proxy_without_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request('https://example.com')
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = 'https://example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_add_proxy_with_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request('https://example.com')
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = 'https://user1:password1@example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_remove_proxy_without_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = None
+        assert middleware.process_request(request, spider) is None
+        self.assertIsNone(request.meta['proxy'])
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_remove_proxy_with_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = None
+        assert middleware.process_request(request, spider) is None
+        self.assertIsNone(request.meta['proxy'])
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_add_credentials(self):
+        """If the proxy request meta switches to a proxy URL with the same
+        proxy and adds credentials (there were no credentials before), the new
+        credentials must be used."""
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+
+        request.meta['proxy'] = 'https://user1:password1@example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_change_credentials(self):
+        """If the proxy request meta switches to a proxy URL with different
+        credentials, those new credentials must be used."""
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = 'https://user2:password2@example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        encoded_credentials = middleware._basic_auth_header(
+            'user2',
+            'password2',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_remove_credentials(self):
+        """If the proxy request meta switches to a proxy URL with the same
+        proxy but no credentials, the original credentials must be still
+        used.
+
+        To remove credentials while keeping the same proxy URL, users must
+        delete the Proxy-Authorization header.
+        """
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+
+        request.meta['proxy'] = 'https://example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+        request.meta['proxy'] = 'https://example.com'
+        del request.headers[b'Proxy-Authorization']
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_change_proxy_add_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+
+        request.meta['proxy'] = 'https://user1:password1@example.org'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.org')
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_change_proxy_keep_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+
+        request.meta['proxy'] = 'https://user1:password1@example.org'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.org')
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+        # Make sure, indirectly, that _auth_proxy is updated.
+        request.meta['proxy'] = 'https://example.com'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_change_proxy_change_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+
+        request.meta['proxy'] = 'https://user2:password2@example.org'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.org')
+        encoded_credentials = middleware._basic_auth_header(
+            'user2',
+            'password2',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_change_proxy_remove_credentials(self):
+        """If the proxy request meta switches to a proxy URL with a different
+        proxy and no credentials, no credentials must be used."""
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = 'https://example.org'
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta, {'proxy': 'https://example.org'})
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_change_proxy_remove_credentials_preremoved_header(self):
+        """Corner case of proxy switch with credentials removal where the
+        credentials have been removed beforehand.
+
+        It ensures that our implementation does not assume that the credentials
+        header exists when trying to remove it.
+        """
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        request.meta['proxy'] = 'https://example.org'
+        del request.headers[b'Proxy-Authorization']
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta, {'proxy': 'https://example.org'})
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_proxy_authentication_header_undefined_proxy(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            headers={'Proxy-Authorization': 'Basic foo'},
+        )
+        assert middleware.process_request(request, spider) is None
+        self.assertNotIn('proxy', request.meta)
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_proxy_authentication_header_disabled_proxy(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            headers={'Proxy-Authorization': 'Basic foo'},
+            meta={'proxy': None},
+        )
+        assert middleware.process_request(request, spider) is None
+        self.assertIsNone(request.meta['proxy'])
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_proxy_authentication_header_proxy_without_credentials(self):
+        middleware = HttpProxyMiddleware()
+        request = Request(
+            'https://example.com',
+            headers={'Proxy-Authorization': 'Basic foo'},
+            meta={'proxy': 'https://example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        self.assertNotIn(b'Proxy-Authorization', request.headers)
+
+    def test_proxy_authentication_header_proxy_with_same_credentials(self):
+        middleware = HttpProxyMiddleware()
+        encoded_credentials = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        request = Request(
+            'https://example.com',
+            headers={'Proxy-Authorization': b'Basic ' + encoded_credentials},
+            meta={'proxy': 'https://user1:password1@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials,
+        )
+
+    def test_proxy_authentication_header_proxy_with_different_credentials(self):
+        middleware = HttpProxyMiddleware()
+        encoded_credentials1 = middleware._basic_auth_header(
+            'user1',
+            'password1',
+        )
+        request = Request(
+            'https://example.com',
+            headers={'Proxy-Authorization': b'Basic ' + encoded_credentials1},
+            meta={'proxy': 'https://user2:password2@example.com'},
+        )
+        assert middleware.process_request(request, spider) is None
+        self.assertEqual(request.meta['proxy'], 'https://example.com')
+        encoded_credentials2 = middleware._basic_auth_header(
+            'user2',
+            'password2',
+        )
+        self.assertEqual(
+            request.headers['Proxy-Authorization'],
+            b'Basic ' + encoded_credentials2,
+        )
diff --git a/tests/test_engine.py b/tests/test_engine.py
index 1bd802bcf..5677052f6 100644
--- a/tests/test_engine.py
+++ b/tests/test_engine.py
@@ -12,8 +12,10 @@ module with the ``runserver`` argument::
 
 import os
 import re
+import subprocess
 import sys
 from collections import defaultdict
+from threading import Timer
 from urllib.parse import urlparse
 from dataclasses import dataclass
 
@@ -484,6 +486,37 @@ class EngineTest(unittest.TestCase):
             yield e.close()
             self.assertTrue(e.has_capacity())
 
+    def test_short_timeout(self):
+        args = (
+            sys.executable,
+            '-m',
+            'scrapy.cmdline',
+            'fetch',
+            '-s',
+            'CLOSESPIDER_TIMEOUT=0.001',
+            '-s',
+            'LOG_LEVEL=DEBUG',
+            'http://toscrape.com',
+        )
+        p = subprocess.Popen(
+            args,
+            stderr=subprocess.PIPE,
+        )
+
+        def kill_proc():
+            p.kill()
+            p.communicate()
+            assert False, 'Command took too much time to complete'
+
+        timer = Timer(15, kill_proc)
+        try:
+            timer.start()
+            _, stderr = p.communicate()
+        finally:
+            timer.cancel()
+
+        self.assertNotIn(b'Traceback', stderr)
+
 
 if __name__ == "__main__":
     if len(sys.argv) > 1 and sys.argv[1] == 'runserver':
diff --git a/tox.ini b/tox.ini
index ab8a715c2..4d1bb574d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -47,7 +47,7 @@ commands =
 [testenv:security]
 basepython = python3
 deps =
-    bandit==1.7.3
+    bandit==1.7.4
 commands =
     bandit -r -c .bandit.yml {posargs:scrapy}
 
@@ -57,16 +57,17 @@ deps =
     {[testenv]deps}
     # Twisted[http2] is required to import some files
     Twisted[http2]>=17.9.0
-    pytest-flake8
-    flake8==3.9.2  # https://github.com/tholo/pytest-flake8/issues/81
+    pytest-flake8==1.1.1
+    flake8==4.0.1
 commands =
     pytest --flake8 {posargs:docs scrapy tests}
 
 [testenv:pylint]
-basepython = python3
+# reppy does not support Python 3.9+
+basepython = python3.8
 deps =
     {[testenv:extra-deps]deps}
-    pylint==2.12.2
+    pylint==2.14.5
 commands =
     pylint conftest.py docs extras scrapy setup.py tests
 
@@ -117,6 +118,8 @@ setenv =
     {[pinned]setenv}
 
 [testenv:extra-deps]
+# reppy does not support Python 3.9+
+basepython = python3.8
 deps =
     {[testenv]deps}
     boto