We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
_sent_failed cut the errback chain in MailSender
`MailSender._sent_failed` return `None`, instead of `failure`.  This cut the errback call chain, making impossible to detect in the code fail in the mails in client code.


--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `scrapy/mail.py`
Content:
```
1 """
2 Mail sending helpers
3 
4 See documentation in docs/topics/email.rst
5 """
6 import logging
7 from email import encoders as Encoders
8 from email.mime.base import MIMEBase
9 from email.mime.multipart import MIMEMultipart
10 from email.mime.nonmultipart import MIMENonMultipart
11 from email.mime.text import MIMEText
12 from email.utils import formatdate
13 from io import BytesIO
14 
15 from twisted import version as twisted_version
16 from twisted.internet import defer, ssl
17 from twisted.python.versions import Version
18 
19 from scrapy.utils.misc import arg_to_iter
20 from scrapy.utils.python import to_bytes
21 
22 logger = logging.getLogger(__name__)
23 
24 
25 # Defined in the email.utils module, but undocumented:
26 # https://github.com/python/cpython/blob/v3.9.0/Lib/email/utils.py#L42
27 COMMASPACE = ", "
28 
29 
30 def _to_bytes_or_none(text):
31     if text is None:
32         return None
33     return to_bytes(text)
34 
35 
36 class MailSender:
37     def __init__(
38         self,
39         smtphost="localhost",
40         mailfrom="scrapy@localhost",
41         smtpuser=None,
42         smtppass=None,
43         smtpport=25,
44         smtptls=False,
45         smtpssl=False,
46         debug=False,
47     ):
48         self.smtphost = smtphost
49         self.smtpport = smtpport
50         self.smtpuser = _to_bytes_or_none(smtpuser)
51         self.smtppass = _to_bytes_or_none(smtppass)
52         self.smtptls = smtptls
53         self.smtpssl = smtpssl
54         self.mailfrom = mailfrom
55         self.debug = debug
56 
57     @classmethod
58     def from_settings(cls, settings):
59         return cls(
60             smtphost=settings["MAIL_HOST"],
61             mailfrom=settings["MAIL_FROM"],
62             smtpuser=settings["MAIL_USER"],
63             smtppass=settings["MAIL_PASS"],
64             smtpport=settings.getint("MAIL_PORT"),
65             smtptls=settings.getbool("MAIL_TLS"),
66             smtpssl=settings.getbool("MAIL_SSL"),
67         )
68 
69     def send(
70         self,
71         to,
72         subject,
73         body,
74         cc=None,
75         attachs=(),
76         mimetype="text/plain",
77         charset=None,
78         _callback=None,
79     ):
80         from twisted.internet import reactor
81 
82         if attachs:
83             msg = MIMEMultipart()
84         else:
85             msg = MIMENonMultipart(*mimetype.split("/", 1))
86 
87         to = list(arg_to_iter(to))
88         cc = list(arg_to_iter(cc))
89 
90         msg["From"] = self.mailfrom
91         msg["To"] = COMMASPACE.join(to)
92         msg["Date"] = formatdate(localtime=True)
93         msg["Subject"] = subject
94         rcpts = to[:]
95         if cc:
96             rcpts.extend(cc)
97             msg["Cc"] = COMMASPACE.join(cc)
98 
99         if charset:
100             msg.set_charset(charset)
101 
102         if attachs:
103             msg.attach(MIMEText(body, "plain", charset or "us-ascii"))
104             for attach_name, mimetype, f in attachs:
105                 part = MIMEBase(*mimetype.split("/"))
106                 part.set_payload(f.read())
107                 Encoders.encode_base64(part)
108                 part.add_header(
109                     "Content-Disposition", "attachment", filename=attach_name
110                 )
111                 msg.attach(part)
112         else:
113             msg.set_payload(body)
114 
115         if _callback:
116             _callback(to=to, subject=subject, body=body, cc=cc, attach=attachs, msg=msg)
117 
118         if self.debug:
119             logger.debug(
120                 "Debug mail sent OK: To=%(mailto)s Cc=%(mailcc)s "
121                 'Subject="%(mailsubject)s" Attachs=%(mailattachs)d',
122                 {
123                     "mailto": to,
124                     "mailcc": cc,
125                     "mailsubject": subject,
126                     "mailattachs": len(attachs),
127                 },
128             )
129             return
130 
131         dfd = self._sendmail(rcpts, msg.as_string().encode(charset or "utf-8"))
132         dfd.addCallbacks(
133             callback=self._sent_ok,
134             errback=self._sent_failed,
135             callbackArgs=[to, cc, subject, len(attachs)],
136             errbackArgs=[to, cc, subject, len(attachs)],
137         )
138         reactor.addSystemEventTrigger("before", "shutdown", lambda: dfd)
139         return dfd
140 
141     def _sent_ok(self, result, to, cc, subject, nattachs):
142         logger.info(
143             "Mail sent OK: To=%(mailto)s Cc=%(mailcc)s "
144             'Subject="%(mailsubject)s" Attachs=%(mailattachs)d',
145             {
146                 "mailto": to,
147                 "mailcc": cc,
148                 "mailsubject": subject,
149                 "mailattachs": nattachs,
150             },
151         )
152 
153     def _sent_failed(self, failure, to, cc, subject, nattachs):
154         errstr = str(failure.value)
155         logger.error(
156             "Unable to send mail: To=%(mailto)s Cc=%(mailcc)s "
157             'Subject="%(mailsubject)s" Attachs=%(mailattachs)d'
158             "- %(mailerr)s",
159             {
160                 "mailto": to,
161                 "mailcc": cc,
162                 "mailsubject": subject,
163                 "mailattachs": nattachs,
164                 "mailerr": errstr,
165             },
166         )
167 
168     def _sendmail(self, to_addrs, msg):
169         from twisted.internet import reactor
170 
171         msg = BytesIO(msg)
172         d = defer.Deferred()
173 
174         factory = self._create_sender_factory(to_addrs, msg, d)
175 
176         if self.smtpssl:
177             reactor.connectSSL(
178                 self.smtphost, self.smtpport, factory, ssl.ClientContextFactory()
179             )
180         else:
181             reactor.connectTCP(self.smtphost, self.smtpport, factory)
182 
183         return d
184 
185     def _create_sender_factory(self, to_addrs, msg, d):
186         from twisted.mail.smtp import ESMTPSenderFactory
187 
188         factory_keywords = {
189             "heloFallback": True,
190             "requireAuthentication": False,
191             "requireTransportSecurity": self.smtptls,
192         }
193 
194         # Newer versions of twisted require the hostname to use STARTTLS
195         if twisted_version >= Version("twisted", 21, 2, 0):
196             factory_keywords["hostname"] = self.smtphost
197 
198         factory = ESMTPSenderFactory(
199             self.smtpuser,
200             self.smtppass,
201             self.mailfrom,
202             to_addrs,
203             msg,
204             d,
205             **factory_keywords
206         )
207         factory.noisy = False
208         return factory
209
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/scrapy/mail.py b/scrapy/mail.py
--- a/scrapy/mail.py
+++ b/scrapy/mail.py
@@ -164,6 +164,7 @@
                 "mailerr": errstr,
             },
         )
+        return failure
 
     def _sendmail(self, to_addrs, msg):
         from twisted.internet import reactor

{"golden_diff": "diff --git a/scrapy/mail.py b/scrapy/mail.py\n--- a/scrapy/mail.py\n+++ b/scrapy/mail.py\n@@ -164,6 +164,7 @@\n                 \"mailerr\": errstr,\n             },\n         )\n+        return failure\n \n     def _sendmail(self, to_addrs, msg):\n         from twisted.internet import reactor\n", "issue": "_sent_failed cut the errback chain in MailSender\n`MailSender._sent_failed` return `None`, instead of `failure`.  This cut the errback call chain, making impossible to detect in the code fail in the mails in client code.\n\n", "before_files": [{"content": "\"\"\"\nMail sending helpers\n\nSee documentation in docs/topics/email.rst\n\"\"\"\nimport logging\nfrom email import encoders as Encoders\nfrom email.mime.base import MIMEBase\nfrom email.mime.multipart import MIMEMultipart\nfrom email.mime.nonmultipart import MIMENonMultipart\nfrom email.mime.text import MIMEText\nfrom email.utils import formatdate\nfrom io import BytesIO\n\nfrom twisted import version as twisted_version\nfrom twisted.internet import defer, ssl\nfrom twisted.python.versions import Version\n\nfrom scrapy.utils.misc import arg_to_iter\nfrom scrapy.utils.python import to_bytes\n\nlogger = logging.getLogger(__name__)\n\n\n# Defined in the email.utils module, but undocumented:\n# https://github.com/python/cpython/blob/v3.9.0/Lib/email/utils.py#L42\nCOMMASPACE = \", \"\n\n\ndef _to_bytes_or_none(text):\n    if text is None:\n        return None\n    return to_bytes(text)\n\n\nclass MailSender:\n    def __init__(\n        self,\n        smtphost=\"localhost\",\n        mailfrom=\"scrapy@localhost\",\n        smtpuser=None,\n        smtppass=None,\n        smtpport=25,\n        smtptls=False,\n        smtpssl=False,\n        debug=False,\n    ):\n        self.smtphost = smtphost\n        self.smtpport = smtpport\n        self.smtpuser = _to_bytes_or_none(smtpuser)\n        self.smtppass = _to_bytes_or_none(smtppass)\n        self.smtptls = smtptls\n        self.smtpssl = smtpssl\n        self.mailfrom = mailfrom\n        self.debug = debug\n\n    @classmethod\n    def from_settings(cls, settings):\n        return cls(\n            smtphost=settings[\"MAIL_HOST\"],\n            mailfrom=settings[\"MAIL_FROM\"],\n            smtpuser=settings[\"MAIL_USER\"],\n            smtppass=settings[\"MAIL_PASS\"],\n            smtpport=settings.getint(\"MAIL_PORT\"),\n            smtptls=settings.getbool(\"MAIL_TLS\"),\n            smtpssl=settings.getbool(\"MAIL_SSL\"),\n        )\n\n    def send(\n        self,\n        to,\n        subject,\n        body,\n        cc=None,\n        attachs=(),\n        mimetype=\"text/plain\",\n        charset=None,\n        _callback=None,\n    ):\n        from twisted.internet import reactor\n\n        if attachs:\n            msg = MIMEMultipart()\n        else:\n            msg = MIMENonMultipart(*mimetype.split(\"/\", 1))\n\n        to = list(arg_to_iter(to))\n        cc = list(arg_to_iter(cc))\n\n        msg[\"From\"] = self.mailfrom\n        msg[\"To\"] = COMMASPACE.join(to)\n        msg[\"Date\"] = formatdate(localtime=True)\n        msg[\"Subject\"] = subject\n        rcpts = to[:]\n        if cc:\n            rcpts.extend(cc)\n            msg[\"Cc\"] = COMMASPACE.join(cc)\n\n        if charset:\n            msg.set_charset(charset)\n\n        if attachs:\n            msg.attach(MIMEText(body, \"plain\", charset or \"us-ascii\"))\n            for attach_name, mimetype, f in attachs:\n                part = MIMEBase(*mimetype.split(\"/\"))\n                part.set_payload(f.read())\n                Encoders.encode_base64(part)\n                part.add_header(\n                    \"Content-Disposition\", \"attachment\", filename=attach_name\n                )\n                msg.attach(part)\n        else:\n            msg.set_payload(body)\n\n        if _callback:\n            _callback(to=to, subject=subject, body=body, cc=cc, attach=attachs, msg=msg)\n\n        if self.debug:\n            logger.debug(\n                \"Debug mail sent OK: To=%(mailto)s Cc=%(mailcc)s \"\n                'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d',\n                {\n                    \"mailto\": to,\n                    \"mailcc\": cc,\n                    \"mailsubject\": subject,\n                    \"mailattachs\": len(attachs),\n                },\n            )\n            return\n\n        dfd = self._sendmail(rcpts, msg.as_string().encode(charset or \"utf-8\"))\n        dfd.addCallbacks(\n            callback=self._sent_ok,\n            errback=self._sent_failed,\n            callbackArgs=[to, cc, subject, len(attachs)],\n            errbackArgs=[to, cc, subject, len(attachs)],\n        )\n        reactor.addSystemEventTrigger(\"before\", \"shutdown\", lambda: dfd)\n        return dfd\n\n    def _sent_ok(self, result, to, cc, subject, nattachs):\n        logger.info(\n            \"Mail sent OK: To=%(mailto)s Cc=%(mailcc)s \"\n            'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d',\n            {\n                \"mailto\": to,\n                \"mailcc\": cc,\n                \"mailsubject\": subject,\n                \"mailattachs\": nattachs,\n            },\n        )\n\n    def _sent_failed(self, failure, to, cc, subject, nattachs):\n        errstr = str(failure.value)\n        logger.error(\n            \"Unable to send mail: To=%(mailto)s Cc=%(mailcc)s \"\n            'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d'\n            \"- %(mailerr)s\",\n            {\n                \"mailto\": to,\n                \"mailcc\": cc,\n                \"mailsubject\": subject,\n                \"mailattachs\": nattachs,\n                \"mailerr\": errstr,\n            },\n        )\n\n    def _sendmail(self, to_addrs, msg):\n        from twisted.internet import reactor\n\n        msg = BytesIO(msg)\n        d = defer.Deferred()\n\n        factory = self._create_sender_factory(to_addrs, msg, d)\n\n        if self.smtpssl:\n            reactor.connectSSL(\n                self.smtphost, self.smtpport, factory, ssl.ClientContextFactory()\n            )\n        else:\n            reactor.connectTCP(self.smtphost, self.smtpport, factory)\n\n        return d\n\n    def _create_sender_factory(self, to_addrs, msg, d):\n        from twisted.mail.smtp import ESMTPSenderFactory\n\n        factory_keywords = {\n            \"heloFallback\": True,\n            \"requireAuthentication\": False,\n            \"requireTransportSecurity\": self.smtptls,\n        }\n\n        # Newer versions of twisted require the hostname to use STARTTLS\n        if twisted_version >= Version(\"twisted\", 21, 2, 0):\n            factory_keywords[\"hostname\"] = self.smtphost\n\n        factory = ESMTPSenderFactory(\n            self.smtpuser,\n            self.smtppass,\n            self.mailfrom,\n            to_addrs,\n            msg,\n            d,\n            **factory_keywords\n        )\n        factory.noisy = False\n        return factory\n", "path": "scrapy/mail.py"}], "after_files": [{"content": "\"\"\"\nMail sending helpers\n\nSee documentation in docs/topics/email.rst\n\"\"\"\nimport logging\nfrom email import encoders as Encoders\nfrom email.mime.base import MIMEBase\nfrom email.mime.multipart import MIMEMultipart\nfrom email.mime.nonmultipart import MIMENonMultipart\nfrom email.mime.text import MIMEText\nfrom email.utils import formatdate\nfrom io import BytesIO\n\nfrom twisted import version as twisted_version\nfrom twisted.internet import defer, ssl\nfrom twisted.python.versions import Version\n\nfrom scrapy.utils.misc import arg_to_iter\nfrom scrapy.utils.python import to_bytes\n\nlogger = logging.getLogger(__name__)\n\n\n# Defined in the email.utils module, but undocumented:\n# https://github.com/python/cpython/blob/v3.9.0/Lib/email/utils.py#L42\nCOMMASPACE = \", \"\n\n\ndef _to_bytes_or_none(text):\n    if text is None:\n        return None\n    return to_bytes(text)\n\n\nclass MailSender:\n    def __init__(\n        self,\n        smtphost=\"localhost\",\n        mailfrom=\"scrapy@localhost\",\n        smtpuser=None,\n        smtppass=None,\n        smtpport=25,\n        smtptls=False,\n        smtpssl=False,\n        debug=False,\n    ):\n        self.smtphost = smtphost\n        self.smtpport = smtpport\n        self.smtpuser = _to_bytes_or_none(smtpuser)\n        self.smtppass = _to_bytes_or_none(smtppass)\n        self.smtptls = smtptls\n        self.smtpssl = smtpssl\n        self.mailfrom = mailfrom\n        self.debug = debug\n\n    @classmethod\n    def from_settings(cls, settings):\n        return cls(\n            smtphost=settings[\"MAIL_HOST\"],\n            mailfrom=settings[\"MAIL_FROM\"],\n            smtpuser=settings[\"MAIL_USER\"],\n            smtppass=settings[\"MAIL_PASS\"],\n            smtpport=settings.getint(\"MAIL_PORT\"),\n            smtptls=settings.getbool(\"MAIL_TLS\"),\n            smtpssl=settings.getbool(\"MAIL_SSL\"),\n        )\n\n    def send(\n        self,\n        to,\n        subject,\n        body,\n        cc=None,\n        attachs=(),\n        mimetype=\"text/plain\",\n        charset=None,\n        _callback=None,\n    ):\n        from twisted.internet import reactor\n\n        if attachs:\n            msg = MIMEMultipart()\n        else:\n            msg = MIMENonMultipart(*mimetype.split(\"/\", 1))\n\n        to = list(arg_to_iter(to))\n        cc = list(arg_to_iter(cc))\n\n        msg[\"From\"] = self.mailfrom\n        msg[\"To\"] = COMMASPACE.join(to)\n        msg[\"Date\"] = formatdate(localtime=True)\n        msg[\"Subject\"] = subject\n        rcpts = to[:]\n        if cc:\n            rcpts.extend(cc)\n            msg[\"Cc\"] = COMMASPACE.join(cc)\n\n        if charset:\n            msg.set_charset(charset)\n\n        if attachs:\n            msg.attach(MIMEText(body, \"plain\", charset or \"us-ascii\"))\n            for attach_name, mimetype, f in attachs:\n                part = MIMEBase(*mimetype.split(\"/\"))\n                part.set_payload(f.read())\n                Encoders.encode_base64(part)\n                part.add_header(\n                    \"Content-Disposition\", \"attachment\", filename=attach_name\n                )\n                msg.attach(part)\n        else:\n            msg.set_payload(body)\n\n        if _callback:\n            _callback(to=to, subject=subject, body=body, cc=cc, attach=attachs, msg=msg)\n\n        if self.debug:\n            logger.debug(\n                \"Debug mail sent OK: To=%(mailto)s Cc=%(mailcc)s \"\n                'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d',\n                {\n                    \"mailto\": to,\n                    \"mailcc\": cc,\n                    \"mailsubject\": subject,\n                    \"mailattachs\": len(attachs),\n                },\n            )\n            return\n\n        dfd = self._sendmail(rcpts, msg.as_string().encode(charset or \"utf-8\"))\n        dfd.addCallbacks(\n            callback=self._sent_ok,\n            errback=self._sent_failed,\n            callbackArgs=[to, cc, subject, len(attachs)],\n            errbackArgs=[to, cc, subject, len(attachs)],\n        )\n        reactor.addSystemEventTrigger(\"before\", \"shutdown\", lambda: dfd)\n        return dfd\n\n    def _sent_ok(self, result, to, cc, subject, nattachs):\n        logger.info(\n            \"Mail sent OK: To=%(mailto)s Cc=%(mailcc)s \"\n            'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d',\n            {\n                \"mailto\": to,\n                \"mailcc\": cc,\n                \"mailsubject\": subject,\n                \"mailattachs\": nattachs,\n            },\n        )\n\n    def _sent_failed(self, failure, to, cc, subject, nattachs):\n        errstr = str(failure.value)\n        logger.error(\n            \"Unable to send mail: To=%(mailto)s Cc=%(mailcc)s \"\n            'Subject=\"%(mailsubject)s\" Attachs=%(mailattachs)d'\n            \"- %(mailerr)s\",\n            {\n                \"mailto\": to,\n                \"mailcc\": cc,\n                \"mailsubject\": subject,\n                \"mailattachs\": nattachs,\n                \"mailerr\": errstr,\n            },\n        )\n        return failure\n\n    def _sendmail(self, to_addrs, msg):\n        from twisted.internet import reactor\n\n        msg = BytesIO(msg)\n        d = defer.Deferred()\n\n        factory = self._create_sender_factory(to_addrs, msg, d)\n\n        if self.smtpssl:\n            reactor.connectSSL(\n                self.smtphost, self.smtpport, factory, ssl.ClientContextFactory()\n            )\n        else:\n            reactor.connectTCP(self.smtphost, self.smtpport, factory)\n\n        return d\n\n    def _create_sender_factory(self, to_addrs, msg, d):\n        from twisted.mail.smtp import ESMTPSenderFactory\n\n        factory_keywords = {\n            \"heloFallback\": True,\n            \"requireAuthentication\": False,\n            \"requireTransportSecurity\": self.smtptls,\n        }\n\n        # Newer versions of twisted require the hostname to use STARTTLS\n        if twisted_version >= Version(\"twisted\", 21, 2, 0):\n            factory_keywords[\"hostname\"] = self.smtphost\n\n        factory = ESMTPSenderFactory(\n            self.smtpuser,\n            self.smtppass,\n            self.mailfrom,\n            to_addrs,\n            msg,\n            d,\n            **factory_keywords\n        )\n        factory.noisy = False\n        return factory\n", "path": "scrapy/mail.py"}]}