from_email_message crashes on non-MIMEBase email.message.Message attachments

[jefftriplett]

RelayEmailData.from_email_message raises AttributeError: 'int' object has no attribute 'lower' when an attachment is a stdlib email.message.Message that is not a MIMEBase subclass.

This is currently the single largest error source in our Sentry org — 465,554 occurrences (LEASE-BACKEND-73, Seer actionability super_high), with a second instance in a sibling task (LEASE-BACKEND-72).

Cause

src/email_relay/email.py branches on isinstance(attachment, MIMEBase):

for attachment in email_message.attachments:
    if isinstance(attachment, MIMEBase):
        ...
    else:
        if isinstance(attachment[1], bytes):   # <-- assumes a tuple

The else branch assumes a (filename, content, mimetype) tuple and subscripts it. MIMEBase is a subclass of email.message.Message, but the reverse is not true — a plain email.message.Message (or email.message.EmailMessage) fails the isinstance check, falls into else, and attachment[1] invokes Message.__getitem__(1), which does name.lower() on the int 1:

  File "email_relay/email.py", line 86, in from_email_message
    if isinstance(attachment[1], bytes):
  File "email/message.py", line 429, in __getitem__
    return self.get(name)
  File "email/message.py", line 510, in get
    name = name.lower()
AttributeError: 'int' object has no attribute 'lower'

Because Message defines __getitem__, this fails deep inside stdlib with a confusing error instead of a clear type error at the boundary.

Suggested fix

Widen the isinstance check to the base class, so every Message subclass takes the message branch:

from email.message import Message

if isinstance(attachment, Message):
    ...

MIMEBase instances still match, and get_payload/get_filename/get_content_type are all defined on Message, so the existing branch body should work unchanged. Worth considering an explicit else guard that raises a clear TypeError when the attachment is neither a Message nor a 3-tuple, so a future mismatch fails legibly.

Reproducing

Attach a plain email.message.Message (not a MIMEBase subclass) to a Django EmailMessage and send it through the relay backend.

Notes

Observed on CPython 3.12.13, environment: production, via django_q workers. Reported from the Sentry issue above; the calling code is in westerveltco/lease (lease/clubs/tasks.py:268), but the defect is here.

[joshuadavidthomas]

#303 is my attempt at fixing this.