The change in D18776 to improve handling of non-utf8 HTML parts broke handling of mail with no HTML parts. Partly, this is because MimeMailParser has a "traditional" PHP-style API where the return type is an exciting surprise.
- Sent a text-only message in Mail.app.
- Used "Show Raw" to copy it to mail.txt, verifying that the raw message contains ONLY a text body.
- Ran cat mail.txt | ./scripts/mail/mail_handler.php --trace --process-duplicates.
- Before patch: error about bad idx() on a non-array.
- After patch: clean mail processing.
- Did the same with a message with both HTML and text bodies to make sure I didn't break anything.
Ideally we'd probably get test coverage on this, but it's been touched roughly once a year since 2013 so it'll probably hold.