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Email::MIME::Kit v3 will fix-and-or-break your code

by rjbs, created 2014-11-20 21:47
last modified 2014-11-20 22:21
tagged with: @markup:md journal perl programming

Ever since its early releases, Email::MIME::Kit had a big problem. It screwed up encodings. Specifically, imagine this manifest (I'm kinda skipping some required junk):

# manifest.yaml
renderer: TemplateToolkit
  - Subject: "Message for [% name %]"
  - type: text/plain
    path: body.txt
  - type: text/html
    path: body.html

The manifest turns into a data structure before it's used, and the subject header is a text string that, later, will get encoded into MIME encoded-words on the assumption that it's all Unicode text.

The files on disk are read with :raw, then filled in as-is, and trusted to already be UTF-8.

If your customer's name is Распутин, strangely enough, you're okay. The header handling encodes it properly, and the wide characters (because Cyrillic codepoints are all above U+00FF) turn into UTF-8 with a warning. On the other hand, for some trouble, consider Ævar Arnfjörð Bjarmason. All those codepoints are below U+0100, so the non-ASCII ones are encoded directly, and you end up with =C6 (Æ) in your quoted-printable body instead of =C3=86 (Æ UTF-8 encoded).

Now, you're probably actually okay. Your email is not correct, but email clients are good at dealing with your (read: my) stupid mistakes. If your email part says it's UTF-8 but it's actually Latin-1, mail clients will usually do the right thing.

The big problem is when you've got both Ævar Arnfjörð Bjarmason and Распутин both in your email. Your body is a mish mash of Latin-1 and UTF-8 data.

In Email::MIME::Kit v3, templates (or non-template bodies) loaded from disk are — if and only if they're for text/* parts — decoded into text and then, when the email is assembled, it's encoded by Email::MIME's usual header_str handling.

There's a case where this can start making things worse, rather than better. If you know that templates in files are treated as bytes, you might be passing in strings pre-encoded into UTF-8. If that was the case, it will now become mojibake.

Finally, plugins that read kit contents for uses as text will need upgrading. The only one I know of like this is my own Email::MIME::Kit::Assembler::Markdown. I will fix it. The trick is: look at what content-type is being built and consider using get_decoded_kit_entry instead of get_kit_entry.

I think this is an important change, and worth the breakage. Please look at your use of EMK and test with v3.