bug#39506: patch

[dick . r . chiang]

> The focus of the sentence is on "before": the previous code already set the
> buffer to unibyte, but it did it afterwards.

Ah, so the buffer in question is merely the working temp-buffer, not the
handle-buffer.

Indeed, the previous code did it afterwards, and therein lies my salvation
because insert-buffer-substring of multibytes to a unibyte buffer corrupts.
I think the Miles Bader code replicated the multibyteness of the
handle-buffer to the temp-buffer, called insert-buffer-substring, and then
converted the temp-buffer to unibyte.

> - How does `mm-with-part` relate to `mm-shr`?

mm-shr calls mm-with-part.

> - Before deciding whether unibyte or multibyte is the right choice, the
>   main question is whether the buffer contains bytes or chars.

My buffer contained some Chinese multibytes.  You can see my unit test in the
patch.

>   AFAIK `mm-with-part` should only ever handle bytes (otherwise calling
>   `mm-decode-content-transfer-encoding` doesn't make much sense).

Okay, maybe mm-decode-content-transfer-encoding is a noop when it doesn't make
sense.

I'm not completely on top of all this, but my individual use case rather
prefers the old Miles Bader order of ops.  I can easily work around it if you 
feel
your 2008 change genuinely fixed something that was broken (my patch is largely
speculative and inconsiderate of broader ramifications).