What does "prepare article headers" mean in message-mode?

What does "prepare article headers" mean in message-mode?

[Vladimir Nikishkin]

Hello, everyone.

The docstring for `message-generate-headers' says:
`"Prepare article HEADERS.
Headers already prepared in the buffer are not modified."`

When I execute `(message-generate-headers '(Date Message-ID))`, for
example right while writing this message to help-gnu-emacs, both Date
and Message-ID are modified.

Apparently, "prepare" does not really mean "generate and insert".
What does it really mean?

Emacs 27.1

-- 
Vladimir Nikishkin (MiEr, lockywolf)
(Laptop)

Re: What does "prepare article headers" mean in message-mode?

[Eric Abrahamsen]

Vladimir Nikishkin <lockywolf@gmail.com> writes:

> Hello, everyone.
>
> The docstring for `message-generate-headers' says:
> `"Prepare article HEADERS.
> Headers already prepared in the buffer are not modified."`
>
> When I execute `(message-generate-headers '(Date Message-ID))`, for
> example right while writing this message to help-gnu-emacs, both Date
> and Message-ID are modified.
>
> Apparently, "prepare" does not really mean "generate and insert".
> What does it really mean?

It almost means that, except for the headers contained in
`message-deletable-headers' which, as the name implies, message-mode
thinks is best to trash re-create as necessary. That variable contains
'(Message-ID Date Lines), which explains what you're seeing. I assume
it's because they ought be regenerated to reflect the newest state of
the message buffer.

Message-ID won't be overwritten if the buffer is marked as modified, if
that helps.

I'm not claiming to fully understand the whys and wherefores of this,
just saying this is how it works.

Eric