bug#38011: 27.0.50; [PATCH] WIP on allowing Gnus backends to return header data directly

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Eric Abrahamsen <eric@ericabrahamsen.net> writes:
>
>> This patch provides the possibility for backends to return their own
>> headers (ie a list of vectors), though it doesn't actually change any of
>> the backends to do that -- that will be another patch.
>
> Great!
>
>> I have one question at this stage: the 'nov or 'headers value gets
>> stored into the `gnus-headers-retrieved-by' variable. That variable is
>> later checked in a couple of places like so:
>>
>>       (when (and gnus-fetch-old-headers
>> 		 (eq gnus-headers-retrieved-by 'nov))
>> 	(if (eq gnus-fetch-old-headers 'invisible)
>> 	    (gnus-build-all-threads)
>> 	  (gnus-build-old-threads)))
>>
>> If the variable is 'headers, the `gnus-build-*-threads' functions don't
>> get called at all.
>>
>> What's the difference between 'nov and 'headers, and why can we build
>> threads in one case and not the other? If backends were to return their
>> own headers, what value should they return?
>
> It's not about threading per se, but about displaying information about
> already-read articles (or more precisely -- about articles not in the
> set that was requested).  Threads are build no matter how the backend
> delivers the data.
>
> If nn-*retrieve-headers supports NOV fetching, then certain Gnus
> variables about filling in threads with "old" articles is switched on,
> because fetching extra NOV headers is fast (you just say "fetch 100-150"
> instead of "fetch 100-110,120-130,150").  With the backends that fetch
> head by head, this is slow, so it's not done.
>
> It's basically a distinction between NNTP and almost all the other
> backends.

Okay, that makes sense. So it's basically a flag saying header retrieval
is cheap enough that we might as well pull in more old messages than we
otherwise would. For example, nnmaildir builds and stores nov data
(which drives people with large maildirs insane because it takes
enormous amounts of time and space), so it returns 'nov, but if we ever
got rid of that (or made it optional) it would return 'headers.

I'm going to assume that any backend capable of returning its own
headers is probably going to... return 'headers. At any rate, as I
implement this for various backends, I'll start with the ones that
return 'headers to begin with. At some point, though, I'd still like to
get rid of this flag and build the distinction into the backend
functions themselves.

Hmmm... nnmaildir needs some love.

Eric