All this Interactor business has been gradually working towards being
able to inform the user _which_ network connection is currently
presenting them with a password prompt (or whatever), in situations
where more than one of them might be, such as an SSH connection being
used as a proxy for another SSH connection when neither one has
one-touch login configured.
At some point, we have to arrange that any attempt to do a user
interaction during connection setup - be it a password prompt, a host
key confirmation dialog, or just displaying an SSH login banner -
makes it clear which host it's come from. That's going to mean calling
some kind of announcement function before doing any of those things.
But there are several of those functions in the Seat API, and calls to
them are scattered far and wide across the SSH backend. (And not even
just there - the Rlogin backend also uses seat_get_userpass_input).
How can we possibly make sure we don't forget a vital call site on
some obscure little-tested code path, and leave the user confused in
just that one case which nobody might notice for years?
Today I thought of a trick to solve that problem. We can use the C
type system to enforce it for us!
The plan is: we invent a new struct type which contains nothing but a
'Seat *'. Then, for every Seat method which does a thing that ought to
be clearly identified as relating to a particular Interactor, we
adjust the API for that function to take the new struct type where it
previously took a plain 'Seat *'. Or rather - doing less violence to
the existing code - we only need to adjust the API of the dispatch
functions inline in putty.h.
How does that help? Because the way you _get_ one of these
struct-wrapped Seat pointers is by calling interactor_announce() on
your Interactor, which will in turn call interactor_get_seat(), and
wrap the returned pointer into one of these structs.
The effect is that whenever the SSH (or Rlogin) code wants to call one
of those particular Seat methods, it _has_ to call
interactor_announce() just beforehand, which (once I finish all of
this) will make sure the user is aware of who is presenting the prompt
or banner or whatever. And you can't forget to call it, because if you
don't call it, then you just don't have a struct of the right type to
give to the Seat method you wanted to call!
(Of course, there's nothing stopping code from _deliberately_ taking a
Seat * it already has and wrapping it into the new struct. In fact
SshProxy has to do that, in order to forward these requests up the
chain of Seats. But the point is that you can't do it _by accident_,
just by forgetting to make a vital function call - when you do that,
you _know_ you're doing it on purpose.)
No functional change: the new interactor_announce() function exists,
and the type-system trick ensures it's called in all the right places,
but it doesn't actually _do_ anything yet.
Thanks to the previous commit, this new parameter can replace two of
the existing ones: instead of passing a LogPolicy and a Seat, we now
pass just an Interactor, from which any proxy implementation can
extract the LogPolicy and the Seat anyway if they need it.
Previously, SSH authentication banners were displayed by calling the
ordinary seat_output function, and passing it a special value in the
SeatOutputType enumeration indicating an auth banner.
The awkwardness of this was already showing a little in SshProxy's
implementation of seat_output, where it had to check for that special
value and do totally different things for SEAT_OUTPUT_AUTH_BANNER and
everything else. Further work in that area is going to make it more
and more awkward if I keep the two output systems unified.
So let's split them up. Now, Seat has separate output() and banner()
methods, which each implementation can override differently if it
wants to.
All the 'end user' Seat implementations use the centralised
implementation function nullseat_banner_to_stderr(), which turns
banner text straight back into SEAT_OUTPUT_STDERR and passes it on to
seat_output. So I didn't have to tediously implement a boring version
of this function in GTK, Windows GUI, consoles, file transfer etc.
There are quite a few of them already, and I'm about to make another
one, so let's start with a bit of tidying up.
The CMake build organisation is unchanged: I haven't put the proxy
object files into a separate library, just moved the locations of the
source files. (Organising proxying as a library would be tricky
anyway, because of the various overrides for tools that want to avoid
cryptography.)
