Implementing a cancelable task

[Alessio Vanni]

Hello,

I have a service doing the following:

* waits for a client to send a specific message
* when the message arrives it opens a CADET channel
* this channel is left open until the client requests it to be closed
* the client sends another specific message
* when the message arrives it is "unpacked" and sent over the channel
* the remote peer replies with some data
* when the data arrives it is "packed" and sent to the client

I'd like to make the parts where the client waits for the service
"cancelable tasks", i.e. have a "cancel" function that stops any further
operations on the canceled message from being executed.  Basically
something like `GNUNET_SCHEDULER_cancel' or what the GNS service
provides.

I've peeked at the implementation of some of those functions, but there
are few things troubling me (might be because I don't know how the code
works, but still.)

The first thing I saw was that those tasks use a unique ID embedded in
the message. I tried something like that, but the CADET part doesn't
make it practical, as I'd have to send the ID to the remote peer and
hope that said peer doesn't change its value and/or use it for some
nefarious purpose.

Ultimately the problem is that I don't have a reliable way to associate
a task with the execution of a certain callback.

I was hoping someone could provide some help on the matter, especially
since GNUnet has a lot of those cancelable tasks.

Thank you,
A.V.

P.S. I tried subscribing to this list, but I can't tell if it succeeded
or not. If it requires a mail confirmation, I didn't get anything after
requesting the subscription.  This is actually the second time I tried
and still nothing.  In this second attempt it might simply be a late
delivery, but the first time I got nothing and I didn't receive anything
of what was sent to the list afterwards.  I can receive mails from other
sources just fine (though it seems some places can't receive from me) so
I really don't know what's going on.