I think this is a good idea. If Tor is already running we can expect that the
user wants to use Tor as much as possible and thus tehre should be no need for
any configuration.

I do not think that we need a new option (except for making --no-use-tor). To
avoid checking for tor with every new connection to Dirmngr, I would do a test
at startup and after each reload.

I will do some rework to make testing for tor easier ....

Okay, that first part has been pushed. Now need to figure out how to test for
Tor in a clean way.

Thanks for this work (and sorry to have just blindly/wrongly assumed that
--no-use-tor already existed without checking it).

On modern debian systems, the default tor daemon will always be listening on
unix domain socket /run/tor/socks. So a simple attempt to connect to that
socket should be sufficient -- it should fail immediately if the socket isn't
present or if no one is listening on it.

This seems cheap and fast enough to be able to do it on every query to me,
rather than introducing additional runtime state to dirmngr. just try to
connect, and if it doesn't work, fall back to a normal connection (you'd want to
do that anyway in case the tor daemon goes away after dirmngr had been launched).

Using a socket conenction would require new code. We use the standard ports
instead. Sometimes the socks5 code (and I assume also the Unix domain socket
code) takes some time to figure out whether Tor is actually running, Thus this
is not done at every request.

Doing a check for every request would also require a lot of new code because we
need to restart a connection attempt at a higher layer. Similar to HTTP 301
handling.

FWIW, using a Debian specific thing is not portable and Unix sockets won't work on Windows. Thus using the standard localhost connection is simpler than adding extra complexity.