bug#70901: 30.0.50; Tramp doesn't use ControlMaster even with (setq tramp-use-connection-share nil)

[Dmitry Gutov <dmitry@gutov.dev>]

I have ~/.ssh/config set up correctly enough that

ssh <user>@<host> in the terminal connects quickly (and when I Ctrl-D
out of it, it finishes with "Shared connection to <...> closed.")

But I haven't been able to make Tramp reuse the connection.

I (setq tramp-use-connection-share nil) - so it would pick up the user
settings from my ssh config - and the first 'C-x C-f /ssh:user@host:'
takes a lot of time anyway with its "Opening connection ...".

For context: I'm on a high-latency network node geographically, and
using an even slower server for this example. I.e. my ping to it is
~170ms, but the full new ssh connection takes around 4 seconds.

On a related note, are asynchronous processes supposed to use
ControlMaster when it's available? I get "Opening connection ..." for
every M-& call, for example.

In GNU Emacs 30.0.50 (build 7, x86_64-pc-linux-gnu, X toolkit, cairo
  version 1.18.0, Xaw scroll bars) of 2024-05-12 built on potemkin
Repository revision: b20d4ab374fb9b3c80b968df6acd6444f763bd40
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12302000
System Description: Ubuntu 23.10