Writing user-friendly message is hard, with that said, let's think up better ones.

First of all, there are two situations where the message will be shown

1. The user wants an xsession-based session, but has no ~/.xsession file.
2. The session selection somehow selected the xsession-based session in yet undetermined conditions. (Unlikely in the following NixOS releases where this will default to false.)

For the first kind of users, the message explains what to do correctly. Though, the phrasing ("the user") makes for a weirdly disjointed experience. "The user" is likely the one reading the message.

For the second kind of users, it does not help them. They are being asked to create a file they may not want, or know how to use. The message needs to somehow explain that they must choose another session in the display manager. Bonus points for detecting that there are other sessions enabled, when there aren't add that they need to enable other sessions. Additional bonus points to show specific instructions for the enabled display manager.

An action is required

Your session started with the xsession session type without a ~/.xsession file.

You will need to do one of the two following actions, depending.

1. You know what is an xsession file and want to use the xsession session type. Please add the xsession script at the expected location (~/.xsession) and log back in.

2. You don't know what is an xsession file and did not expect to see this message. $$$

And for $$$

When there are more sessions enabled:

At the login prompt, select another session type.

When only xsession is enabled:

Configure the desired services.xserver.desktopManager or services.xserver.windowManager in your configuration.nix and rebuild the system. You will be able to select the desired session type at the login prompt.

Does this sound feasible? I'm thinking though that this may be a bit abrupt to only show those messages without giving the ability to act on them within the session... which would only be possible if we launched a terminal for the user, in addition to the prompt. And now I'm thinking we may need to figure out the WM with the smallest closure and somehow shoe-horn it there and I'm not so sure about anything :/.

Suddenly we've created a fallback xterm session for the user with very helpful messages @samueldr
(not that this is a bad thing)
And I'm still not sure the issue with the xterm session being selected as a default was just an issue with upstream xsessions in nixpkgs.

I believe that we'll see some users enable xserver without enabling a particular WM or DM. In those cases the message will help steer those the right way.

Though, I think there's one difference in having a wm+xterm, compared to xterm only: user friendlyness. xterm doesn't allow nicely launching graphical tools, as there is no wm to resize the windows. Additionally, writing in the terminal doesn't work unless your mouse hovers it. Even if the session only starts a fullscreen-type wm, which has some knowledge about managing windows, it ends up being more user friendly than a raw xterm.

~/tmp/nixpkgs/nixpkgs $ nix path-info -hS ./result*
/nix/store/4l1dm6fm5408gzxxd7jjfil5q7dd4853-twm-1.0.10            34.7M
/nix/store/lp9hifv43iic2shsrwvg9j7512m1sdpq-matchbox-1.2          36.9M

Shouldn't this also include removing the lines

@rycee I think we discussed that this change is just a starting point to eventually do this.
i.e it would be much easier to have this change right away instead of trying to do that all in one go.

@worldofpeace Fair enough, I haven't followed the discussions and I agree that this change by itself would be very useful for somebody who's not having an ~/.xsession file.

I'm still down to do this or something similar for 20.03.

Sounds good to me @Ericson2314. I'll add it to the milestone, hopefully we can circle back to get it finished. It was just difficult when it came in when 19.09 was released/releasing.

Edit: already in the milestone.

Am I correct that this means that to get no desktopManager at all, you will need to explicitly set services.xserver.desktopManager.session.enable to false again?

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