How can I test this?

It doesn't seem to work if all you do is nix-build -A sway and run result/bin/sway. I'm not exactly sure why. But if you add sway and xwayland to environment.systemPackages and run nixos-rebuild -I nixpkgs=/path/to/nixpkgs and then stop X11 and run sway as a user, you should have the sway background show up. hitting super+enter will give you a terminal. You can also switch workspaces using super+# (where # is the workspace you want). The sway bar doesn't seem to work with the default config on 0.14.0. Works great with 0.15-rc2 though, so maybe we just wait until that releases.

I added a module with the capabilities as described in the project page, but I get a segfault as well:

gdb> bt
#0  0x00007f4798edf0f7 in wl_list_remove () from /nix/store/xyajc0iyp9z1pkmssfazjjsvw1r9kc68-wayland-1.12.0/lib/libwayland-server.so.0
#1  0x00007f47997ce3a7 in wlc_xwm_release () from /nix/store/ia6jgf3yw2nrz0704dbnnk09bcrx6rsj-wlc-0.0.10/lib/libwlc.so.0
#2  0x00007f47997b34c0 in xwayland_event () from /nix/store/ia6jgf3yw2nrz0704dbnnk09bcrx6rsj-wlc-0.0.10/lib/libwlc.so.0
#3  0x00007f47997cbafc in wlc_xwayland_terminate () from /nix/store/ia6jgf3yw2nrz0704dbnnk09bcrx6rsj-wlc-0.0.10/lib/libwlc.so.0
#4  0x00007f47997cc422 in destroy_event () from /nix/store/ia6jgf3yw2nrz0704dbnnk09bcrx6rsj-wlc-0.0.10/lib/libwlc.so.0
#5  0x00007f4798eda7bc in wl_client_destroy () from /nix/store/xyajc0iyp9z1pkmssfazjjsvw1r9kc68-wayland-1.12.0/lib/libwayland-server.so.0
#6  0x00007f4798eda8d8 in wl_client_connection_data () from /nix/store/xyajc0iyp9z1pkmssfazjjsvw1r9kc68-wayland-1.12.0/lib/libwayland-server.so.0
#7  0x00007f4798edcae2 in wl_event_loop_dispatch () from /nix/store/xyajc0iyp9z1pkmssfazjjsvw1r9kc68-wayland-1.12.0/lib/libwayland-server.so.0
#8  0x00007f4798edaf8a in wl_display_run () from /nix/store/xyajc0iyp9z1pkmssfazjjsvw1r9kc68-wayland-1.12.0/lib/libwayland-server.so.0
#9  0x00007f47997c992c in wlc_run () from /nix/store/ia6jgf3yw2nrz0704dbnnk09bcrx6rsj-wlc-0.0.10/lib/libwlc.so.0
#10 0x000000000040bc81 in main ()
#11 0x00007f47974f4560 in __libc_start_main () from /nix/store/yydnhs7migvlbl48wpsxan1yvq2icbr9-glibc-2.25-49/lib/libc.so.6
#12 0x000000000040be3a in _start ()

Do you have something regarding opengl disabled/enabled? What graphic card do you use?

From 5e6491ac2ac33476fdc5afd681ba1c191b118629 Mon Sep 17 00:00:00 2001
From: Joerg Thalheim <joerg@thalheim.io>
Date: Tue, 17 Oct 2017 15:09:42 +0100
Subject: [PATCH] nixos/sway: add module
Signed-off-by: Joerg Thalheim <joerg@thalheim.io>
---
 nixos/modules/module-list.nix   |  1 +
 nixos/modules/programs/sway.nix | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 nixos/modules/programs/sway.nix
diff --git a/nixos/modules/module-list.nix b/nixos/modules/module-list.nix
index a8cb957ffe2..e8fecafb2c8 100644
--- a/nixos/modules/module-list.nix
+++ b/nixos/modules/module-list.nix
@@ -99,6 +99,7 @@
   ./programs/ssh.nix
   ./programs/ssmtp.nix
   ./programs/sysdig.nix
+  ./programs/sway.nix
   ./programs/thefuck.nix
   ./programs/tmux.nix
   ./programs/venus.nix
diff --git a/nixos/modules/programs/sway.nix b/nixos/modules/programs/sway.nix
new file mode 100644
index 00000000000..e0fcd0c1deb
--- /dev/null
+++ b/nixos/modules/programs/sway.nix
@@ -0,0 +1,19 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+{
+  options.programs.sway.enable = mkEnableOption "sway";
+
+  config = mkIf config.programs.sway.enable {
+    environment.systemPackages = [ pkgs.sway ];
+    security.wrappers.sway = {
+      source = "${pkgs.sway}/bin/sway";
+      capabilities = "cap_sys_ptrace,cap_sys_tty_config=eip";
+      owner = "root";
+      group = "sway";
+      permissions = "u+rx,g+rx";
+    };
+
+    users.extraGroups.sway = {};
+  };
+}
--
2.14.2

Graphics card is Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09) I have this in my configuration.nix:

      opengl.enable = true;                                                               
      opengl.extraPackages = [ pkgs.vaapiIntel ];

I have a Broadwell CPU + graphics. The version here keeps segfaulting while the old version seems to work. Can you make sure, you also tested the updated version and not the old?

using programs.sway.enable = true; I get a segfault as well, but if I just put sway and xwayland in environment.systemPackages it works fine for me. Here's the configuration.nix I used: https://gist.github.com/anonymous/6fab3b96d18cf6c808fafc5d0eeb05a5. Command to install was sudo nixos-rebuild switch -I nixpkgs=/home/sam/nixpkgs

proof of newer version of sway running:

sam      28275  1.0  1.5 252592 62856 tty1     S+   21:58   0:03 /run/current-system/sw/bin/sway                                                   
sam      28277  0.0  0.0 177456  1232 tty1     S+   21:58   0:00 /run/current-system/sw/bin/sway                                                   
sam      28279  0.0  0.5 150456 22384 tty1     S+   21:58   0:00 swaybg 0 /nix/store/r4g9pwhs6biyfsyaxxa2vnwbhlfwi8a0-sway-0.14.0/share/sway/Sway_Wallpaper_Blue_1920x1080.png fill
$ ls -hal `which sway`                                                                                                                             
lrwxrwxrwx 1 root root 64 Dec 31  1969 /run/current-system/sw/bin/sway -> /nix/store/r4g9pwhs6biyfsyaxxa2vnwbhlfwi8a0-sway-0.14.0/bin/sway         

Ok. With xwayland in my systemPackages it works. But adding just adding ${xwayland}/bin in the sway wrapper does not solve the problem. So it must be a different dependency, which gets propagated by this package.

@Mic92 I tried propagating the buildInputs for wlc, and it works for me now! Can you test?

I updated the nixos module to include xwayland and things work for me.

Should we pick this into stable?

Probably a good idea. This capability stuff was security relevant.

I am trying to figure out how this module is supposed to work...
If I enable it and remove everything I have that is XMonad and XServer related, I land on a plain tty1 prompt upon reboot.
I log in, I say sway and it complains about X not running.

Ideally, this module would just replace the xserver module and give me a UI to log in to etc. right?
Am I misunderstanding its purpose?

Also, would it not be better to sit in services next to xserver than in programs?

@NeQuissimus There were efforts on this, but none got finished and merged:
#19663
#23059

Ah, OK.

So if we mixed #19663 with a little bit of #20385 and this one here, we should have all the ingredients to make this work, correct?

Most likely. Here's someone was able to login via slim #19663 (comment)
It's important not to miss extraSessionCommands, because many things are configured this way starting from https://github.com/swaywm/sway/wiki#input-configuration

I mix-and-matched the PRs together but cannot get past SLiM or SDDM...
https://gist.github.com/NeQuissimus/bce7119e985bf077eafc16d9f99bcbcb

@NeQuissimus Your patch works for me if services.xserver.enable = true;. I'm able to login into sway and it works.
Regarding your patch:

1. package option is not necessary atm, since there are no sway forks, while there is valuable i3 fork - i3gaps.
2. Would be nice to also add extraPackages option https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/x11/window-managers/i3.nix#L40 to your patch

@NeQuissimus Perhaps something is wrong with your setup. If not using your patch:

If I enable it and remove everything I have that is XMonad and XServer related, I land on a plain tty1 prompt upon reboot.
I log in, I say sway and it complains about X not running.

If you disable xserver, then your should add hardware.opengl.enable = true; to your config. But actually you don't need to disable xserver and xmonad to test sway. Just switch to console tty, login and run sway command.

I use this to configure sway: (I import it and have no other xserver-related config)
https://github.com/NeQuissimus/DevSetup/blob/master/nixos-sway.nix

I updated the Gist to reflect your changes.
I still cannot log in... And I see no errors anywhere either.

@NeQuissimus Thanks. I've took your updated patch and nixos-sway.nix, did

I import it and have no other xserver-related config

the same. And it works fine for me. To debug try following:

1. Use your normal config with xserver and xmonad
2. Put programs.sway.enable = true; there, do rebuild
3. Switch to console tty (ctrl+alt+f1), login here
4. Run sway -Vd >/tmp/sway.log 2>&1 (you would like to start it via dbus-launch to have bar)

I will spend some more time with this over the weekend if I can. Once I have things working, I will send a PR with the contents of my Gist...