wireplumber: ensure ALSA_UCM_CONF2 env var is on PATH

this is critical for pipewire/wireplumber to work on moby

hosts/by-name/moby/default.nix

@@ -126,40 +126,19 @@
   # inject specialized alsa configs via the environment.
   # specifically, this gets the pinephone headphones & internal earpiece working.
   # see pkgs/patched/alsa-ucm-conf for more info.
-  environment.variables.ALSA_CONFIG_UCM2 = "/run/current-system/sw/share/alsa/ucm2";
+  # TODO: move this directly into `sane.programs.alsa-ucm-conf`?
-  environment.pathsToLink = [ "/share/alsa/ucm2" ];
+  sane.programs.alsa-ucm-conf.packageUnwrapped = pkgs.alsa-ucm-conf-sane.override {
-  environment.systemPackages = [
-    (pkgs.alsa-ucm-conf-sane.override {
     # internal speaker has a tendency to break :(
     preferEarpiece = true;
-    })
+  };
-  ];
-  systemd = let
-    ucm-env = config.environment.variables.ALSA_CONFIG_UCM2;
-  in {
-    # cribbed from <repo:nixos/mobile-nixos:modules/quirks/audio.nix>
-
-    # pipewire
-    user.services.pipewire.environment.ALSA_CONFIG_UCM2       = ucm-env;
-    user.services.pipewire-pulse.environment.ALSA_CONFIG_UCM2 = ucm-env;
-    user.services.wireplumber.environment.ALSA_CONFIG_UCM2    = ucm-env;
-    services.pipewire.environment.ALSA_CONFIG_UCM2            = ucm-env;
-    services.pipewire-pulse.environment.ALSA_CONFIG_UCM2      = ucm-env;
-    services.wireplumber.environment.ALSA_CONFIG_UCM2         = ucm-env;
-
-    # pulseaudio
-    # user.services.pulseaudio.environment.ALSA_CONFIG_UCM2 = ucm-env;
-    # services.pulseaudio.environment.ALSA_CONFIG_UCM2      = ucm-env;
-
 
   # TODO: move elsewhere...
-    services.ModemManager.serviceConfig = {
+  systemd.services.ModemManager.serviceConfig = {
     # N.B.: the extra "" in ExecStart serves to force upstream ExecStart to be ignored
     ExecStart = [ "" "${pkgs.modemmanager}/bin/ModemManager --debug" ];
     # --debug sets DEBUG level logging: so reset
     ExecStartPost = [ "${pkgs.modemmanager}/bin/mmcli --set-logging=INFO" ];
   };
-  };
 
   services.udev.extraRules = let
     chmod = "${pkgs.coreutils}/bin/chmod";

hosts/common/programs/alsa-ucm-conf.nix

@@ -0,0 +1,22 @@
+{ config, lib, ... }:
+let
+  cfg = config.sane.programs.alsa-ucm-conf;
+in
+{
+  sane.programs.alsa-ucm-conf = {
+    sandbox.enable = false;  #< only provides #out/share/alsa
+
+    # alsa-lib package only looks in its $out/share/alsa to find runtime config data, by default.
+    # but ALSA_CONFIG_UCM2 is an env var that can override that.
+    # this is particularly needed by wireplumber;
+    #   also *maybe* pipewire and pipewire-pulse.
+    # taken from <repo:nixos/mobile-nixos:modules/quirks/audio.nix>
+    env.ALSA_CONFIG_UCM2 = "/run/current-system/sw/share/alsa/ucm2";
+
+    enableFor.system = lib.mkIf (builtins.any (en: en) (builtins.attrValues cfg.enableFor.user)) true;
+  };
+
+  environment.pathsToLink = lib.mkIf cfg.enabled [
+    "/share/alsa/ucm2"
+  ];
+}

hosts/common/programs/default.nix

@@ -5,6 +5,7 @@
     ./abaddon.nix
     ./aerc.nix
     ./alacritty.nix
+    ./alsa-ucm-conf.nix
     ./animatch.nix
     ./assorted.nix
     ./audacity.nix

hosts/common/programs/wireplumber.nix

@@ -25,6 +25,8 @@ in
       "/sys/devices"
     ];
 
+    suggestedPrograms = [ "alsa-ucm-conf" ];
+
     persist.byStore.plaintext = [
       # persist per-device volume levels
       ".local/state/wireplumber"