nixos: unify wallpapers via syncthing

Start hyprpaper via a user service and set wallpaper via IPC on session start.\nPoint xmonad random-background at the same Syncthing wallpaper directory (X11 only).

nixos/imalison.nix

@@ -4,6 +4,120 @@
       ./emacs.nix
     ];
 
+    # Hyprland config starts this target on login (see `dotfiles/config/hypr/hyprland.conf`).
+    systemd.user.targets.hyprland-session = {
+      Unit = {
+        Description = "Hyprland session (custom)";
+      };
+    };
+
+    systemd.user.services.hyprpaper = let
+      wallpaperDir = "/var/lib/syncthing/sync/Wallpaper";
+      waitForWayland = pkgs.writeShellScript "wait-for-wayland" ''
+        # Hyprpaper needs a Wayland socket. systemd "Condition*" checks are
+        # brittle here (they skip the unit entirely, with no retry) and also
+        # don't support expanding $WAYLAND_DISPLAY in paths.
+        for _ in {1..50}; do
+          if [ -n "$WAYLAND_DISPLAY" ] && [ -n "$XDG_RUNTIME_DIR" ] && [ -S "$XDG_RUNTIME_DIR/$WAYLAND_DISPLAY" ]; then
+            exit 0
+          fi
+          ${pkgs.coreutils}/bin/sleep 0.1
+        done
+
+        echo "Wayland socket not ready: WAYLAND_DISPLAY=$WAYLAND_DISPLAY XDG_RUNTIME_DIR=$XDG_RUNTIME_DIR" >&2
+        exit 1
+      '';
+      setWallpaper = pkgs.writeShellScript "set-hyprpaper-wallpaper" ''
+        # hyprpaper 0.8.x doesn't seem to apply `wallpaper = ...` entries from
+        # its config on startup reliably. Explicitly set the wallpaper via IPC
+        # once the hyprpaper socket is ready.
+        #
+        # NOTE: hyprctl hyprpaper currently ignores `--instance` and uses
+        # $HYPRLAND_INSTANCE_SIGNATURE to find hyprpaper's socket, so we rely on
+        # the environment imported by Hyprland on login.
+        set -u
+
+        runtimeDir="''${XDG_RUNTIME_DIR:-}"
+        sig="''${HYPRLAND_INSTANCE_SIGNATURE:-}"
+        sockPath=""
+        if [ -n "$runtimeDir" ] && [ -n "$sig" ]; then
+          sockPath="$runtimeDir/hypr/$sig/.hyprpaper.sock"
+        fi
+
+        # Don't fail the service if env isn't available yet.
+        if [ -z "$sockPath" ]; then
+          echo "set-hyprpaper-wallpaper: missing XDG_RUNTIME_DIR/HYPRLAND_INSTANCE_SIGNATURE" >&2
+          exit 0
+        fi
+
+        for _ in {1..50}; do
+          if [ -S "$sockPath" ]; then
+            break
+          fi
+          ${pkgs.coreutils}/bin/sleep 0.1
+        done
+
+        if [ ! -S "$sockPath" ]; then
+          echo "set-hyprpaper-wallpaper: hyprpaper socket not ready at $sockPath" >&2
+          exit 0
+        fi
+
+        if [ ! -d "${wallpaperDir}" ]; then
+          echo "set-hyprpaper-wallpaper: wallpaper directory missing: ${wallpaperDir}" >&2
+          exit 0
+        fi
+
+        wallpaper="$(${pkgs.findutils}/bin/find "${wallpaperDir}" -type f -regextype posix-extended -iregex '.*\.(png|jpe?g|webp)$' -print0 | ${pkgs.coreutils}/bin/shuf -z -n 1 | ${pkgs.coreutils}/bin/tr -d '\0')"
+
+        if [ -z "$wallpaper" ]; then
+          echo "set-hyprpaper-wallpaper: no wallpapers found in ${wallpaperDir}" >&2
+          exit 0
+        fi
+
+        # Apply to all current monitors.
+        mons=$(/run/current-system/sw/bin/hyprctl -j monitors 2>/dev/null | ${pkgs.jq}/bin/jq -r '.[].name' 2>/dev/null || true)
+        if [ -z "$mons" ]; then
+          # Fallback to parsing non-JSON output.
+          mons=$(/run/current-system/sw/bin/hyprctl monitors 2>/dev/null | ${pkgs.gnugrep}/bin/grep -oE '^Monitor [^ ]+' | ${pkgs.coreutils}/bin/cut -d' ' -f2 || true)
+        fi
+
+        for mon in $mons; do
+          # The socket file can exist before hyprpaper is actually ready to
+          # accept connections, so retry briefly.
+          for _ in {1..50}; do
+            if /run/current-system/sw/bin/hyprctl hyprpaper wallpaper "$mon,$wallpaper" >/dev/null 2>&1; then
+              break
+            fi
+            ${pkgs.coreutils}/bin/sleep 0.1
+          done
+        done
+
+        exit 0
+      '';
+      hyprpaperConf = pkgs.writeText "hyprpaper.conf" ''
+        ipc = true
+        splash = false
+      '';
+    in {
+      Unit = {
+        Description = "Hyprpaper (managed by home-manager)";
+        PartOf = [ "hyprland-session.target" ];
+        After = [ "hyprland-session.target" ];
+      };
+
+      Service = {
+        ExecStartPre = waitForWayland;
+        ExecStart = "${pkgs.hyprpaper}/bin/hyprpaper -c ${hyprpaperConf}";
+        ExecStartPost = setWallpaper;
+        Restart = "on-failure";
+        RestartSec = 1;
+      };
+
+      Install = {
+        WantedBy = [ "hyprland-session.target" ];
+      };
+    };
+
     xdg.desktopEntries.google-chrome-devtools = {
       name = "Google Chrome (DevTools)";
       genericName = "Web Browser";