[presets] add steamos files

presets/steam/steamos/bin/startplasma-steamos-oneshot

@@ -0,0 +1,46 @@
+#!/bin/bash
+
+set -e
+
+die() { echo >&2 "!! $*"; exit 1; }
+
+SENTINEL_FILE="steamos-session-select"
+SENTINEL_VALUE="plasma-steamos-oneshot.desktop"
+
+# If we proceed, execute this
+CHAINED_SESSION="/usr/bin/startplasma-x11"
+# If we decide the sentinel is consumed, execute this command instead and fail
+RESTORE_SESSION=(steamos-session-select) # No arguments restores the session
+
+# Find or check config sentinel
+check_sentinel()
+{
+  if [[ -z ${HOME+x} ]]; then
+    echo >&2 "$0: No \$HOME variable!"
+    # Rather than break we'll just launch plasma and hope for the best?
+    return 0
+  fi
+
+  local config_dir="${XDG_CONF_DIR:-"$HOME/.config"}"
+  (
+    cd "$HOME"
+    cd "$config_dir"
+    sentinel_value="$(cat "$SENTINEL_FILE")"
+    [[ $sentinel_value = "$SENTINEL_VALUE" ]] || return 1
+    rm "$SENTINEL_FILE"
+  ) || return 1 # If we couldn't read the value or it wasn't what we wanted
+
+  # Found value and removed it, we're good to continue
+  return 0
+}
+
+if check_sentinel; then
+  # We found and consumed the oneshot sentinel, proceed to launch plasma
+  echo >&2 "$0: Found and removed sentinel file for one-shot plasma, proceeding to launch"
+  exec "$CHAINED_SESSION"
+else
+  echo >&2 "$0: Sentinel value not found, executing session-select to restore session"
+  "${RESTORE_SESSION[@]}" || echo >&2 "$0: !! Failed to restore previous session, executing chained session"
+  # Session restore should've stopped us, if it is broken at least let plasma continue to open
+  exec "$CHAINED_SESSION"
+fi